handler:message:looper机制

Handler/Message/Looper机制

Handler作用

android开发过程中，我们都知道在一个异步线程中完成一段耗时操作，利用handler把结果send到UI主线程，从而更新ui；Android应用在初始化启动时，在ActivityThread中也定义了一个Handler的派生类H，用来进行Activity和Service生命周期的管理等；

private class H extends Handler {
...//省略
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
case LAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
final ActivityClientRecord r = (ActivityClientRecord) msg.obj;
r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);
handleLaunchActivity(r, null);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
case RELAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart");
ActivityClientRecord r = (ActivityClientRecord)msg.obj;
handleRelaunchActivity(r);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
case PAUSE_ACTIVITY:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
handlePauseActivity((IBinder)msg.obj, false, (msg.arg1&1) != 0, msg.arg2,
(msg.arg1&2) != 0);
maybeSnapshot();
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
...
}
if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what));
}
...//省略
}

Message的send机制

public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}

往下追踪

public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

继续往下追踪

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);
}

这个方法中我们大致可以看出消息Message是进入一个队列mQueue当中，mQueue是Handler实例化时，从UI主线程里面的Looper中获取到的

public Handler(Callback callback, boolean async) {
...//省略
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;
}

每个线程只能对应一个Looper，UI主线程中的Looper在应用中是全局的，所有Message发送后都存储在这个Looper的MessageQueue中

继续往下看

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}

以上这些都发生在Handler中，下面进入MessageQueue，看看消息Message是如何进入队列的

boolean enqueueMessage(Message msg, long when) {
//处理消息msg的Handler不能为空，所有的消息最终还是交给Handler处理的
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
//如果消息msg正在使用中，无需继续执行下去，同时抛出一个异常
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}

synchronized (this) {
//判断MessageQueue是否已停止，停止抛出一个所在线程死亡的异常
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;
}
//标志这个消息正在使用中
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
//如果在队列中msg前面没有其他消息或者msg不是最新的消息，把当前msg放入队列的头部，如果队列是阻塞状态就唤醒队列
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 {//找到合适的位置把msg放入到队列中
// 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;
//队列中没有更多消息，跳出死循环
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;
}

Handler是如何拿到Message的

应用在启动初始化时，在ActivityThread中开启了一个无限循环检查的操作，当有新的Message时，就从消息队列中MessageQueue中拿出来

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(); // 有可能阻塞
if (msg == null) {
// msg为空说明队列已经停止不可用，不执行后面的操作了
return;
}

// This must be in a local variable, in case a UI event sets the logger
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}

//把消息msg交给它的目标Handler来分配处理消息，消息在哪个页面中处理就是通过Message中的target来确定是哪个Activity中的Handler，从而在正确页面中处理msg
msg.target.dispatchMessage(msg);

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调用dispatchMessage(msg)来处理消息

public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

我们终于看到熟悉的handleMessage()了

Handler消息机制UML图