总结
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消息循环基础
其中绿色的部分与蓝色可能在一个线程，也可能不在一个线程
初始化handler时，指定了对应的Looper以及消息队列MessageQueue
public Handler(Callback callback, boolean async) { //可能的内存泄露 //所以尽可能用static 的handler,用弱引用引用外部类 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()); } } //所以必须在一个调用了Looper.prepare()的线程中初始化handler //android的主线程在启动是就已经调用过了 mLooper = Looper.myLooper(); if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; mCallback = callback; mAsynchronous = async; }在主线程或工作线程中，将要传递的数据或者要执行的Runable包装成一个Message对象，然后Handler将这个Message对象加到主线程的消息队列中
//Handler 代码 public boolean sendMessageAtTime(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); }Message除了包含必要的信息外，还有一个target，这个target就是这个Handler对象本身
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { //指定了消息处理者target,也就是handler本身 msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
消息循环
Looper不断的从消息队列中取出消息，然后调用消息的target去分发消息，也就是调用Handler去处理消息，处理完后，将Message放到Message对象池中，循环使用
//Looper代码 public static void loop() { final Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } final MessageQueue queue = me.mQueue; // Make sure the identity of this thread is that of the local process, // and keep track of what that identity token actually is. Binder.clearCallingIdentity(); final long ident = Binder.clearCallingIdentity(); for (;;) { Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return; } // This must be in a local variable, in case a UI event sets the logger final Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } final long traceTag = me.mTraceTag; if (traceTag != 0) { Trace.traceBegin(traceTag, msg.target.getTraceName(msg)); } try { //取出消息，然后调用target去处理消息，实际也就是handler去处理消息 msg.target.dispatchMessage(msg); } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } if (logging != null) { logging.println("<<<<< Finished to " + msg.target + " " + msg.callback); } // Make sure that during the course of dispatching the // identity of the thread wasn't corrupted. final long newIdent = Binder.clearCallingIdentity(); if (ident != newIdent) { Log.wtf(TAG, "Thread identity changed from 0x" + Long.toHexString(ident) + " to 0x" + Long.toHexString(newIdent) + " while dispatching to " + msg.target.getClass().getName() + " " + msg.callback + " what=" + msg.what); } msg.recycleUnchecked(); } }
Handler分发
public void dispatchMessage(Message msg) {
//如果消息的callback不为null,实际上就是这个消息是包装了一个Runable
//就去执行这个Runable
if (msg.callback != null) {
handleCallback(msg);
} else {
//如果handler的callback不为空，并且返回true，表示处理了消息，就结束了
//否则就由自身的handleMessage函数去处理
//handler的callback主要目的是不用重载函数，只用传入一个callback对象就行
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
MessageQueue的next函数
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) {
// 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.
// 异步消息，如果开头是一个SyncBarrier,则会找到第一个异步消息进行分发
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.
// 在没有消息体要分发，所要做的事情
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;
}
}
异步消息
SyncBarrier是指target为空的Message
- Asynchronous指的是标识位为1的消息
- SyncBarrier会阻止之后的同步消息的执行，只允许异步消息执行，postSyncBarrier与removeSyncBarrie必须要成对调用
- SyncBarrier在ViewRootImpl执行Traversals相关的代码时有被设置