handler 源代码分析

handler
Looper 轮询器
MessageQueue 消息对象

1 主线程在一创建的时候就会调用，    public static void prepareMainLooper() {}构造方法。

public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

2 在prepareMainLooper(){} 内部调用了 prepare(false);方法，这就是在子线程中new Handler()会抱错的关键

    prepare(quitAllowed) {}方法里面设置了一个Looper对象。假设已经有了 Looper 对象，会抛出异常 Only one Looper may be created per thread
    所以说一个 Handler仅仅能有一个Looper对象
    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));//创建一个 Looper构造器
    }

3 在 Looper 的构造器中
    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);//创建了 MessageQueue对象
        mRun = true;
        mThread = Thread.currentThread();//线程对象
    }

4 可是在 handler(){}的源代码构造方法中
    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();//仅仅运行这种方法
        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;
    }

5 查看 Looper.myLooper();
    public static Looper myLooper() {
        return sThreadLocal.get();//返回的是一个 Looper对象。这里就跟 2的结果一样了
    }
所以在4 中抛出异常。跟2 也一样了
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
所以敢肯定（1 2 3）的原理就是主线程 Handler的工作原理
而 （4 5）就是我们手动创建 Handler的时候的工作原理。

handler.sendMessage(msg);他做的是将消息入队操作
6 经过源代码跟踪。会发如今调用enqueueMessage(){}构造方法的时候。所做的事情就是将消息即可。入栈处理
   private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);//注意这里。想必从字面意思理解，enqueueMessage就是入栈的意思吧
    }

7 看enqueueMessage()所做的事情
    final boolean enqueueMessage(Message msg, long when) {
        if (msg.isInUse()) {
            throw new AndroidRuntimeException(msg + " This message is already in use.");
        }
        if (msg.target == null) {
            throw new AndroidRuntimeException("Message must have a target.");
        }

        boolean needWake;
        synchronized (this) {
            if (mQuiting) {
                RuntimeException e = new RuntimeException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w("MessageQueue", e.getMessage(), e);
                return false;
            }

            msg.when = when;
            Message p = mMessages;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg; //将消息对象的引用赋值给 Message
                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; //将消息对象的引用赋值给 Message
            }
        }
        if (needWake) {
            nativeWake(mPtr);
        }
        return true;
    }
8 那么问题来了。消息引用都传递给Message对象了。那是怎样从 Message中吧消息分发出去。并响应呢？这就得看 Looper的源代码中的 public static void loop() {}方法
事实上 loop就是一个轮询器，在不断的从  MessageQueue中获取消息，能够看 loop()中的 Message msg = queue.next(); 内部实现源代码，next() 方法就是消息队列的出队方法。

只是因为这种方法的代码略微有点长，我就不贴出来了，它的简单逻辑就是假设当前MessageQueue中存在mMessages(即待处理消息)，就将这个消息出队，然后让下一条消息成为mMessages。否则就进入一个堵塞状态，一直等到有新的消息入队
    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 (;;) {//天呐，在这里竟然是 for的空循环
            //queue.next() 出现了，有兴趣的能够点进去看看
            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);//我们又发现了什么？对。msg.target代表的是Handler，调用了dispatchMessage方法
            // 这样我相信大家就都明确了为什么handleMessage()方法中能够获取到之前发送的消息了吧！

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.recycle();
        }
    }