Handler的底层实现

Handler的底层实现

1.查看Handler源码的基本流程（其他类似）

首先查看Handler的构造方法，对初始化变量有一个大致的了解，然后开始走调用逻辑，Handler的sendMessage（），handleMessage（）等等

2.Handler的大致原理

Handler：发送消息send，处理消息handle

Message：消息的实体，Handler通过sendMessage（）将Message发送给消息队列MessageQueue

MessageQueue：消息队列，存储Message的消息队列

Looper：轮询(for(;;))消息队列中的消息，依次取出，然后交给handler处理

3.源码分析

Handler

public Handler(Callback callback) {
    this(callback, false);
}

public Handler(Callback callback, boolean async) {
    if (FIND_POTENTIAL_LEAKS) {
        final Classextends 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;
}

看关键的地方，构造器主要初始化了Looper（mLooper）和MessageQueue（mQueue），并且mCallback存储了CallBack接口，也就是我们创建Handler传入的接口

Handler handler = new Handler(new Handler.Callback() {
    @Override
    public boolean handleMessage(Message msg) {
        return false;
    }
});

然后来看一下sendMessage（）方法

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

可以看出最终调用了sendMessageDelayed（）

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

sendMessageDelayed只做了简单的判断，然后调用sendMessageAtTime（），需要说明一下SystemClock.uptimeMillis()指的是从开机到现在的毫秒数，不包括睡眠时间，下面看一下sendMessageAtTime（）

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);
}

往下看enqueueMessage（）

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

需要注意msg.target，将msg和handler绑定在了一起，然后调用了MessageQueue中的enqueueMessage（）方法，将消息放入MessageQueue消息队列中

MessageQueue

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 (;;) {
                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;
}

因为是直接在主线程中创建的Handler，所以Looper.loop()在ActivityThread中执行的

public static void main(String[] args) {

    Looper.prepareMainLooper();
    ActivityThread thread = new ActivityThread();
    thread.attach(false);
    Looper.loop();
}

接着看Looper中的loop方法

Looper

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();
    }
}

找到关键的方法msg.target.dispatchMessage()方法，msg.target就是前面讲的Handler对象，调用了handler的dispatchMessage()方法，接着看这个方法

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

在讲dispatchMessage（）方法之前，咱们先来看一下Handler的post方法，看一个最简单的

public final boolean post(Runnable r)
{
   return  sendMessageDelayed(getPostMessage(r), 0);
}

调用了可以看出，同sendMessage一样，最后也是调用了sendMessageDelayed（），接着看一下getPostMessage（）

private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}

可以看出，Message中的callback存储了r，下面接着分析dispatchMessage()方法的条件

第一个if条件

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

现在就知道了msg.callback实际上就是我们通过handler的post方法传入的runnable对象，所以第一个if其实就是最后回调了post中的runnable对象，并且runnble所在的线程和handler在同一个线程（注意）

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

else条件

mCallback对象，就是之前讲到的，构造器中储存的Handler的回调，

@Override
public boolean handleMessage(Message msg) {
    return false;
}

最后一个handleMessage（），通常是Handler的子类去重写handleMessage（）