Handler 原理分析

核心类，在主线程中定义：

Handler，ActivityThread，Looper，MessageQueue。

Looper类

包含MessageQueue，Thread，ThreadLocal
通过ThreadLocal.set（key) 保存线程唯一的Looper
每个线程仅有一个Looper

    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));
    }

Looper.loop()

从 Looper类中的MessageQueue，不断取出message，调用msg.tartget.dispathMessage方法

1、取出message

/**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {
        final Looper me = myLooper();final MessageQueue queue = me.mQueue;for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }try {
                msg.target.dispatchMessage(msg);
            } catch (Exception exception) {
               
            } 
            msg.recycleUnchecked();
        }
    }

2、dispatchMessage方法走向

    /**
     * Handle system messages here.
     */
    public void dispatchMessage(@NonNull Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

Handler

内部维护一个MessageQueue，指向Looper类中的MessageQueue
sendMessage()添加到MessageQueue中

    public boolean sendMessageAtTime(@NonNull 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);
    }

    private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
            long uptimeMillis) {
        msg.target = this;
        msg.workSourceUid = ThreadLocalWorkSource.getUid();

        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }