android面试准备之==>Handler

Handler主要借助内部的三个类完成线程间的通信:

Looper,MessageQueue,Message

在android中:

• Handler创建:

//出自Handler
 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 " + Thread.currentThread()
                        + " that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

//=================================出自Looper============================

public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

public static void prepare() {
        prepare(true);
    }

private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

非UI线程创建hander对象需要提前调用Looper.prepare();

创建完对象之后需要调用Looper.loop();(如果不调用的话handler将不能接收到消息)

(注:UI线程不需要手动调用的原因是在初始化的时候已经调用了)

• 消息的发送:

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

Handler的消息的发送是sendMessage最终调用的是MessageQueue的enqueueMessage()将消息压入MessageQueue中

• 消息的接收:

 /**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {
        final Looper me = myLooper(); 
        final MessageQueue queue = me.mQueue;

        ...

        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

         ...

            try {
                //这里条用了handler的dispatchMessage方法
                msg.target.dispatchMessage(msg);
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

         ...

            msg.recycleUnchecked();
        }
    }

Handler的消息是通过Looper.loop()方法(里面有个死循环) 不停的检测MessageQueue中是否有新的Message消息,如果有就调用Message对象保存的handler的dispatchMessage(msg)方法,dispatchMessage()方法最终会调用handleMessage(msg)方法.

• ps:

其中,Looper保存在线程的本地变量threadLocal中,Looper保存有当前线程和MessageQueue,通过Lopper.loop()方法循环的从MessageQueue中读取消息,一旦发现MessageQueue中有新的Message消息,就会调用target(就是当前线程的handler)的dispatchMessage方法分发消息