Android 消息机制主要指 Handler 的运行机制，包括了 MessageQueue、Looper 和 Handler 的共同作用。其中 MessageQueue 以队列的形式对外提供插入和删除，它内部由单链表实现。Looper 的作用是处理 MessageQueue 中存储的消息。

通常，Handler 被我们用来更新 UI，有如下两种常见的用法

public class MainActivity extends AppCompactActivity implements View.OnClickListener {

    ...

    private Handler handler = new Handler() {
        public void handleMessage(Message msg) {
            // 进行 UI 操作
        }
    };

    ...

    @Override
    public void onClick(View v) {
        new Thread(new Runnable() {
            @Override
            public void run() {

                ...

                handler.sendMessage(message);
            }
        }).start();
    }
}

或者

public class MainActivity extends AppCompactActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Handler handler = new Handler();
        new Thread(new Runnable() {
            @Override
            public void run() {
                handler.post(new Runnable() {
                    @Override
                    public void run() {
                        // 进行 UI 操作
                    }
                });
            }
        }).start();
    }
}

用法一

在子线程完成耗时操作后，通过 sendMessage 方法发送消息，回到 Handler 所在的主线程，通过 handleMessage 方法完成 UI 操作。

来看 sendMessage 和它的后续方法

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

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

可以看到，经过这几个方法，发送的消息最终都传入了 MessageQueue 的 enqueueMessage 方法中，于是我们找到 enqueueMessage 方法

boolean enqueueMessage(Message msg, long when) {

    ...

    synchronized (this) {

        ...

        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;
}

在 enqueueMessage 方法中，它主要执行的是单链表的插入操作，并没有对消息队列里的消息执行，那执行操作在哪呢？开头说到 Looper 的作用是执行 MessageQueue 中的消息，于是找到 Looper 的 loop 方法

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();
    }
}

可以看到，loop 方法是一个死循环，它会调用 MessageQueue 的 next 方法来获取新消息，当 next 方法返回空时会退出循环。来看 next 方法源码

Message next() {

    ...

    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }

        nativePollOnce(ptr, nextPollTimeoutMillis);

        synchronized (this) {
            // Try to retrieve the next message.  Return if found.
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;
            if (msg != null && msg.target == null) {
                // Stalled by a barrier.
                // Find the next asynchronous message in the queue.
                do {
                    prevMsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isAsynchronous());
            }
            if (msg != null) {
                if (now < msg.when) {
                    // Next message is not ready.
                    // Set a timeout to wake up when it is ready.
                    nextPollTimeoutMillis = (int) Math.min(msg.when - now,
                        Integer.MAX_VALUE);
                } else {
                    // Got a message.
                    mBlocked = false;
                    if (prevMsg != null) {
                        prevMsg.next = msg.next;
                    } else {
                        mMessages = msg.next;
                    }
                    msg.next = null;
                    if (false) Log.v("MessageQueue", "Returning message: " + msg);
                    return msg;
                }
            } else {
                // No more messages.
                nextPollTimeoutMillis = -1;
            }

            // Process the quit message now that all pending messages have been handled.
            if (mQuitting) {
                dispose();
                return null;
            }

            ...

        }

        ...

    }
}

next 方法用 mMessage 管理消息队列。当消息队列不为空时，返回并删除当前消息，并将下一条消息置前。若消息队列为空，next 方法会一直阻塞，直到有新消息到来。那 next 方法何时返回空呢？当 Looper 的 quit 方法调用时，next 会返回空。因为当 Looper 调用 quit 方法，MessageQueue 的 quit 或 quitSafely 方法会被调用，此时消息队列会被标记为退出状态，next 判断到消息队列的状态便会返回空。

接着看 loop 方法，如果 next 方法返回了消息，它会调用msg.target.dispatchMessage(msg)来处理，其中msg.target就是发送消息的 Handler 对象。也就是说通过 sendMessage 方法发送的消息会来到 Handler 对象的 dispatchMessage 方法，来看 dispatchMessage 方法

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

dispatchMessage 方法会检查 Message 的 callback 是否为空，不为空就通过 handleCallback 方法来处理消息。Message 的 callback 是一个 Runnable 对象，它实际上就是 post 方法传来的 Runnable 参数。在 handleCallback 方法中会执行传来的 Runnable 对象

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

接着 dispatchMessage 方法会判断 mCallback 是否为空，若不为空，调用 mCallback 的 handleMessage 方法，若为空，则调用 Handler 的 handleMessage 方法。其中 mCallback 的 handleMessage 方法指的是通过 Callback 接口实现的 handleMessage 方法

/**
 * Callback interface you can use when instantiating a Handler to avoid
 * having to implement your own subclass of Handler.
 *
 * @param msg A {@link android.os.Message Message} object
 * @return True if no further handling is desired
 */
public interface Callback {
    public boolean handleMessage(Message msg);
}

就这样，通过 sendMessage 发送的消息最终回到了 handleMessage 方法进行处理。

用法二

在子线程中通过 post 方法传入一个 Runnable 对象，在该对象中实现 UI 操作。

来看 post 方法

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

这里使用了 sendMessageDelayed 方法发送了一条消息。getPostMessage 方法的作用是将 Runnable 对象转换成一条消息

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

sendMessageDelayed 方法经过的流程在上面已经分析过。最终，通过 post 传入的 Runnable 对象会来到 dispatchMessage 方法中，被 handleCallback 方法调用。

补充

在子线程中直接创建 Handler 会报错，正确的用法是

class LooperThread extends Thread {
    
    public Handler mHandler;

    public void run() {
      Looper.prepare();

      mHandler = new Handler() {
          public void handleMessage(Message msg) {
              // process incoming messages here
          }
      };

      Looper.loop();
    }
}

在主线程中不必这样，因为它会调用Looper.prepareMainLooper方法来创建 Looper。

相关参考 郭霖的博客