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Android是一种基于Linux的自由及开放源代码的操作系统,主要使用于移动设备,如智能手机和平板电脑,由Google公司和开放手机联盟领导及开发。尚未有统一中文名称,中国大陆地区较多人使用“安卓”或“安致”。
这篇文章主要为大家详细分析了Android Handler消息派发机制源码,感兴趣的小伙伴们可以参考一下
注:这里只是说一下sendmessage的一个过程,post就类似的
如果我们需要发送消息,会调用sendMessage方法
- public final boolean sendMessage(Message msg)
- {
- return sendMessageDelayed(msg, 0);
- }
这个方法会调用如下的这个方法
- public final boolean sendMessageDelayed(Message msg, long delayMillis)
- {
- if (delayMillis < 0) {
- delayMillis = 0;
- }
- return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
- }
接下来设定延迟时间,然后继续调用sendMessageAtTime方法
- 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);
- }
这里获得了消息队列,检查队列是否存在,然后返回enqueMessage的方法的执行结果,这个结果是说明消息能否进入队列的一个布尔值
- private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
- msg.target = this;
- if (mAsynchronous) {
- msg.setAsynchronous(true);
- }
- return queue.enqueueMessage(msg, uptimeMillis);
- }
这里是对消息进行入队处理,下面就是在MessageQueue中对消息进行入队
- 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;
- 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 {
- // 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;
- }
就是对传递过来的消息进行一些封装然后放到队列中,至此我们的sendMessage处理完毕,返回的结果是进队是否成功的布尔值,那么究竟消息之后是如何被处理的呢?
我们可以看到在Handler构造的时候记录了一个Looper对象,也记录了一个回掉函数
- public Handler(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 that has not called Looper.prepare()");
- }
- mQueue = mLooper.mQueue;
- mCallback = callback;
- mAsynchronous = async;
- }
这里的myLooper方法返回的是当前线程关联的一个Looper对象
- public static@Nullable Looper myLooper() {
- return sThreadLocal.get();
- }
当Looper实例化了以后会执行自己的prepare方法然后执行loop方法,loop方法就是不断的读取消息队列中的消息然后执行相应的操作的方法,因为是在其他线程中执行的循环所以不会影响其他线程
- 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
- Printer logging = me.mLogging;
- if (logging != null) {
- logging.println(">>>>> Dispatching to " + msg.target + " " +
- msg.callback + ": " + msg.what);
- }
- 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();
- }
- }
在循环中如果读取到了消息,就会执行dispatchMessage方法,然后分派完消息之后再执行一次recycleUnchecked方法来重用这个Message,我们看到dispatchMessage方法
- public void dispatchMessage(Message msg) {
- if (msg.callback != null) {
- handleCallback(msg);
- } else {
- if (mCallback != null) {
- if (mCallback.handleMessage(msg)) {
- return;
- }
- }
- handleMessage(msg);
- }
- }
这里看到直接执行了一个handlerMessage方法,这个方法是一个回调方法,我们是必须实现的,否则Handler什么都不会做,为什么呢?还记得刚刚说构造Handler的时候我们记录了一个CallBack的回掉吗?Handler中的这个handlerMessage方法是一个空方法,如果我们重写了这个方法,在回调的时候就会执行我们先写下的代码,也就是接收到消息之后要做什么。
- public interface Callback {
- public boolean handleMessage(Message msg);
- }
- public void handleMessage(Message msg) {
- }
这里简单说下整个过程:
当我们实例化一个Handler的子类并重写handleMessage方法之后,这个时候系统已经帮我们做了几个事情
1.实例化了一个消息队列MessageQueue
2.实例化了一个关联的Looper对象,并让Looper不断的读取消息队列
3.把我们重写的handleMessage方法记录为我们需要回调的方法
当我们执行Handler的sendMessage方法的时候,系统会把我们传过去的Message对象添加到消息队列,这个时候如果Looper读取到了消息,就会把消息派发出去,然后回调handleMessage方法,执行我们设定的代码。
来源: http://www.phperz.com/article/17/0915/348574.html