Handler源码解析

Handler是Android提供用于更新ui的一套机制，也是处理消息的机制。Android在设计的时候，封装了一套消息创建，传递，处理机制，如果不遵循这套机制就不能更新ui,会抛出异常信息。

Handler部分代码：

public class Handler {
    @UnsupportedAppUsage
    final Looper mLooper;
    final MessageQueue mQueue;
    @UnsupportedAppUsage
    final Callback mCallback;
    final boolean mAsynchronous;
    @UnsupportedAppUsage
    IMessenger mMessenger;

    public interface Callback {
        boolean handleMessage(@NonNull Message msg);
    }

        public Handler(@Nullable 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;
    }
}

首先，创建Handler，调用构造函数，调用关键句mLooper = Looper.myLooper()

myLooper方法代码：

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

该方法返回一个sThreadLocal对象中保存的Looper。除非已经调用过prepare，否则sThreadLocal.get()会返回null。

Looper中prepare方法代码：

    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

    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));
    }

调用prepare会创建Looper对象存入sThreadLocal中，并且一个Looper只能调用一次prepare，否则抛出异常：每个线程只能创建一个looper。

Looper构造方法：

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

创建Looper时，创建了MessageQueue对象并保存，MessageQueue中使用一个单链表来维护消息队列。

    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();

        // Allow overriding a threshold with a system prop. e.g.
        // adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
        final int thresholdOverride =
                SystemProperties.getInt("log.looper."
                        + Process.myUid() + "."
                        + Thread.currentThread().getName()
                        + ".slow", 0);

        boolean slowDeliveryDetected = false;

        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
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }
            // Make sure the observer won't change while processing a transaction.
            final Observer observer = sObserver;

            final long traceTag = me.mTraceTag;
            long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
            long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
            if (thresholdOverride > 0) {
                slowDispatchThresholdMs = thresholdOverride;
                slowDeliveryThresholdMs = thresholdOverride;
            }
            final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
            final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);

            final boolean needStartTime = logSlowDelivery || logSlowDispatch;
            final boolean needEndTime = logSlowDispatch;

            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }

            final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
            final long dispatchEnd;
            Object token = null;
            if (observer != null) {
                token = observer.messageDispatchStarting();
            }
            long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
            try {
                msg.target.dispatchMessage(msg);
                if (observer != null) {
                    observer.messageDispatched(token, msg);
                }
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } catch (Exception exception) {
                if (observer != null) {
                    observer.dispatchingThrewException(token, msg, exception);
                }
                throw exception;
            } finally {
                ThreadLocalWorkSource.restore(origWorkSource);
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (logSlowDelivery) {
                if (slowDeliveryDetected) {
                    if ((dispatchStart - msg.when) <= 10) {
                        Slog.w(TAG, "Drained");
                        slowDeliveryDetected = false;
                    }
                } else {
                    if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
                            msg)) {
                        // Once we write a slow delivery log, suppress until the queue drains.
                        slowDeliveryDetected = true;
                    }
                }
            }

            msg.recycleUnchecked();
        }
    }

可将长长的Looper.loop()方法理解成一个不断检测messageQueue是否有数据，若有即取出并执行回调的死循环。如果取到null就说明消息队列已经退出或被释放，此时loop终止。

接下来，就需要handler对象来发送消息message了，Message类部分代码：

public final class Message implements Parcelable {
    public int what;
    public int arg1;
    public int arg2;
    public Object obj;
    Bundle data;
    Handler target;
    Message next;

    public static Message obtain() {
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                m.flags = 0; // clear in-use flag
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }
}

当我们调用Message.obtain()时，获取一个Message对象。Message对象使用完毕后，调用recycle()方法将其回收。

handler发送消息：

    public final boolean sendMessage(@NonNull Message msg) {
        return sendMessageDelayed(msg, 0);
    }

      public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

    private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
            long uptimeMillis) {
        msg.target = this;
        msg.workSourceUid = ThreadLocalWorkSource.getUid();

        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

最终，sendMessage会调用queue.enqueueMessage(msg, uptimeMillis)将msg对象保存至messageQueue中。

到这再回到Looper的loop方法中：

          Message msg = queue.next()；
          if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }
          msg.target.dispatchMessage(msg)，

当从队列读取消息不为空时，就会将该msg通过msg.target.dispatchMessage回调，而msg.target正是发送该调消息的handler对象。

进入到Handler的dispatchMessage处理消息回调方法：

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

handler会将读取到的消息通过handleMessage回调进行处理。

关系图表示：

关于Handler的问题总结：

1、首先Looper.prepare()在本线程中保存一个Looper实例，然后该实例中保存一个MessageQueue对象；因为Looper.prepare()在一个线程中只能调用一次，所以MessageQueue在一个线程中只会存在一个。

2、Looper.loop()会让当前线程进入一个无限循环，不端从MessageQueue的实例中读取消息，然后回调msg.target.dispatchMessage(msg)方法。

3、Handler的构造方法，会首先得到当前线程中保存的Looper实例，进而与Looper实例中的MessageQueue想关联。

4、Handler的sendMessage方法，会给msg的target赋值为handler自身，然后加入MessageQueue中。

5、在Activity中，我们并没有显示的调用Looper.prepare()和Looper.loop()方法，为啥Handler可以成功创建呢，这是因为在Activity的启动代码中，已经在当前UI线程调用了Looper.prepare()和Looper.loop()方法。

参考文章

Handler源码解析 (Java层)

Android 异步消息处理机制