文章目录
- 一、Handler 构造函数
- 二、Handler 消息分发
- 三、MessageQueue 消息队列相关函数
一、Handler 构造函数
一般使用 Handler 时 , 调用 Handler 的普通 无参构造函数 ,
代码语言：javascript复制public class Handler {
/**
* 默认的构造函数 , 与当前线程相关联.
* 如果该线程没有 Looper , 该 Handler 不能接受 Message 消息 , 并抛出异常
*/
public Handler() {
this(null, false);
}
}上面的无参构造函数调用了下面的构造方法 ,
第一个参数 Callback callback 是一个回调 , mCallback = callback , 该回调直接设置给了 mCallback 成员变量 ,
在该方法中 , 调用 mLooper = Looper.myLooper() 获取线程本地变量 Looper ;
获取 Looper 中的消息队列 MessageQueue , mQueue = mLooper.mQueue ;
主线程的 Looper 是在 ActivityThread 中的 main 函数 中 , 使用 Looper.prepareMainLooper() 创建的 ,
在 ActivityThread 的 main 函数最后调用了 Looper.loop() , 无限循环获取主线程 Looper 中封装的 MessageQueue 消息队列中的消息 ;
参考 : 【Android 异步操作】Handler ( 主线程中的 Handler 与 Looper | Handler 原理简介 ) ,
代码语言：javascript复制public class Handler {
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
public Handler(@Nullable Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: "
klass.getCanonicalName());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " Thread.currentThread()
" that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
}Handler 中的 Callack 回调 接口 ;
代码语言：javascript复制public class Handler {
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*/
public interface Callback {
/**
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
boolean handleMessage(@NonNull Message msg);
}
}二、Handler 消息分发
Handler 中的消息分发 , 在 Looper 的 loop 方法中 , 调用该消息 dispatchMessage 分发消息的方法 ,
在该分发消息方法中 , 首先会查看 消息 Message 中 是否有 Callback 回调 ,
如果有执行该回调 , 就是构造函数中赋值的 mCallback ,
如果没有就调用 Handler 中的 handleMessage 方法 ;
代码语言：javascript复制public class Handler {
/**
* 在这里处理 Message 消息.
*/
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
}使用 Handler 发送消息时 , 会 调用各种发送消息的方法 , 如
- public final boolean sendMessage(@NonNull Message msg)
- public final boolean sendEmptyMessage(int what)
- public final boolean sendEmptyMessageDelayed
- public final boolean sendEmptyMessageAtTime
- public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis)
等方法 , 所有的发送消息的方法 , 最终都会调用 public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) 方法 ,
在该方法中 , 调用 MessageQueue queue = mQueue , 获取 消息队列 MessageQueue ,
然后调用 enqueueMessage(queue, msg, uptimeMillis) 方法 , 将消息加入到 消息队列 MessageQueue 中 ;
代码语言：javascript复制public class Handler {
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
}三、MessageQueue 消息队列相关函数
下面的代码是将消息存储到消息队列中的 enqueueMessage 方法 ;
代码语言：javascript复制public final class MessageQueue {
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg " This message is already in use.");
}
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
// 如果当前消息为空 , 时间小于当前该消息的发送时间 , 需要马上将该消息发送出去
// 将表头设置成发送进来的消息
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// 如果该消息不急着发送 , 那么将该消息放在消息队列列表尾部
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
}从链表中取出数据 , 调用的是 消息队列 MessageQueue 的 next 方法 , 获取消息时 , 需要获取当前的时间 , 用于判定是否有需要延迟发送的消息 ;
代码语言：javascript复制public final class MessageQueue {
@UnsupportedAppUsage
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// 获取当前的时间 , 需要判定是否有需要延迟发送的消息
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i ) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
}
