Handler机制分析

背景: Handler解决线程之间发送消息

解决的问题:

1:主线程不能执行耗时操作，ANR

2:子线程不能操作主线程的UI视图

写在前面, 首先需要明确涉及到了 4个类:

Handler的核心类:

Message: 对发送的消息的封装

MessageQueue: 消息队列,存放所有的消息

Looper: 可以循环读取消息(从MessageQueue中读取)

Handler: 处理消息,同时也是发送消息的

1.
Message: 消息包装类, 并且包含一个target属性成员(Handler在发送它时Handler自己就被赋值: msg.target=handler)

Message msg = Message.obtain();

msg.obj ="hello,你好!";

handler.sendMessage(msg);

2. MessageQueue: 是由Looper创建, 它主要存放Message

3. Looper: 在本线程中保存Looper实例和MessageQueue实例, 它的loop方法在MessageQueue当中取Message

Looper类:

private Looper(boolean quitAllowed) {

mQueue = new MessageQueue(quitAllowed);

mRun = true;

mThread = Thread.currentThread();

}

public static final void prepare() {

if (sThreadLocal.get() != null) {

throw new RuntimeException("Only one Looper may be created per thread");

}

sThreadLocal.set(new Looper(true));

}

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.recycle();

}

}

Looper主要作用：

1. 与当前线程绑定，保证一个线程只会有一个Looper实例，同时一个Looper实例也只有一个MessageQueue。

2. loop()方法，不断从MessageQueue中去取消息，交给消息的target属性的dispatchMessage去处理。

还记得上面说的Message的target属性吧, 没错就是Handler调用了dispatchMessage, dispatchMessage是个什么鬼?下面看源码:

public void dispatchMessage(Message msg) {

if (msg.callback != null) {

handleCallback(msg);

} else {

if (mCallback != null) {

if (mCallback.handleMessage(msg)) {

return;

}

}

handleMessage(msg);

}

}

handleMessage(msg);这句就是关键所在, 对这个是要我们接收方重写的, 也就是处理消息

最后注意:

1 主线程必须要有 handler的引用(线程之间使用同一个handler)

2 主线程一启动就会创建主线程的looper: 调用Looper.prepare()和Looper.loop()方法