Handler机制总结

handler的主要作用是将一个任务切换到指定的线程中去执行，最常用的就是在子线程中通过handler将任务切换到UI线程去更新UI。

主要用法：
我们知道Android不允许在子线程中更新UI的，这是因为UI线程不是线程安全的，为什么这么说呢，如果在多线程并发访问UI，那么UI可能不处于不可预期的状态，Android是没有给UI控件访问加锁机制的，若果这样做首先UI的访问逻辑无疑会变得复杂，其次会降低Ui的访问效率，加了锁可能就会阻塞某些线程的执行。
一般的我们在子线程进行了某个耗时操作，然后会需要在UI线程中去改变UI的状态，这是一个非常常见的需求。就可以这样做

public class MainActivity extends AppCompatActivity {private TextView mTextView;Handler mHandler = new Handler(){@Overridepublic void handleMessage(Message msg) {if (msg.what == 0){mTextView.setText("update");}        }    };@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);mTextView = (TextView)findViewById(R.id.textView);        new Thread(){@Overridepublic void run() {try {                    Thread.sleep(2000);mHandler.sendEmptyMessage(0);} catch (InterruptedException e) {                }            }        }.start();}}

除了发送一个msg之外，handler还可以发送一个Runnable对象，跟上边发送一个msg实现的功能一样

Handler mHandler = new Handler();new Thread(){@Overridepublic void run() {try {            Thread.sleep(2000);mHandler.post(new Runnable() {@Overridepublic void run() {mTextView.setText("update");}            });} catch (InterruptedException e) {        }    }}.start();

也可以通过sendMessage(msg)方法发送一个Message对象，Message对象可以携带参数，也可以携带一个Object

Message message = new Message();message.what = 0;message.arg1 = 1000;mHandler.sendMessage(message);public void handleMessage(Message msg) {if (msg.what == 0){mTextView.setText(msg.arg1+"");}}

除了使用new Massage()的方式创建一个Message之外，还可以通过
Message message = mHandler.obtainMessage();获取一个系统的message。也可以使用message.sendToTarget();去发送给handler。看源码：

public void sendToTarget() {target.sendMessage(this);}

target是什么呢，打开obtainMessage(),看到target就是刚才获取Message的mHandler自己。

public final Message obtainMessage(){return Message.obtain(this);}public static Message obtain(Handler h) {    Message m = obtain();m.target = h;    return m;}

还有一种新建handler的方式通过这种传入Callback的方式，在返回值为true或者false时分别截获和不截获消息。

Handler mHandler = new Handler(new Handler.Callback() {@Overridepublic boolean handleMessage(Message msg) {        Toast.makeText(getApplicationContext(),"1",1).show();        return false;}}){@Overridepublic void handleMessage(Message msg) {        Toast.makeText(getApplicationContext(), "2", 1).show();}};

handler，message，messageQueue，Looper之间的关系

**
handle的机制主要靠MessageQueue和Looper来实现，每当handler发送一个消息msg或者post一个Runnable对象(会被包装成message)，msg会通过MessageQueue的enqueueMessage()方法加入到消息队列，然后Looper通过无限循环的方式查找消息队列是否有新消息，有的话就处理，最终消息中的Runnable或者handleMessage方法会被调用。Looper是与创建Handler的线程绑定的，主线程中默认创建了Looper，但是子线程不会自己创建，若要在子线程中创建handler,需要自己创建一个Looper，方法是Looper.prepare()创建成功后调用Looper.loop开启循环。如下，从一个子线程将任务切换到另外一个子线程。

new Thread("Thread1"){@Overridepublic void run() {        Looper.prepare();handler = new Handler();Looper.loop();}}.start();new Thread("Thread2"){@Overridepublic void run() {try {            Thread.sleep(2000);handler.post(new Runnable() {@Overridepublic void run() {                    Log.e("fwc",Thread.currentThread().getName());}            });} catch (InterruptedException e) {            e.printStackTrace();}    }}.start();

MessageQueue工作机制

主要有两个方法，enqueueMessage将一个消息加入消息队列，可以看到内部并不是一个队列，而是以一个单连表的方式实现的，这样比较有利于增加和删除

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("MessageQueue", 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;}

另外一个是next方法，next方法是一个无限循环的方法，当消息队列中没有消息的时候会一直阻塞在这里，当有新消息到来时，next方法会返回这条消息并且将消息从队列中移除。

Message next() {    // Return here if the message loop has already quit and been disposed.    // This can happen if the application tries to restart a looper after quit    // which is not supported.    final long ptr = mPtr;    if (ptr == 0) {        return null;    }    int pendingIdleHandlerCount = -1; // -1 only during first iteration    int nextPollTimeoutMillis = 0;    for (;;) {        if (nextPollTimeoutMillis != 0) {            Binder.flushPendingCommands();        }        nativePollOnce(ptr, nextPollTimeoutMillis);        synchronized (this) {            // Try to retrieve the next message.  Return if found.            final long now = SystemClock.uptimeMillis();            Message prevMsg = null;            Message msg = mMessages;            if (msg != null && msg.target == null) {                // Stalled by a barrier.  Find the next asynchronous message in the queue.                do {                    prevMsg = msg;                    msg = msg.next;                } while (msg != null && !msg.isAsynchronous());            }            if (msg != null) {                if (now < msg.when) {                    // Next message is not ready.  Set a timeout to wake up when it is ready.                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);                } else {                    // Got a message.                    mBlocked = false;                    if (prevMsg != null) {                        prevMsg.next = msg.next;                    } else {                        mMessages = msg.next;                    }                    msg.next = null;                    if (false) Log.v("MessageQueue", "Returning message: " + msg);                    return msg;                }            } else {                // No more messages.                nextPollTimeoutMillis = -1;            }            // Process the quit message now that all pending messages have been handled.            if (mQuitting) {                dispose();                return null;            }            // If first time idle, then get the number of idlers to run.            // Idle handles only run if the queue is empty or if the first message            // in the queue (possibly a barrier) is due to be handled in the future.            if (pendingIdleHandlerCount < 0                    && (mMessages == null || now < mMessages.when)) {                pendingIdleHandlerCount = mIdleHandlers.size();            }            if (pendingIdleHandlerCount <= 0) {                // No idle handlers to run.  Loop and wait some more.                mBlocked = true;                continue;            }            if (mPendingIdleHandlers == null) {                mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];            }            mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);        }        // Run the idle handlers.        // We only ever reach this code block during the first iteration.        for (int i = 0; i < pendingIdleHandlerCount; i++) {            final IdleHandler idler = mPendingIdleHandlers[i];            mPendingIdleHandlers[i] = null; // release the reference to the handler            boolean keep = false;            try {                keep = idler.queueIdle();            } catch (Throwable t) {                Log.wtf("MessageQueue", "IdleHandler threw exception", t);            }            if (!keep) {                synchronized (this) {                    mIdleHandlers.remove(idler);                }            }        }        // Reset the idle handler count to 0 so we do not run them again.        pendingIdleHandlerCount = 0;        // While calling an idle handler, a new message could have been delivered        // so go back and look again for a pending message without waiting.        nextPollTimeoutMillis = 0;    }}

Looper工作机制

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

通过构造方法可以看到Looper内部有MessageQueue，并且与当前线程绑定。looper的主要方法是loop();无限循环去调用messageQueue的next方法，当返回值为null的时候跳出循环，否则，msg.target.dispatchMessage(msg);之前说了这个target就是Handler本身，继续跟源码看到，如果msg.callback != null（这个callback是一个Runnable的，就是之前post一个Runnable对象的callback）就执行callback的run方法。否则执行自己的callback的handleMessage方法，就是之前说的返回值为true的时候截获消息，最后才是handler的handleMessage方法。这样就从一个线程切换到了handler所在的线程。Looper可以调用quit方法停止，会调用messageQueue的quit方法，将消息队列标记为退出。标记之后next方法会返回null；

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

handler工作原理
sendMessage方法最终只是调用了enqueueMessage方法将消息插入到了MessageQueue中，Looper的loop方法循环调用messageQueue的next方法从消息队列中取出消息，调用handler的dispatchMessage方法，之后就与上边分析的一致了。
前边说了子线程中创建handler需要自己调用Looper的prepare方法创建一个与当前线程绑定的Looper，并调用loop方法才能让整个handler运转起来，那么主线程的Looper是什么时候创建的呢，Android的主线程是ActivityThread，入口是main方法，看到在main方法中调用了Looper.prepareMainLooper();创建了一个主线程的Looper并且开启了循环。

handler，message，messageQueue，Looper之间的关系

MessageQueue工作机制

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