1.include
include就是为了解决重复定义相同布局的问题
代码语言：javascript复制<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:id="@ id/my_title_parent_id"
android:layout_height="wrap_content" >
<ImageButton
android:id="@ id/back_btn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/ic_launcher" />
<TextView
android:id="@ id/title_tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerVertical="true"
android:layout_marginLeft="20dp"
android:layout_toRightOf="@ id/back_btn"
android:gravity="center"
android:text="我的title"
android:textSize="18sp" />
</RelativeLayout> include布局文件：
代码语言：javascript复制<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<include
android:id="@ id/my_title_ly"
android:layout_width="match_parent"
android:layout_height="wrap_content"
layout="@layout/my_title_layout" />
<!-- 代码省略 -->
</LinearLayout> 这样就可以使用my_title_layout了。
注意事项 使用include最常见的问题就是findViewById查找不到目标控件，这个问题出现的前提是在include时设置了id，而在findViewById时却用了被include进来的布局的根元素id。例如上述例子中，include时设置了该布局的id为my_title_ly，而my_title_layout.xml中的根视图的id为my_title_parent_id。此时如果通过findViewById来找my_title_parent_id这个控件，然后再查找my_title_parent_id下的子控件则会抛出空指针。代码如下 :
代码语言：javascript复制View titleView = findViewById(R.id.my_title_parent_id) ;
// 此时 titleView 为空，找不到。此时空指针
TextView titleTextView = (TextView)titleView.findViewById(R.id.title_tv) ;
titleTextView.setText("new Title"); 看源码： LayoutInflater#rInflate
代码语言：javascript复制void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
boolean pendingRequestFocus = false;
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
if (type != XmlPullParser.START_TAG) {
continue;
}
final String name = parser.getName();
if (TAG_REQUEST_FOCUS.equals(name)) {
pendingRequestFocus = true;
consumeChildElements(parser);
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else {
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflateChildren(parser, view, attrs, true);
viewGroup.addView(view, params);
}
}
if (pendingRequestFocus) {
parent.restoreDefaultFocus();
}
if (finishInflate) {
parent.onFinishInflate();
}
}匹配到TAG_INCLUDE，执行parseInclude方法，主要做以下几件事 1.获取属性集，即在include标签中设置的属性 2.解析include的布局的根元素 3.获取include包含布局的布局属性并设置 4.解析根元素下的子view 5. 将include中设置的id设置给根view,因此实际上my_title_layout.xml中的RelativeLayout的id会变成include标签中的id，include不设置id，那么也可以通过relative的找到. 6. 将根view添加到父控件中
2.ViewStub
其实ViewStub就是一个宽高都为0的一个View，它默认是不可见的，只有通过调用setVisibility函数或者Inflate函数才会将其要装载的目标布局给加载出来，从而达到延迟加载的效果，这个要被加载的布局通过android:layout属性来设置。例如我们通过一个ViewStub来惰性加载一个消息流的评论列表，因为一个帖子可能并没有评论，此时我可以不加载这个评论的ListView，只有当有评论时我才把它加载出来，这样就去除了加载ListView带来的资源消耗以及延时，示例如下 :
代码语言：javascript复制<ViewStub
android:id="@ id/stub_import"
android:inflatedId="@ id/stub_comm_lv"
android:layout="@layout/my_comment_layout"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_gravity="bottom"> my_comment_layout.xml如下:
代码语言：javascript复制<?xml version="1.0" encoding="utf-8"?>
<ListView xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:id="@ id/my_comm_lv"
android:layout_height="match_parent" >
</ListView> 在运行时，我们只需要控制id为stub_import的ViewStub的可见性或者调用inflate()函数来控制是否加载这个评论列表即可。示例如下 :
代码语言：javascript复制public class MainActivity extends Activity {
public void onCreate(Bundle b){
// main.xml中包含上面的ViewStub
setContentView(R.layout.main);
// 方式1，获取ViewStub,
ViewStub listStub = (ViewStub) findViewById(R.id.stub_import);
// 加载评论列表布局
listStub.setVisibility(View.VISIBLE);
// 获取到评论ListView，注意这里是通过ViewStub的inflatedId来获取
ListView commLv = findViewById(R.id.stub_comm_lv);
if ( listStub.getVisibility() == View.VISIBLE ) {
// 已经加载, 否则还没有加载
}
}
} 通过setVisibility(View.VISIBILITY)来加载评论列表，此时你要获取到评论ListView对象的话，则需要通过findViewById来查找，而这个id并不是就是ViewStub的id，而是inflatedId。 这是为什么呢 ？ 查看ViewStub代码
代码语言：javascript复制 private void initialize(Context context) {
mContext = context;
setVisibility(GONE);// 设置不可见
setWillNotDraw(true);// 设置不绘制
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(0, 0);// 宽高都为0
} setVisibility中如果还没渲染view会调用inflate()方法渲染
代码语言：javascript复制 @Override
@android.view.RemotableViewMethod(asyncImpl = "setVisibilityAsync")
public void setVisibility(int visibility) {
if (mInflatedViewRef != null) {
View view = mInflatedViewRef.get();
if (view != null) {
view.setVisibility(visibility);
} else {
throw new IllegalStateException("setVisibility called on un-referenced view");
}
} else {
super.setVisibility(visibility);
if (visibility == VISIBLE || visibility == INVISIBLE) {
inflate();
}
}
} public View inflate() {
final ViewParent viewParent = getParent();
if (viewParent != null && viewParent instanceof ViewGroup) {
if (mLayoutResource != 0) {
final ViewGroup parent = (ViewGroup) viewParent;
final View view = inflateViewNoAdd(parent);
replaceSelfWithView(view, parent);
mInflatedViewRef = new WeakReference<>(view);
if (mInflateListener != null) {
mInflateListener.onInflate(this, view);
}
return view;
} else {
throw new IllegalArgumentException("ViewStub must have a valid layoutResource");
}
} else {
throw new IllegalStateException("ViewStub must have a non-null ViewGroup viewParent");
}
}主要看inflateViewNoAdd
代码语言：javascript复制 private View inflateViewNoAdd(ViewGroup parent) {
final LayoutInflater factory;
if (mInflater != null) {
factory = mInflater;
} else {
factory = LayoutInflater.from(mContext);
}
final View view = factory.inflate(mLayoutResource, parent, false);
if (mInflatedId != NO_ID) {
view.setId(mInflatedId);
}
return view;
}这里如果设置了mInflatedId，会把view的id改为mInflatedId
然后是replaceSelfWithView方法
代码语言：javascript复制 private void replaceSelfWithView(View view, ViewGroup parent) {
final int index = parent.indexOfChild(this);
parent.removeViewInLayout(this);
final ViewGroup.LayoutParams layoutParams = getLayoutParams();
if (layoutParams != null) {
parent.addView(view, index, layoutParams);
} else {
parent.addView(view, index);
}
}里面会通过父控件把viewstub自己移除，将最终渲染的控件插入会对应的index
注意事项 判断是否已经加载过， 如果通过setVisibility来加载，那么通过判断可见性即可；如果通过inflate()来加载是不可以通过判断可见性来处理的，而需要使用方式2来进行判断。 findViewById的问题，注意ViewStub中是否设置了inflatedId，如果设置了则需要通过inflatedId来查找目标布局的根元素。
Merge
其实就是减少在include布局文件时的层级。标签是这几个标签中最让我费解的，大家可能想不到，标签竟然会是一个Activity，里面有一个LinearLayout对象。
代码语言：javascript复制/**
* Exercise <merge /> tag in XML files.
*/
public class Merge extends Activity {
private LinearLayout mLayout;
@Override
protected void onCreate(Bundle icicle) {
super.onCreate(icicle);
mLayout = new LinearLayout(this);
mLayout.setOrientation(LinearLayout.VERTICAL);
LayoutInflater.from(this).inflate(R.layout.merge_tag, mLayout);
setContentView(mLayout);
}
public ViewGroup getLayout() {
return mLayout;
}
} 使用merge来组织子元素可以减少布局的层级。例如我们在复用一个含有多个子控件的布局时，肯定需要一个ViewGroup来管理，例如这样 :
代码语言：javascript复制<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<ImageView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:scaleType="center"
android:src="@drawable/golden_gate" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="20dip"
android:layout_gravity="center_horizontal|bottom"
android:padding="12dip"
android:background="#AA000000"
android:textColor="#ffffffff"
android:text="Golden Gate" />
</FrameLayout> 使用merge标签就会消除上图中蓝色的FrameLayout层级。示例如下 :
代码语言：javascript复制<merge xmlns:android="http://schemas.android.com/apk/res/android">
<ImageView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:scaleType="center"
android:src="@drawable/golden_gate" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="20dip"
android:layout_gravity="center_horizontal|bottom"
android:padding="12dip"
android:background="#AA000000"
android:textColor="#ffffffff"
android:text="Golden Gate" />
</merge> public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context)mConstructorArgs[0];
mConstructorArgs[0] = mContext;
View result = root;
try {
// Look for the root node.
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
if (type != XmlPullParser.START_TAG) {
throw new InflateException(parser.getPositionDescription()
": No start tag found!");
}
final String name = parser.getName();
// m如果是erge标签，那么调用rInflate进行解析
if (TAG_MERGE.equals(name)) {
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
"ViewGroup root and attachToRoot=true");
}
// 解析merge标签
rInflate(parser, root, attrs, false);
} else {
// 代码省略
}
} catch (XmlPullParserException e) {
// 代码省略
}
return result;
}
}
void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
if (type != XmlPullParser.START_TAG) {
continue;
}
final String name = parser.getName();
if (TAG_REQUEST_FOCUS.equals(name)) {
parseRequestFocus(parser, parent);
} else if (TAG_INCLUDE.equals(name)) {
// 代码省略
parseInclude(parser, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else if (TAG_1995.equals(name)) {
final View view = new BlinkLayout(mContext, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflate(parser, view, attrs, true);
viewGroup.addView(view, params);
} else { // 我们的例子会进入这里
final View view = createViewFromTag(parent, name, attrs);
// 获取merge标签的parent
final ViewGroup viewGroup = (ViewGroup) parent;
// 获取布局参数
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
// 递归解析每个子元素
rInflate(parser, view, attrs, true);
// 将子元素直接添加到merge标签的parent view中
viewGroup.addView(view, params);
}
}
if (finishInflate) parent.onFinishInflate();
} 其实就是如果是merge标签，那么直接将其中的子元素添加到merge标签parent中，这样就保证了不会引入额外的层级。
