Handler source code analysis

Handler Yes 7 A construction method , Commonly used ：

public Handler() {
    this(null, false);
}

public Handler(Looper looper) {
    this(looper, null, false);
}

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());
        }
    }

    // obtain Looper example 
    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;
}

When created in a child thread , You need to call Looper.prepare() initialization Looper object , Then you must call Looper.loop() Method ;

When created in the main thread , From ThreadLocal In order to get mainLooper example .

Common methods of sending messages , There are other overloaded methods , I won't list it here ：

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

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

These methods will call sendMessageDelayed(msg,delayMillis)

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;
    }
    // Store the message in the message queue 
    return enqueueMessage(queue, msg, uptimeMillis);
}

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

summary ：handler.sendMessage(msg) In fact, it's just our Message Added message queue , Queue adopted

It's a linked list , according to  when  That is, time sequencing . No more logic .

How to process messages , stay looper Example of loop() Method ：

public void dispatchMessage(Message msg) {
    // if msg.callback Not empty , It means that post（Runnable r） Send a message 
    // Execute at this time handleCallback(msg), Call back Runnable Copied from the object run（）
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        //handler The callback of the instance is not empty 
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        
        // So let's go over here , explain handler The callback of the instance is empty ,
        // Execute at this time handleMessage(msg) Method ,
        // establish Handler Instance replication method 
        handleMessage(msg);
    }
}

Handler Analysis of finished , Then look at Looper class , Its construction method ：

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

Looper The message queue is initialized in the construction method of , Also get the current thread , You can see it here ,Looper Is bound to a thread , And there is only one future looper example .

Get the child thread's looper example ：

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

Get the main thread's looper example ：

public static Looper getMainLooper() {
    synchronized (Looper.class) {
        return sMainLooper;
    }
}

Looper The most important thing in the class is the message loop method ：

public static void loop() {
    // obtain looper example 
    final Looper me = myLooper();
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
     // obtain Looper The message queue object in the instance （MessageQueue）
    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 the retrieved message is empty , The thread is blocked 
        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);
        }

        // Send a message to the corresponding Handler, then handler Will process messages 
        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);
        }

        // Release the resources occupied by messages 
        msg.recycleUnchecked();
    }
}

loop() Method keeps getting messages in a loop , Send the message to handler Handle .

MessageQueue Class has two main methods , No detailed analysis here ：

enqueueMessage(queue, msg, uptimeMillis)： Insert a message into the message queue

next()： Get messages from the message queue

thus ,Handler Relevant source code analysis is completed .