Handler源码学习（一）流程

本系列笔记是我阅读Handler及相关类的源码时所记录的一些之前所不知道的知识点，该系列分为三部分，整体流程，Message对象回收原理，MessageQueue管理队列
Handler源码学习（一）流程
Handler源码学习（二）Message对象池
Handler源码学习（三）MessageQueue入队插队

1.创建handler — 默认构造方法会获取当前线程的looper，也可以传入指定的looper

/**
 * Default constructor associates this handler with the {@link Looper} for the
 * current thread.
 *
 * If this thread does not have a looper, this handler won't be able to receive messages
 * so an exception is thrown.
 */
public Handler() {
    this(null, false);
}
public Handler(Callback callback, boolean async) {
        //警告潜在内存泄漏风险
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
          //如果是匿名类，内部类等等并且不是static 有内存泄露风险
            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());
            }
        }
        //创建looper
        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;
    }

2.Looper

/** Initialize the current thread as a looper.
  * This gives you a chance to create handlers that then reference
  * this looper, before actually starting the loop. Be sure to call
  * {@link #loop()} after calling this method, and end it by calling
  * {@link #quit()}.
  */
public static void prepare() {
    prepare(true);
}
//用ThreadLocal来存放looper 如果当前线程已经有一个looper则抛异常
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();
    }

//主线程的looper
/**
     * Initialize the current thread as a looper, marking it as an
     * application's main looper. The main looper for your application
     * is created by the Android environment, so you should never need
     * to call this function yourself.  See also: {@link #prepare()}
     */
    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }
//构造方法，创建了一个消息队列
private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

3.sendMessage

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);
}
//handler中将message添加到消息队列
//将这个message的target设置成当前的这个handler
 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

4.loop() — 取出消息

//target是handler，这里是个无限循环
msg.target.dispatchMessage(msg);

5.dispatchMessage() — 分发消息

//三种情况，
//第一种Message对象的callback不为空(runnable)，交给callback处理，第一种大多是使用post方法传入runnable对象时会调用
//第二种是handler的callback不为空，交给callback处理，callback
//前两种都没有的情况下交给handleMessag去处理
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

private static void handleCallback(Message message) {
        message.callback.run();
    }

public interface Callback {
        public boolean handleMessage(Message msg);
    }
//Handler的callback可以在构造方法中传入
//Message的callback可以在obtain方法中作为参数传入，注意，一个消息被使用完毕后会被recycle，callback也会被移除，所以只能使用一次

6.post — 各种post方法

//注意传入的runnable的run方法会在handler所绑定的looper所在线程中执行，最终还是sendMessageDelayed()
public final boolean post(Runnable r)
{
   return  sendMessageDelayed(getPostMessage(r), 0);
}

//不被推荐使用的方法,很容易导致消息队列清空，排序问题，以及其它副作用
//Posts a message to an object that implements Runnable.
//Causes the Runnable r to executed on the next iteration through the
//message queue.
//This method is only for use in very special circumstances -- it
//can easily starve the message queue, cause ordering problems, or have
//other unexpected side-effects.</b>
public final boolean sendMessageAtFrontOfQueue(Message msg) {
        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, 0);
    }

8.扩展

• runOnUiThread()

//如果当前线程是ui线程就直接调用run方法，如果不是当前线程则通过handler发送消息
public final void runOnUiThread(Runnable action) {
    if (Thread.currentThread() != mUiThread) {
        mHandler.post(action);
    } else {
        action.run();
    }
}

• view.post()

//首先会判断这个view是否已经attach上了window，如果是的话会使用handler来发送消息
//否则会加入到消息队列中，等到attach了才会一次取出执行
public boolean post(Runnable action) {
    final AttachInfo attachInfo = mAttachInfo;
    if (attachInfo != null) {
        return attachInfo.mHandler.post(action);
    }
    // Assume that post will succeed later
    ViewRootImpl.getRunQueue().post(action);
    return true;
}

7.收获

1.looper是通过 ThreadLoacal来保存 ，保证looper只被创建一次，且跟当前线程绑定，一个looper。

2.handler有多个构造方法，并不是必须要重写handleMessage方法，可以通过传入callback对象来处理消息，同时也可以给handler指定looper，因此handler可以给其它线程的looper发送消息。

3.MessageQueue不由handler创建，而是从looper中获取，一个looper对应有一个MessageQueue，因此一个looper必然可以对应多个handler，它们都是往这同一个消息队列中发送消息而已

4.Android中的主线程会调用Looper.prepareMainLooper() 方法来创建一个looper，因此不需要我们手动创建looper

5.handler发送延迟消息时，是通过但前时间＋delayed时间，在某个时间去发送，因此如果在插入message时的时间是0，会容易导致排序问题

6.每一个Activity在创建的时候其实就已经创建了一个handler，runOnUiThread方法中使用的就是这个handler

7.view.post除了异步更新ui外还有一个作用，可以判断当前view是否已经attach window，在这个runnable任务被执行时，可以拿到尺寸等等，避免因为disattach造成异常