代码语言：javascript复制界面窗口的根布局是
DecorView，该类继承自FrameLayout，而FrameLayout继承自ViewGroup。感觉绘制肯定会在ViewGroup或者View中 但是木有找到。发现ViewGroup实现ViewParent接口，而ViewParent有一个实现类是ViewRootImpl，ViewGruop中会使用ViewRootImpl
/**
* The top of a view hierarchy, implementing the needed protocol between View
* and the WindowManager. This is for the most part an internal implementation
* detail of {@link WindowManagerGlobal}.
*
* {@hide}
*/
@SuppressWarnings({"EmptyCatchBlock", "PointlessBooleanExpression"})
public final class ViewRootImpl implements ViewParent,
View.AttachInfo.Callbacks, HardwareRenderer.HardwareDrawCallbacks {
}View的绘制过程从ViewRootImpl.performTraversals()方法开始。
private void performTraversals() {
// ... 此处省略源代码N行
// 是否需要Measure
if (!mStopped) {
boolean focusChangedDueToTouchMode = ensureTouchModeLocally(
(relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);
if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()
|| mHeight != host.getMeasuredHeight() || contentInsetsChanged) {
// 这里是获取widthMeasureSpec,这俩参数不是一般的尺寸数值，而是将模式和尺寸组合在一起的数值.
// getRootMeasureSpec方法内部会使用MeasureSpec.makeMeasureSpec()方法来组装一个MeasureSpec，
// 当lp.width参数等于MATCH_PARENT的时候，MeasureSpec的specMode就等于EXACTLY，当lp.width等于WRAP_CONTENT的时候，MeasureSpec的specMode就等于AT_MOST。
// 并且MATCH_PARENT和WRAP_CONTENT时的specSize都是等于windowSize的，也就意味着根视图总是会充满全屏的。
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
if (DEBUG_LAYOUT) Log.v(TAG, "Ooops, something changed! mWidth="
mWidth " measuredWidth=" host.getMeasuredWidth()
" mHeight=" mHeight
" measuredHeight=" host.getMeasuredHeight()
" coveredInsetsChanged=" contentInsetsChanged);
// 调用PerformMeasure方法。
// Ask host how big it wants to be
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
// Implementation of weights from WindowManager.LayoutParams
// We just grow the dimensions as needed and re-measure if
// needs be
int width = host.getMeasuredWidth();
int height = host.getMeasuredHeight();
boolean measureAgain = false;
if (lp.horizontalWeight > 0.0f) {
width = (int) ((mWidth - width) * lp.horizontalWeight);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(width,
MeasureSpec.EXACTLY);
measureAgain = true;
}
if (lp.verticalWeight > 0.0f) {
height = (int) ((mHeight - height) * lp.verticalWeight);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(height,
MeasureSpec.EXACTLY);
measureAgain = true;
}
if (measureAgain) {
if (DEBUG_LAYOUT) Log.v(TAG,
"And hey let's measure once more: width=" width
" height=" height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
}
layoutRequested = true;
}
}
final boolean didLayout = layoutRequested && !mStopped;
boolean triggerGlobalLayoutListener = didLayout
|| mAttachInfo.mRecomputeGlobalAttributes;
// 是否需要Layout
if (didLayout) {
// 调用performLayout方法。
performLayout(lp, desiredWindowWidth, desiredWindowHeight);
// By this point all views have been sized and positioned
// We can compute the transparent area
if ((host.mPrivateFlags & View.PFLAG_REQUEST_TRANSPARENT_REGIONS) != 0) {
// start out transparent
// TODO: AVOID THAT CALL BY CACHING THE RESULT?
host.getLocationInWindow(mTmpLocation);
mTransparentRegion.set(mTmpLocation[0], mTmpLocation[1],
mTmpLocation[0] host.mRight - host.mLeft,
mTmpLocation[1] host.mBottom - host.mTop);
host.gatherTransparentRegion(mTransparentRegion);
if (mTranslator != null) {
mTranslator.translateRegionInWindowToScreen(mTransparentRegion);
}
if (!mTransparentRegion.equals(mPreviousTransparentRegion)) {
mPreviousTransparentRegion.set(mTransparentRegion);
mFullRedrawNeeded = true;
// reconfigure window manager
try {
mWindowSession.setTransparentRegion(mWindow, mTransparentRegion);
} catch (RemoteException e) {
}
}
}
if (DBG) {
System.out.println("======================================");
System.out.println("performTraversals -- after setFrame");
host.debug();
}
}
// 是否需要Draw
if (!cancelDraw && !newSurface) {
if (!skipDraw || mReportNextDraw) {
if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
for (int i = 0; i < mPendingTransitions.size(); i) {
mPendingTransitions.get(i).startChangingAnimations();
}
mPendingTransitions.clear();
}
// 调用performDraw方法
performDraw();
}
} else {
if (viewVisibility == View.VISIBLE) {
// Try again
scheduleTraversals();
} else if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
for (int i = 0; i < mPendingTransitions.size(); i) {
mPendingTransitions.get(i).endChangingAnimations();
}
mPendingTransitions.clear();
}
}
mIsInTraversal = false;
}从上面源码可以看出,performTraversals()方法中会依次做三件事：
performMeasure(), 内部是mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);测量View大小。这里顺便提一下，这个mView是什么？它就是Window最顶成的View(DecorView),它是FrameLayout的子类。performLayout(), 内部是mView.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());视图布局，确定View位置。performDraw(), 内部是draw(fullRedrawNeeded);绘制界面。
View绘制的三个过程Measure、Layout、Draw
下面是这三个过程的详细说明
Measure
performMeasure方法如下：
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}在performMeasure()方法中会调用View.measure()方法， 源码如下：
/**
* <p>
* This is called to find out how big a view should be. The parent
* supplies constraint information in the width and height parameters.
* </p>
*
* <p>
* The actual measurement work of a view is performed in
* {@link #onMeasure(int, int)}, called by this method. Therefore, only
* {@link #onMeasure(int, int)} can and must be overridden by subclasses.
* </p>
*
*
* @param widthMeasureSpec Horizontal space requirements as imposed by the
* parent
* @param heightMeasureSpec Vertical space requirements as imposed by the
* parent
*
* @see #onMeasure(int, int)
*/
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int oWidth = insets.left insets.right;
int oHeight = insets.top insets.bottom;
widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
}
// Suppress sign extension for the low bytes
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
mMeasureCache.indexOfKey(key);
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// 调用onMeasure方法
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
long value = mMeasureCache.valueAt(cacheIndex);
// Casting a long to int drops the high 32 bits, no mask needed
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
// 重写onMeausre方法的时，必须调用setMeasuredDimension或者super.onMeasure方法，不然就会走到这里报错。
// setMeasuredDimension中回去改变mPrivateFlags的值
throw new IllegalStateException("onMeasure() did not set the"
" measured dimension by calling"
" setMeasuredDimension()");
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}在measure方法中会调用onMeasure方法。ViewGroup的子类会重写该方法来进行测量大小，因为mView是DecorView，
而DecorView是FrameLayout的子类。所以我们看一下FrameLayout.onMeasure方法：
FrameLayout.onMeasure源码如下：
/**
* {@inheritDoc}
*/
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i ) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
// 调用该方法去测量每个子View
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() lp.leftMargin lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() lp.topMargin lp.bottomMargin);
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too
maxWidth = getPaddingLeftWithForeground() getPaddingRightWithForeground();
maxHeight = getPaddingTopWithForeground() getPaddingBottomWithForeground();
// Check against our minimum height and width
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
// Check against our foreground's minimum height and width
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
count = mMatchParentChildren.size();
if (count > 1) {
for (int i = 0; i < count; i ) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
int childWidthMeasureSpec;
int childHeightMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -
getPaddingLeftWithForeground() - getPaddingRightWithForeground() -
lp.leftMargin - lp.rightMargin,
MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() getPaddingRightWithForeground()
lp.leftMargin lp.rightMargin,
lp.width);
}
if (lp.height == LayoutParams.MATCH_PARENT) {
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -
getPaddingTopWithForeground() - getPaddingBottomWithForeground() -
lp.topMargin - lp.bottomMargin,
MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() getPaddingBottomWithForeground()
lp.topMargin lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}我们看到内部会调用measureChildWithMargins()方法,该方法源码如下：
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding
* and margins. The child must have MarginLayoutParams The heavy lifting is
* done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param widthUsed Extra space that has been used up by the parent
* horizontally (possibly by other children of the parent)
* @param parentHeightMeasureSpec The height requirements for this view
* @param heightUsed Extra space that has been used up by the parent
* vertically (possibly by other children of the parent)
*/
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft mPaddingRight lp.leftMargin lp.rightMargin
widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop mPaddingBottom lp.topMargin lp.bottomMargin
heightUsed, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}里面就是对该子View调用了measure方法，我们假设这个View已经不是ViewGroup了，就会又和上面一样，又调用onMeasure方法，
下面我们直接看一下View.onMeasure()方法：
View.onMeasure()方法的源码如下：
/**
* <p>
* Measure the view and its content to determine the measured width and the
* measured height. This method is invoked by {@link #measure(int, int)} and
* should be overriden by subclasses to provide accurate and efficient
* measurement of their contents.
* </p>
*
* <p>
* <strong>CONTRACT:</strong> When overriding this method, you
* <em>must</em> call {@link #setMeasuredDimension(int, int)} to store the
* measured width and height of this view. Failure to do so will trigger an
* <code>IllegalStateException</code>, thrown by
* {@link #measure(int, int)}. Calling the superclass'
* {@link #onMeasure(int, int)} is a valid use.
* </p>
*
* <p>
* The base class implementation of measure defaults to the background size,
* unless a larger size is allowed by the MeasureSpec. Subclasses should
* override {@link #onMeasure(int, int)} to provide better measurements of
* their content.
* </p>
*
* <p>
* If this method is overridden, it is the subclass's responsibility to make
* sure the measured height and width are at least the view's minimum height
* and width ({@link #getSuggestedMinimumHeight()} and
* {@link #getSuggestedMinimumWidth()}).
* </p>
*
* @param widthMeasureSpec horizontal space requirements as imposed by the parent.
* The requirements are encoded with
* {@link android.view.View.MeasureSpec}.
* @param heightMeasureSpec vertical space requirements as imposed by the parent.
* The requirements are encoded with
* {@link android.view.View.MeasureSpec}.
*
* @see #getMeasuredWidth()
* @see #getMeasuredHeight()
* @see #setMeasuredDimension(int, int)
* @see #getSuggestedMinimumHeight()
* @see #getSuggestedMinimumWidth()
* @see android.view.View.MeasureSpec#getMode(int)
* @see android.view.View.MeasureSpec#getSize(int)
*/
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// 如果不重写onMeasure方法，默认会调用getDefaultSize获取大小，下面会说getDefaultSize这个方法。
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}setMeasuredDimension()方法如下：
/**
* <p>This method must be called by {@link #onMeasure(int, int)} to store the
* measured width and measured height. Failing to do so will trigger an
* exception at measurement time.</p>
*
* @param measuredWidth The measured width of this view. May be a complex
* bit mask as defined by {@link #MEASURED_SIZE_MASK} and
* {@link #MEASURED_STATE_TOO_SMALL}.
* @param measuredHeight The measured height of this view. May be a complex
* bit mask as defined by {@link #MEASURED_SIZE_MASK} and
* {@link #MEASURED_STATE_TOO_SMALL}.
*/
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int opticalWidth = insets.left insets.right;
int opticalHeight = insets.top insets.bottom;
measuredWidth = optical ? opticalWidth : -opticalWidth;
measuredHeight = optical ? opticalHeight : -opticalHeight;
}
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}setMeasuredDimensionRaw()方法如下：
/**
* Sets the measured dimension without extra processing for things like optical bounds.
* Useful for reapplying consistent values that have already been cooked with adjustments
* for optical bounds, etc. such as those from the measurement cache.
*
* @param measuredWidth The measured width of this view. May be a complex
* bit mask as defined by {@link #MEASURED_SIZE_MASK} and
* {@link #MEASURED_STATE_TOO_SMALL}.
* @param measuredHeight The measured height of this view. May be a complex
* bit mask as defined by {@link #MEASURED_SIZE_MASK} and
* {@link #MEASURED_STATE_TOO_SMALL}.
*/
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
// 赋值给mMeasuredWidth，getMeasuredWidth就会调用该值。
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
// 这就是重写onMeasure方法时如果不调用setMeasuredDimension方法时为什么会报错的原因。
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}我们接着看一下上面用到的getDefaultSize()方法，源码如下：
/**
* Utility to return a default size. Uses the supplied size if the
* MeasureSpec imposed no constraints. Will get larger if allowed
* by the MeasureSpec.
*
* @param size Default size for this view
* @param measureSpec Constraints imposed by the parent
* @return The size this view should be.
*/
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
// measureSpec值用于获取宽度(高度)的规格和大小，解析出对应的size和mode
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}getDefaultSize方法又会使用到MeasureSpec类
- MeasureSpec.EXACTLY The parent has determined an exact size for the child. The child is going to be given those bounds regardless of how big it wants to be. 理解成MATCH_PARENT或者在布局中指定了宽高值，如layout:width=‘50dp’
- MeasureSpec.AT_MOST The child can be as large as it wants up to the specified size.理解成WRAP_CONTENT,这是的值是父View可以允许的最大的值，只要不超过这个值都可以。
- MeasureSpec.UNSPECIFIED The parent has not imposed any constraint on the child. It can be whatever size it wants. 这种情况比较少，一般用不到。
这里简单总结一下上面的过程:
代码语言：javascript复制performMeasure() {
- 1.调用View.measure方法
mView.measure():
- 2.measure内部会调用onMeasure方法,但是因为这里mView是DecorView，所以会调用FrameLayout的onMeasure方法。
onMeasure(FrameLayout)
- 3. 内部设置ViewGroup的宽高
setMeasuredDimension
并且对每个子View进行遍历测量
for (int i = 0; i < count; i ) {
final View child = getChildAt(i);
- 4. 对每个子View调用measureChildWithMargins方法
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
-5. measureChildWithMargins内部调用子View的measure方法
meausre
- 6. measure方法内部又调用onMeasure方法
onMeasure(View)
- 7. onMeasure方法内部调用setMeasuredDimension
setMeasuredDimension
- 8. setMeasuredDimension内部调用setMeasuredDimensionRaw
setMeasuredDimensionRaw
}
}从上面代码中能看到measure是final的，我们可以重写onMeasure来实现measure过程。
到这里基本都讲完了，我们在开发中会按照需要重写onMeasure方法，然后调用setMeasuredDimension方法设置大小，
ps:譬如我们设置了setMeasuredDimension(10, 10),那么不管布局中怎么设置这个View的大小
都是没用的，最后显示出来大小都是10*10。
Layout
performLayout方法源码如下：
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mLayoutRequested = false;
mScrollMayChange = true;
mInLayout = true;
final View host = mView;
if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
Log.v(TAG, "Laying out " host " to ("
host.getMeasuredWidth() ", " host.getMeasuredHeight() ")");
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
try {
// 把刚才测量的宽高设置进来
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
mInLayout = false;
int numViewsRequestingLayout = mLayoutRequesters.size();
if (numViewsRequestingLayout > 0) {
// requestLayout() was called during layout.
// If no layout-request flags are set on the requesting views, there is no problem.
// If some requests are still pending, then we need to clear those flags and do
// a full request/measure/layout pass to handle this situation.
ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters,
false);
if (validLayoutRequesters != null) {
// Set this flag to indicate that any further requests are happening during
// the second pass, which may result in posting those requests to the next
// frame instead
mHandlingLayoutInLayoutRequest = true;
// Process fresh layout requests, then measure and layout
int numValidRequests = validLayoutRequesters.size();
for (int i = 0; i < numValidRequests; i) {
final View view = validLayoutRequesters.get(i);
Log.w("View", "requestLayout() improperly called by " view
" during layout: running second layout pass");
view.requestLayout();
}
measureHierarchy(host, lp, mView.getContext().getResources(),
desiredWindowWidth, desiredWindowHeight);
mInLayout = true;
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
mHandlingLayoutInLayoutRequest = false;
// Check the valid requests again, this time without checking/clearing the
// layout flags, since requests happening during the second pass get noop'd
validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);
if (validLayoutRequesters != null) {
final ArrayList<View> finalRequesters = validLayoutRequesters;
// Post second-pass requests to the next frame
getRunQueue().post(new Runnable() {
@Override
public void run() {
int numValidRequests = finalRequesters.size();
for (int i = 0; i < numValidRequests; i) {
final View view = finalRequesters.get(i);
Log.w("View", "requestLayout() improperly called by " view
" during second layout pass: posting in next frame");
view.requestLayout();
}
}
});
}
}
}
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
mInLayout = false;
}内部会调用layout()方法，因为host是mView，ViewGroup中重写了layout方法，并调用了super.layout.
所以我们直接看View.layout()方法，该方法源码如下：
/**
* Assign a size and position to a view and all of its
* descendants
*
* <p>This is the second phase of the layout mechanism.
* (The first is measuring). In this phase, each parent calls
* layout on all of its children to position them.
* This is typically done using the child measurements
* that were stored in the measure pass().</p>
*
* <p>Derived classes should not override this method.
* Derived classes with children should override
* onLayout. In that method, they should
* call layout on each of their children.</p>
*
* @param l Left position, relative to parent
* @param t Top position, relative to parent
* @param r Right position, relative to parent
* @param b Bottom position, relative to parent
*/
@SuppressWarnings({"unchecked"})
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
// 这部分是判断这个View的大小是否已经发生了变化，来判断是否需要重绘。
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
// 内部调用onLayout方法
onLayout(changed, l, t, r, b);
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList<OnLayoutChangeListener> listenersCopy =
(ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i < numListeners; i) {
listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}这里会调用onLayout方法，同样因为mView是FrameLayout的子类，所以我们要看FrameLayout的onLayout方法，
这里我们先看一下ViewGroup.onLayout方法:
/**
* {@inheritDoc}
*/
@Override
protected abstract void onLayout(boolean changed,
int l, int t, int r, int b);是个抽象方法，所以ViewGroup的子类都需要实现该方法。
我们看一下FrameLayout.onLayout方法,源码如下：
/**
* {@inheritDoc}
*/
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
void layoutChildren(int left, int top, int right, int bottom,
boolean forceLeftGravity) {
final int count = getChildCount();
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
mForegroundBoundsChanged = true;
for (int i = 0; i < count; i ) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
int gravity = lp.gravity;
if (gravity == -1) {
gravity = DEFAULT_CHILD_GRAVITY;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft (parentRight - parentLeft - width) / 2
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
childLeft = parentLeft lp.leftMargin;
}
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop (parentBottom - parentTop - height) / 2
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop lp.topMargin;
}
//调用子View的layout方法
child.layout(childLeft, childTop, childLeft width, childTop height);
}
}
}而View.layout方法，又会调用到View.onLayout方法，我们假设这个子View不是ViewGroup.
看一下View.onLayout方法源码如下：
/**
* Called from layout when this view should
* assign a size and position to each of its children.
*
* Derived classes with children should override
* this method and call layout on each of
* their children.
* @param changed This is a new size or position for this view
* @param left Left position, relative to parent
* @param top Top position, relative to parent
* @param right Right position, relative to parent
* @param bottom Bottom position, relative to parent
*/
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}是一个空方法，这是因为Layout需要ViewGroup来控制进行。
这里也总结一下layout的过程。
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
- 1. host.layout
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
-2. layout方法会分别调用setFrame()和onLayout()方法
setFrame()
onLayout()
-3. 因为host是mView也就是DecorView也就是FrameLayout的子类。FrameLayout的onLayout方法如下
for (int i = 0; i < count; i ) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
-4. 遍历每个子View，并分别调用layout方法。
child.layout(childLeft, childTop, childLeft width, childTop height);
}
}
}Draw
绘制阶段是从ViewRootImpl中的performDraw方法开始的：
private void performDraw() {
if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
return;
}
final boolean fullRedrawNeeded = mFullRedrawNeeded;
mFullRedrawNeeded = false;
mIsDrawing = true;
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
try {
// 开始draw了
draw(fullRedrawNeeded);
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
// For whatever reason we didn't create a HardwareRenderer, end any
// hardware animations that are now dangling
if (mAttachInfo.mPendingAnimatingRenderNodes != null) {
final int count = mAttachInfo.mPendingAnimatingRenderNodes.size();
for (int i = 0; i < count; i ) {
mAttachInfo.mPendingAnimatingRenderNodes.get(i).endAllAnimators();
}
mAttachInfo.mPendingAnimatingRenderNodes.clear();
}
if (mReportNextDraw) {
mReportNextDraw = false;
if (mAttachInfo.mHardwareRenderer != null) {
mAttachInfo.mHardwareRenderer.fence();
}
if (LOCAL_LOGV) {
Log.v(TAG, "FINISHED DRAWING: " mWindowAttributes.getTitle());
}
if (mSurfaceHolder != null && mSurface.isValid()) {
mSurfaceHolderCallback.surfaceRedrawNeeded(mSurfaceHolder);
SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();
if (callbacks != null) {
for (SurfaceHolder.Callback c : callbacks) {
if (c instanceof SurfaceHolder.Callback2) {
((SurfaceHolder.Callback2)c).surfaceRedrawNeeded(
mSurfaceHolder);
}
}
}
}
try {
mWindowSession.finishDrawing(mWindow);
} catch (RemoteException e) {
}
}
}内部会调用draw方法，draw方法源码如下：
private void draw(boolean fullRedrawNeeded) {
Surface surface = mSurface;
if (!surface.isValid()) {
return;
}
if (DEBUG_FPS) {
trackFPS();
}
if (!sFirstDrawComplete) {
synchronized (sFirstDrawHandlers) {
sFirstDrawComplete = true;
final int count = sFirstDrawHandlers.size();
for (int i = 0; i< count; i ) {
mHandler.post(sFirstDrawHandlers.get(i));
}
}
}
scrollToRectOrFocus(null, false);
if (mAttachInfo.mViewScrollChanged) {
mAttachInfo.mViewScrollChanged = false;
mAttachInfo.mTreeObserver.dispatchOnScrollChanged();
}
boolean animating = mScroller != null && mScroller.computeScrollOffset();
final int curScrollY;
if (animating) {
curScrollY = mScroller.getCurrY();
} else {
curScrollY = mScrollY;
}
if (mCurScrollY != curScrollY) {
mCurScrollY = curScrollY;
fullRedrawNeeded = true;
}
final float appScale = mAttachInfo.mApplicationScale;
final boolean scalingRequired = mAttachInfo.mScalingRequired;
int resizeAlpha = 0;
if (mResizeBuffer != null) {
long deltaTime = SystemClock.uptimeMillis() - mResizeBufferStartTime;
if (deltaTime < mResizeBufferDuration) {
float amt = deltaTime/(float) mResizeBufferDuration;
amt = mResizeInterpolator.getInterpolation(amt);
animating = true;
resizeAlpha = 255 - (int)(amt*255);
} else {
disposeResizeBuffer();
}
}
final Rect dirty = mDirty;
if (mSurfaceHolder != null) {
// The app owns the surface, we won't draw.
dirty.setEmpty();
if (animating) {
if (mScroller != null) {
mScroller.abortAnimation();
}
disposeResizeBuffer();
}
return;
}
if (fullRedrawNeeded) {
mAttachInfo.mIgnoreDirtyState = true;
dirty.set(0, 0, (int) (mWidth * appScale 0.5f), (int) (mHeight * appScale 0.5f));
}
if (DEBUG_ORIENTATION || DEBUG_DRAW) {
Log.v(TAG, "Draw " mView "/"
mWindowAttributes.getTitle()
": dirty={" dirty.left "," dirty.top
"," dirty.right "," dirty.bottom "} surface="
surface " surface.isValid()=" surface.isValid() ", appScale:"
appScale ", width=" mWidth ", height=" mHeight);
}
mAttachInfo.mTreeObserver.dispatchOnDraw();
int xOffset = 0;
int yOffset = curScrollY;
final WindowManager.LayoutParams params = mWindowAttributes;
final Rect surfaceInsets = params != null ? params.surfaceInsets : null;
if (surfaceInsets != null) {
xOffset -= surfaceInsets.left;
yOffset -= surfaceInsets.top;
// Offset dirty rect for surface insets.
dirty.offset(surfaceInsets.left, surfaceInsets.right);
}
if (!dirty.isEmpty() || mIsAnimating) {
if (mAttachInfo.mHardwareRenderer != null && mAttachInfo.mHardwareRenderer.isEnabled()) {
// Draw with hardware renderer.
mIsAnimating = false;
boolean invalidateRoot = false;
if (mHardwareYOffset != yOffset || mHardwareXOffset != xOffset) {
mHardwareYOffset = yOffset;
mHardwareXOffset = xOffset;
mAttachInfo.mHardwareRenderer.invalidateRoot();
}
mResizeAlpha = resizeAlpha;
dirty.setEmpty();
mBlockResizeBuffer = false;
mAttachInfo.mHardwareRenderer.draw(mView, mAttachInfo, this);
} else {
// If we get here with a disabled & requested hardware renderer, something went
// wrong (an invalidate posted right before we destroyed the hardware surface
// for instance) so we should just bail out. Locking the surface with software
// rendering at this point would lock it forever and prevent hardware renderer
// from doing its job when it comes back.
// Before we request a new frame we must however attempt to reinitiliaze the
// hardware renderer if it's in requested state. This would happen after an
// eglTerminate() for instance.
if (mAttachInfo.mHardwareRenderer != null &&
!mAttachInfo.mHardwareRenderer.isEnabled() &&
mAttachInfo.mHardwareRenderer.isRequested()) {
try {
mAttachInfo.mHardwareRenderer.initializeIfNeeded(
mWidth, mHeight, mSurface, surfaceInsets);
} catch (OutOfResourcesException e) {
handleOutOfResourcesException(e);
return;
}
mFullRedrawNeeded = true;
scheduleTraversals();
return;
}
// draw的部分在这里。。。内部会用canvas去画
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
}
}
if (animating) {
mFullRedrawNeeded = true;
scheduleTraversals();
}
}我们看一下drawSoftware方法：
/**
* @return true if drawing was successful, false if an error occurred
*/
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
// Draw with software renderer.
final Canvas canvas;
try {
final int left = dirty.left;
final int top = dirty.top;
final int right = dirty.right;
final int bottom = dirty.bottom;
canvas = mSurface.lockCanvas(dirty);
// The dirty rectangle can be modified by Surface.lockCanvas()
//noinspection ConstantConditions
if (left != dirty.left || top != dirty.top || right != dirty.right
|| bottom != dirty.bottom) {
attachInfo.mIgnoreDirtyState = true;
}
// TODO: Do this in native
canvas.setDensity(mDensity);
} catch (Surface.OutOfResourcesException e) {
handleOutOfResourcesException(e);
return false;
} catch (IllegalArgumentException e) {
Log.e(TAG, "Could not lock surface", e);
// Don't assume this is due to out of memory, it could be
// something else, and if it is something else then we could
// kill stuff (or ourself) for no reason.
mLayoutRequested = true; // ask wm for a new surface next time.
return false;
}
try {
if (DEBUG_ORIENTATION || DEBUG_DRAW) {
Log.v(TAG, "Surface " surface " drawing to bitmap w="
canvas.getWidth() ", h=" canvas.getHeight());
//canvas.drawARGB(255, 255, 0, 0);
}
// If this bitmap's format includes an alpha channel, we
// need to clear it before drawing so that the child will
// properly re-composite its drawing on a transparent
// background. This automatically respects the clip/dirty region
// or
// If we are applying an offset, we need to clear the area
// where the offset doesn't appear to avoid having garbage
// left in the blank areas.
if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
canvas.drawColor(0, PorterDuff.Mode.CLEAR);
}
dirty.setEmpty();
mIsAnimating = false;
attachInfo.mDrawingTime = SystemClock.uptimeMillis();
mView.mPrivateFlags |= View.PFLAG_DRAWN;
if (DEBUG_DRAW) {
Context cxt = mView.getContext();
Log.i(TAG, "Drawing: package:" cxt.getPackageName()
", metrics=" cxt.getResources().getDisplayMetrics()
", compatibilityInfo=" cxt.getResources().getCompatibilityInfo());
}
try {
canvas.translate(-xoff, -yoff);
if (mTranslator != null) {
mTranslator.translateCanvas(canvas);
}
canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
attachInfo.mSetIgnoreDirtyState = false;
// 内部会去调用View.draw()；
mView.draw(canvas);
} finally {
if (!attachInfo.mSetIgnoreDirtyState) {
// Only clear the flag if it was not set during the mView.draw() call
attachInfo.mIgnoreDirtyState = false;
}
}
} finally {
try {
surface.unlockCanvasAndPost(canvas);
} catch (IllegalArgumentException e) {
Log.e(TAG, "Could not unlock surface", e);
mLayoutRequested = true; // ask wm for a new surface next time.
//noinspection ReturnInsideFinallyBlock
return false;
}
if (LOCAL_LOGV) {
Log.v(TAG, "Surface " surface " unlockCanvasAndPost");
}
}
return true;
}代码中调用了mView.draw()方法，所以我们看一下FrameLayout.draw()方法：
/**
* {@inheritDoc}
*/
@Override
public void draw(Canvas canvas) {
super.draw(canvas);
if (mForeground != null) {
final Drawable foreground = mForeground;
if (mForegroundBoundsChanged) {
mForegroundBoundsChanged = false;
final Rect selfBounds = mSelfBounds;
final Rect overlayBounds = mOverlayBounds;
final int w = mRight-mLeft;
final int h = mBottom-mTop;
if (mForegroundInPadding) {
selfBounds.set(0, 0, w, h);
} else {
selfBounds.set(mPaddingLeft, mPaddingTop, w - mPaddingRight, h - mPaddingBottom);
}
final int layoutDirection = getLayoutDirection();
Gravity.apply(mForegroundGravity, foreground.getIntrinsicWidth(),
foreground.getIntrinsicHeight(), selfBounds, overlayBounds,
layoutDirection);
foreground.setBounds(overlayBounds);
}
foreground.draw(canvas);
}
}内部调用了super.draw()，而ViewGroup没有重写该方法，所以直接看View的draw()方法.
View.draw()方法如下：
/**
* Manually render this view (and all of its children) to the given Canvas.
* The view must have already done a full layout before this function is
* called. When implementing a view, implement
* {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.
* If you do need to override this method, call the superclass version.
*
* @param canvas The Canvas to which the View is rendered.
*/
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
// 这里注释说的很明白了，draw的6个步骤。
/*
* Draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. Draw the background
* 2. If necessary, save the canvas' layers to prepare for fading
* 3. Draw view's content, 调用onDraw方法绘制自身
* 4. Draw children, 调用dispatchDraw方法绘制子View
* 5. If necessary, draw the fading edges and restore layers
* 6. Draw decorations (scrollbars for instance)
*/
// Step 1, draw the background, if needed
int saveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
// skip step 2 & 5 if possible (common case)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas);
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// we're done...
return;
}
/*
* Here we do the full fledged routine...
* (this is an uncommon case where speed matters less,
* this is why we repeat some of the tests that have been
* done above)
*/
boolean drawTop = false;
boolean drawBottom = false;
boolean drawLeft = false;
boolean drawRight = false;
float topFadeStrength = 0.0f;
float bottomFadeStrength = 0.0f;
float leftFadeStrength = 0.0f;
float rightFadeStrength = 0.0f;
// Step 2, save the canvas' layers
int paddingLeft = mPaddingLeft;
final boolean offsetRequired = isPaddingOffsetRequired();
if (offsetRequired) {
paddingLeft = getLeftPaddingOffset();
}
int left = mScrollX paddingLeft;
int right = left mRight - mLeft - mPaddingRight - paddingLeft;
int top = mScrollY getFadeTop(offsetRequired);
int bottom = top getFadeHeight(offsetRequired);
if (offsetRequired) {
right = getRightPaddingOffset();
bottom = getBottomPaddingOffset();
}
final ScrollabilityCache scrollabilityCache = mScrollCache;
final float fadeHeight = scrollabilityCache.fadingEdgeLength;
int length = (int) fadeHeight;
// clip the fade length if top and bottom fades overlap
// overlapping fades produce odd-looking artifacts
if (verticalEdges && (top length > bottom - length)) {
length = (bottom - top) / 2;
}
// also clip horizontal fades if necessary
if (horizontalEdges && (left length > right - length)) {
length = (right - left) / 2;
}
if (verticalEdges) {
topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
drawTop = topFadeStrength * fadeHeight > 1.0f;
bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
}
if (horizontalEdges) {
leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
drawLeft = leftFadeStrength * fadeHeight > 1.0f;
rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
drawRight = rightFadeStrength * fadeHeight > 1.0f;
}
saveCount = canvas.getSaveCount();
int solidColor = getSolidColor();
if (solidColor == 0) {
final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;
if (drawTop) {
canvas.saveLayer(left, top, right, top length, null, flags);
}
if (drawBottom) {
canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
}
if (drawLeft) {
canvas.saveLayer(left, top, left length, bottom, null, flags);
}
if (drawRight) {
canvas.saveLayer(right - length, top, right, bottom, null, flags);
}
} else {
scrollabilityCache.setFadeColor(solidColor);
}
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Step 5, draw the fade effect and restore layers
final Paint p = scrollabilityCache.paint;
final Matrix matrix = scrollabilityCache.matrix;
final Shader fade = scrollabilityCache.shader;
if (drawTop) {
matrix.setScale(1, fadeHeight * topFadeStrength);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, right, top length, p);
}
if (drawBottom) {
matrix.setScale(1, fadeHeight * bottomFadeStrength);
matrix.postRotate(180);
matrix.postTranslate(left, bottom);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, bottom - length, right, bottom, p);
}
if (drawLeft) {
matrix.setScale(1, fadeHeight * leftFadeStrength);
matrix.postRotate(-90);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, left length, bottom, p);
}
if (drawRight) {
matrix.setScale(1, fadeHeight * rightFadeStrength);
matrix.postRotate(90);
matrix.postTranslate(right, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(right - length, top, right, bottom, p);
}
canvas.restoreToCount(saveCount);
// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas);
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
}上面会调用onDraw和dispatchDraw方法。
我们先看一下View.onDraw方法：
/**
* Implement this to do your drawing.
*
* @param canvas the canvas on which the background will be drawn
*/
protected void onDraw(Canvas canvas) {
}是空方法，这是也很好理解，因为每个View的展现都不一样，例如TextView、ProgressBar等，
所以View不会去实现onDraw方法，具体是要子类去根据自己的显示要求实现该方法。
再看一下dispatchDraw方法，这个方法是用来绘制子View的，所以要看ViewGroup.dispatchDraw方法，View.dispatchDraw是空的。
/**
* {@inheritDoc}
*/
@Override
protected void dispatchDraw(Canvas canvas) {
boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
final int childrenCount = mChildrenCount;
final View[] children = mChildren;
int flags = mGroupFlags;
if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {
final boolean cache = (mGroupFlags & FLAG_ANIMATION_CACHE) == FLAG_ANIMATION_CACHE;
final boolean buildCache = !isHardwareAccelerated();
for (int i = 0; i < childrenCount; i ) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {
final LayoutParams params = child.getLayoutParams();
attachLayoutAnimationParameters(child, params, i, childrenCount);
bindLayoutAnimation(child);
if (cache) {
child.setDrawingCacheEnabled(true);
if (buildCache) {
child.buildDrawingCache(true);
}
}
}
}
final LayoutAnimationController controller = mLayoutAnimationController;
if (controller.willOverlap()) {
mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;
}
controller.start();
mGroupFlags &= ~FLAG_RUN_ANIMATION;
mGroupFlags &= ~FLAG_ANIMATION_DONE;
if (cache) {
mGroupFlags |= FLAG_CHILDREN_DRAWN_WITH_CACHE;
}
if (mAnimationListener != null) {
mAnimationListener.onAnimationStart(controller.getAnimation());
}
}
int clipSaveCount = 0;
final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
if (clipToPadding) {
clipSaveCount = canvas.save();
canvas.clipRect(mScrollX mPaddingLeft, mScrollY mPaddingTop,
mScrollX mRight - mLeft - mPaddingRight,
mScrollY mBottom - mTop - mPaddingBottom);
}
// We will draw our child's animation, let's reset the flag
mPrivateFlags &= ~PFLAG_DRAW_ANIMATION;
mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;
boolean more = false;
final long drawingTime = getDrawingTime();
if (usingRenderNodeProperties) canvas.insertReorderBarrier();
// Only use the preordered list if not HW accelerated, since the HW pipeline will do the
// draw reordering internally
final ArrayList<View> preorderedList = usingRenderNodeProperties
? null : buildOrderedChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
for (int i = 0; i < childrenCount; i ) {
int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i;
final View child = (preorderedList == null)
? children[childIndex] : preorderedList.get(childIndex);
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
// 调用drawChild方法
more |= drawChild(canvas, child, drawingTime);
}
}
if (preorderedList != null) preorderedList.clear();
// Draw any disappearing views that have animations
if (mDisappearingChildren != null) {
final ArrayList<View> disappearingChildren = mDisappearingChildren;
final int disappearingCount = disappearingChildren.size() - 1;
// Go backwards -- we may delete as animations finish
for (int i = disappearingCount; i >= 0; i--) {
final View child = disappearingChildren.get(i);
more |= drawChild(canvas, child, drawingTime);
}
}
if (usingRenderNodeProperties) canvas.insertInorderBarrier();
if (debugDraw()) {
onDebugDraw(canvas);
}
if (clipToPadding) {
canvas.restoreToCount(clipSaveCount);
}
// mGroupFlags might have been updated by drawChild()
flags = mGroupFlags;
if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {
invalidate(true);
}
if ((flags & FLAG_ANIMATION_DONE) == 0 && (flags & FLAG_NOTIFY_ANIMATION_LISTENER) == 0 &&
mLayoutAnimationController.isDone() && !more) {
// We want to erase the drawing cache and notify the listener after the
// next frame is drawn because one extra invalidate() is caused by
// drawChild() after the animation is over
mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;
final Runnable end = new Runnable() {
public void run() {
notifyAnimationListener();
}
};
post(end);
}
}可以看到上面的方法中会调用drawChild方法，该方法如下：
/**
* Draw one child of this View Group. This method is responsible for getting
* the canvas in the right state. This includes clipping, translating so
* that the child's scrolled origin is at 0, 0, and applying any animation
* transformations.
*
* @param canvas The canvas on which to draw the child
* @param child Who to draw
* @param drawingTime The time at which draw is occurring
* @return True if an invalidate() was issued
*/
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}这里也简单总结一下draw的过程：
// 1. ViewRootImpl.performDraw()
private void performDraw() {
// 2. ViewRootImpl.draw()
draw(fullRedrawNeeded);
// 3. ViewRootImpl.drawSoftware
drawSoftware
// 4. 内部调用mView.draw,也就是FrameLayout.draw().
mView.draw()(FrameLayout)
// 5. FrameLayout.draw方法内部会调用super.draw方法，也就是View.draw方法.
super.draw(canvas);
// 6. View.draw方法内部会分别调用onDraw绘制自己以及dispatchDraw绘制子View.
onDraw
// 绘制子View
dispatchDraw
// 7. dispatchDraw方法内部会遍历所有子View.
for (int i = 0; i < childrenCount; i ) {
// 8. 对每个子View分别调用drawChild方法
drawChild()
// 9. drawChild方法内部会对该子View调用draw方法，进行绘制。然后draw又会调用onDraw等，循环就开始了。
child.draw()
}
}
getWidth()与getMeasuredWidth()有什么区别呢？ 一般情况下这两个的值是相同的，getMeasureWidth()方法在measure()过程结束后就可以获取到了，而getWidth()方法要在layout()过程结束后才能获取到。 而且getMeasureWidth()的值是通过setMeasuredDimension()设置的，但是getWidth()的值是通过视图右边的坐标减去左边的坐标计算出来的。如果我们在layout的时候将宽高 不传getMeasureWidth的值，那么这时候getWidth()与getMeasuredWidth的值就不会再相同了，当然一般也不会这么干…
