Handler通信机制分析

Handler通信机制

首先，在分析Handler之前，我们必须先了解为啥需要Handler，可以不需要Handler吗？答案毫无疑问，当然是不行。我们知道在主线程是不能进行耗时操作的，子线程可以进行耗时操作但不能更新UI，那怎么办呢？只能子线程进行耗时操作，任务完成后，再通知主线程进行UI更新。其实更新UI只是一方面，Handler最主要的作用是线程间的通信，所以我们需要了解去Handler。

• Handler：负责消息的发送和处理；
• Looper：负责消息的循环，即将消息从队列中取出，并发送给Handler处理；
• MessageQueue：消息队列，负责消息的存放；
• Message：消息，即信息的载体；

我们看下Google使用Handler的方式

 class LooperThread extends Thread {
        
        public Handler mHandler;

        public void run() {
            Looper.prepare();

            mHandler = new Handler() {
                public void handleMessage(Message msg) {
                    // process incoming messages here
                }
            };

            Looper.loop();
        }
    }

问题①：大家可以思考下为什么Looper.prepare()方法为什么一定要在Handler之前调用？

我们将根据官方给出的示例，按照顺序分析。在线程的run()方法中，先调用Looper.prepare()方法，我们看下其源码

 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));
    }

可以看到，在prepare()方法中创建Looper并加入到ThreadLocal中，我们看下Looper的构造方法

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

在Looper的构造方法中可以看到，将初始化MessageQueue并获得当前线程Thread.currentThread()。大家看到这里，应该可以回答上面的问题①了。因为如果不先调用Looper.prepare()方法，MessageQueue将不会初始化，当Handler发送消息到消息队列时为空，将会导致空指针异常。

下面，我们看下Handler的构造

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 that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

在构造方法中可以看到，将会获取Looper,Looper为空将会报错，并进行一些赋值操作。

继续看Looper.Loop()方法，

 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 (;;) {   // 1
            Message msg = queue.next(); // 2
            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);
            }

            final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            final long end;
            try {
                msg.target.dispatchMessage(msg); // 3
                end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (slowDispatchThresholdMs > 0) {
                final long time = end - start;
                if (time > slowDispatchThresholdMs) {
                    Slog.w(TAG, "Dispatch took " + time + "ms on "
                            + Thread.currentThread().getName() + ", h=" +
                            msg.target + " cb=" + msg.callback + " msg=" + msg.what);
                }
            }

            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();
        }
    }

代码很多，我们这里只看比较重要的1,2,3处。在1处，是一个for无限循环；在2处，通过MessageQueue.next()方法获取消息；在3处，调用msg.target.dispatchMessage(msg)方法，将消息传给Handler处理；我们看下Message的target是什么？

Message的源码
我们发现target是一个Handler,那么我们在Handler中看下这个方法

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

我们可以看到，最后调用的是handleMessage()方法。毫无疑问，上面的Loop()方法是从MessageQueue中取出Message交给handleMessage()处理，那么Message从何而来，或者说从哪儿将Message加入到MessageQueue中？当然是Handler发送的消息的方法，我们看下

Handler发送消息的方式
通过追踪，我们最后发现，它们最终调用的是enqueueMessage()方法

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

在这里看到，Message的target在这儿进行赋值的，我们继续看下MessageQueue的enqueueMessage()方法

 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(TAG, 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;//1
                mMessages = msg;//2
                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;//3
                    p = p.next;//4
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; //5
                prev.next = msg;//6
            }

            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }

方法里面的内容很多，其实通过1,2,3,4,5,6可以发现，主要的是将传过来的Message进行赋值，赋值给MessageQueue的mMessage,达到保存消息的目的。

到这里，Handler通信机制基本上分析完了，最后画了一张图进行一个总结

Handler机制流程图