Handler解析

Handler是Android的消息传递机制上层(应用层)的实现，通过它可以把一个任务切换到Handler所在的线程中执行，只不过我们经常使用Hnadler来更新UI。

首先介绍一下消息机制中包括的四个要素

• Message（消息）：需要被传递的消息对象，其中包含了消息ID，消息相应方以及回调方法等，由MessageQueue统一列队，最终由Handler处理。

• MessageQueue（消息队列）：用来存放Handler发送过来的消息（Message），虽然叫队列，但是内部是通过单链表的数据结构来维护消息列表，等待Looper的抽取。

• Handler（处理者）：负责Message的发送及处理。通过 Handler.sendMessage() 向消息池发送各种消息事件；通过 Handler.handleMessage() 处理相应的消息事件。

• Looper（消息泵）：通过Looper.loop()不断地从MessageQueue中抽取Message，按分发机制将消息分发给目标处理者（Handler）。

下面是一张Handler的流程图:

 

他们的数量关系是：一个Thread只能有一个Looper，可以有多个Handler，而一个Looper中又维护了一个MessageQueue队列。

这里就要提一下ThreadLocal，是一个线程内部的数据存储类，通过它可以在指定的线程中存储数据。对于Handler来说就是获取当前线程的Looper。

下面就从整个流程上来分析一下：

1、发送消息

// 直接创建可能会导致内存泄露
public Handler() {
    this(null, false);
}

// 一般创建的时候会选择这个构造方法
public Handler(Callback callback) {
    this(callback, false);
}

// 上面两个构造方法的最终实现
public Handler(Callback callback, boolean async) {
    if (FIND_POTENTIAL_LEAKS) {
        final Class 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(); // 从当前线程的ThreadLocal中获取Looper对象
    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;
}

这里说一下为什么直接new Handler()会存在内存泄露，我们一般会使用下面的方式创建。这就相当于在Activity中声明了一个内部类，内部类默认会持有外部类的引用，也就是Handler持有Activity的引用而Message又持有对Handler的引用，如果在退出Activity的时候，有一个消息还没有处理的话，那么这时候Activity是没法回收的，就会造成内存泄露

// 每次创建Android Studio都会提出会出现内存泄露的警告
private Handler handler=new Handler(){
    @Override
    public void handleMessage(Message msg) {

        super.handleMessage(msg);
    }
};

发消息包括sendMessage和postMessage两种，但最终都是调用到了sendMessageAtTime方法

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

public final boolean sendEmptyMessage(int what) {
    return sendEmptyMessageDelayed(what, 0);
}

public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
    Message msg = Message.obtain(); // 从消息池中获取一个消息
    msg.what = what;
    return sendMessageDelayed(msg, delayMillis);
}

public final boolean sendMessageDelayed(Message msg, long delayMillis) {
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); // 系统时间加上delay的时间就是执行的时间
}

public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
    Message msg = Message.obtain();
    msg.what = what;
    return sendMessageAtTime(msg, uptimeMillis);
}

// sendMessage系列方法最终调用的地方
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);
}

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

// 其余的postDelay等等方法都和上面的send差不多，最终也是调用了sendMessage，只不过差别在于传入了一个Runnable对象
public final boolean post(Runnable r) {
    return  sendMessageDelayed(getPostMessage(r), 0);
}

private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r; // 回调对象是Runnable，直接在Runnbale中的run方法中写上要执行的方法
    return m;
}

 post()直接在runnable里面完成更新操作(大家可能会有疑问，在Runnable的run方法里执行那不就相当于新建了一个线程执行么，其实不然，因为在下面接收消息的时候调用的是线程的run方法而不是start方法)，这个任务会被添加到handler所在线程的消息队列中，即主线程的消息队列中

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;
            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 (;;) { // 单链表的插入操作
                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;
}

2、接收消息

public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg); // post调用这个方法
    } else {
        if (mCallback != null) { // 这个mCallback就是我们new Handler构造函数中传入的callback
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

// 直接调用的Runnable的run方法，并不是strat()，所以仅仅是当做一个有run()的普通类使用而已，并不是开启了一个新的线程
private static void handleCallback(Message message) {
    message.callback.run();
}

3、轮询消息

Looper.prepare()在每个线程只允许执行一次，该方法会创建Looper对象，Looper的构造方法中会创建一个MessageQueue对象，再将Looper对象保存到ThreadLocal。

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();
}

消息的轮询主要是靠Looper.loop()来实现的，它会不停地从MessageQueue中查看是否有新的消息，如果有的话就立即处理，否则就一直阻塞在那里。

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;
                }
            }
        }
        if (logSlowDispatch) {
            showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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();
    }
}

上面代码最主要的就是一个死循环，当MessageQueue的next方法返回null时跳出循环。那么一直在主线程循环的话会不会造成系统卡死呢，当然不会，因为loop就是把消息发送到该去的地方，具体执行的地方不在这里，而且也不会消耗cpu的资源（具体可以了解一下Linux的pipe/epoll机制）

最后是quit和quitSafely，顾名思义，两个方法的区别在与是否是安全地退出Looper，quit是直接退出，而quitSafely会把消息队列中的消息处理完毕之后才退出

public void quit() {
    mQueue.quit(false);
}

public void quitSafely() {
    mQueue.quit(true);
}

void quit(boolean safe) {
    if (!mQuitAllowed) {
        throw new IllegalStateException("Main thread not allowed to quit.");
    }

    synchronized (this) {
        if (mQuitting) {
            return;
        }
        mQuitting = true;

        if (safe) {
            removeAllFutureMessagesLocked(); // 移除尚未触发的所有消息
        } else {
            removeAllMessagesLocked(); // 移除所有消息
        }

        // We can assume mPtr != 0 because mQuitting was previously false.
        nativeWake(mPtr);
    }
}

 

另外拓展一下，感觉IPC调用中的Messager和这个实现原理差不多

参考自 http://gityuan.com/2015/12/26/handler-message-framework/