看完这篇，别再说你不了解 Handler 消息机制了

var mHandler: Handler? = null
// 1. 开启子线程
Thread(Runnable {
  	// 2. 调用 prepare
    Looper.prepare()
  	// 3. 实例化子线程 Hanlder
    mHandler = Handler()
  	// 4. 开启消息循环
    Looper.loop()
}).start()

Thread.sleep(1000)

// 5. 往子线程发送消息
mHandler?.post {
    println(Thread.currentThread())
}

   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));
   }

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

   // 隐式传入 Looper 的构造方法已经被废弃, Google 推荐显式传入 Looper, 这样可以避免使用了错误的线程造成意想不到的 bug
   @Deprecated
   public Handler(@Nullable Callback callback, boolean async) {
          ...
           mLooper = Looper.myLooper();
           if (mLooper == null) {
               throw new RuntimeException(
                   "Can't create handler inside thread " + Thread.currentThread()
                           + " that has not called Looper.prepare()");
           }
           mQueue = mLooper.mQueue;
           mCallback = callback;
           mAsynchronous = async;
   }

   public static void loop() {
           final Looper me = myLooper();
           if (me == null) {
               throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
           }
   	       // ... 
           for (;;) {
               if (!loopOnce(me, ident, thresholdOverride)) {
                   return;
               }
           }
   }
   
   private static boolean loopOnce(final Looper me,
               final long ident, final int thresholdOverride) {
           Message msg = me.mQueue.next(); // might block
           if (msg == null) {
               // No message indicates that the message queue is quitting.
               return false;
           }
           // ...
           msg.target.dispatchMessage(msg);
           return true;
   }

   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);
   }

     public void dispatchMessage(@NonNull Message msg) {
         if (msg.callback != null) {
             // 1. post runnable 或者 Message 带 callback 会走到这里来
             handleCallback(msg);
         } else {
             // 2. 发送一个不带 callback 的 Message 会走到这里来
             // 3. 如果实例化 Handler 的时候传了 callback, 就在 callback 中处理消息
             if (mCallback != null) {
                 // 4. 如果 Handler 构造时传入的 callback 处理完消息返回了 true, 则消息处理完毕
                 if (mCallback.handleMessage(msg)) {
                     return;
                 }
             }
             // 5. 如果 Handler 构造时传入的 callback 处理完消息返回了 false, 则消息继续交给 Handler 子类实现的 handleMessage 处理
             // 6. Hanlder 默认的 handleMessage 是空实现
             handleMessage(msg);
         }
     }

public static Message obtain() {
    synchronized (sPoolSync) {
        if (sPool != null) {
            Message m = sPool;
            sPool = m.next;
            m.next = null;
            m.flags = 0; // clear in-use flag
            sPoolSize--;
            return m;
        }
    }
    return new Message();
}

private static boolean gCheckRecycle = true;

/** @hide */
public static void updateCheckRecycle(int targetSdkVersion) {
    if (targetSdkVersion < Build.VERSION_CODES.LOLLIPOP) {
        gCheckRecycle = false;
    }
}

/**
 * Return a Message instance to the global pool.
 * <p>
 * You MUST NOT touch the Message after calling this function because it has
 * effectively been freed.  It is an error to recycle a message that is currently
 * enqueued or that is in the process of being delivered to a Handler.
 * </p>
 */
public void recycle() {
    if (isInUse()) {
        if (gCheckRecycle) {
            throw new IllegalStateException("This message cannot be recycled because it "
                    + "is still in use.");
        }
        return;
    }
    recycleUnchecked();
}

/**
 * Recycles a Message that may be in-use.
 * Used internally by the MessageQueue and Looper when disposing of queued Messages.
 */
void recycleUnchecked() {
    // Mark the message as in use while it remains in the recycled object pool.
    // Clear out all other details.
    flags = FLAG_IN_USE;
    what = 0;
    arg1 = 0;
    arg2 = 0;
    obj = null;
    replyTo = null;
    sendingUid = -1;
    when = 0;
    target = null;
    callback = null;
    data = null;

    synchronized (sPoolSync) {
        if (sPoolSize < MAX_POOL_SIZE) {
            next = sPool;
            sPool = this;
            sPoolSize++;
        }
    }
}

nativePollOnce(ptr, nextPollTimeoutMillis);

    synchronized (this) {
        // Try to retrieve the next message.  Return if found.
        final long now = SystemClock.uptimeMillis();
        // 指向前一个message
        Message prevMsg = null;
        // 初始时指向第一个message
        Message msg = mMessages;
        // 1 msg.target == null说明遇到消息屏障
        if (msg != null && msg.target == null) {
                // 能进入这个if，说明此时的msg是屏障消息
                // 循环遍历，退出循环的条件是，message到末尾了，或者
                // msg是异步消息
            do {
                prevMsg = msg;
                msg = msg.next;
            } while (msg != null && !msg.isAsynchronous());
        }

// 这个方法是专门为主线程用的，手动再调用一次会抛异常
@Deprecated
    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
}

// 给子线程调用的
public static void prepare() {
        prepare(true);
}

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));
}

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

void quit(boolean safe) {
        if (!mQuitAllowed) {
            throw new IllegalStateException("Main thread not allowed to quit.");
        }

        synchronized (this) {
            if (mQuitting) {
                return;
            }
            mQuitting = true;

            if (safe) {
                removeAllFutureMessagesLocked();
            } else {
                removeAllMessagesLocked();
            }

            // We can assume mPtr != 0 because mQuitting was previously false.
            nativeWake(mPtr);
        }
}

private void removeAllMessagesLocked() {
        Message p = mMessages;
        while (p != null) {
            Message n = p.next;
            p.recycleUnchecked();
            p = n;
        }
        mMessages = null;
}

private void removeAllFutureMessagesLocked() {
        final long now = SystemClock.uptimeMillis();
        Message p = mMessages;
        if (p != null) {
            if (p.when > now) {
                removeAllMessagesLocked();
            } else {
                Message n;
                for (;;) {
                    n = p.next;
                    if (n == null) {
                        return;
                    }
                    if (n.when > now) {
                        break;
                    }
                    p = n;
                }
                p.next = null;
                do {
                    p = n;
                    n = p.next;
                    p.recycleUnchecked();
                } while (n != null);
            }
        }
 }

boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }

        synchronized (this) {
            if (msg.isInUse()) {
                throw new IllegalStateException(msg + " This message is already in use.");
            }

            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;
            }
		// ...
}

Message next() {
        // Return here if the message loop has already quit and been disposed.
        // This can happen if the application tries to restart a looper after quit
        // which is not supported.
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        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 (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        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;
                }

                // If first time idle, then get the number of idlers to run.
                // Idle handles only run if the queue is empty or if the first message
                // in the queue (possibly a barrier) is due to be handled in the future.
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    mBlocked = true;
                    continue;
                }

                if (mPendingIdleHandlers == null) {
                    mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
                }
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }

            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            for (int i = 0; i < pendingIdleHandlerCount; i++) {
                final IdleHandler idler = mPendingIdleHandlers[i];
                mPendingIdleHandlers[i] = null; // release the reference to the handler

                boolean keep = false;
                try {
                    keep = idler.queueIdle();
                } catch (Throwable t) {
                    Log.wtf(TAG, "IdleHandler threw exception", t);
                }

                if (!keep) {
                    synchronized (this) {
                        mIdleHandlers.remove(idler);
                    }
                }
            }

            // Reset the idle handler count to 0 so we do not run them again.
            pendingIdleHandlerCount = 0;

            // While calling an idle handler, a new message could have been delivered
            // so go back and look again for a pending message without waiting.
            nextPollTimeoutMillis = 0;
        }
    }

  public boolean post(Runnable action) {
      final AttachInfo attachInfo = mAttachInfo;
      if (attachInfo != null) {
          return attachInfo.mHandler.post(action);
      }

      // Postpone the runnable until we know on which thread it needs to run.
      // Assume that the runnable will be successfully placed after attach.
      getRunQueue().post(action);
      return true;
  }

private void performTraversals() {
	// cache mView since it is used so much below...
  final View host = mView;
	// ...
	host.dispatchAttachedToWindow(mAttachInfo, 0);
 	//...
  performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);//执行测量
  performLayout(lp, mWidth, mHeight);//执行布局
  performDraw();//执行绘制
}

void dispatchAttachedToWindow(AttachInfo info, int visibility) {
      mAttachInfo = info;
      //...
      // Transfer all pending runnables.
      if (mRunQueue != null) {
          mRunQueue.executeActions(info.mHandler);
          mRunQueue = null;
      }
      //...
      onAttachedToWindow();

      //...
}

public class HandlerActionQueue {
    private HandlerAction[] mActions;
    private int mCount;

    public void post(Runnable action) {
        postDelayed(action, 0);
    }

    public void postDelayed(Runnable action, long delayMillis) {
        final HandlerAction handlerAction = new HandlerAction(action, delayMillis);

        synchronized (this) {
            if (mActions == null) {
                mActions = new HandlerAction[4];
            }
            mActions = GrowingArrayUtils.append(mActions, mCount, handlerAction);
            mCount++;
        }
    }

   //...

    public void executeActions(Handler handler) {
        synchronized (this) {
            final HandlerAction[] actions = mActions;
            for (int i = 0, count = mCount; i < count; i++) {
                final HandlerAction handlerAction = actions[i];
                handler.postDelayed(handlerAction.action, handlerAction.delay);
            }

            mActions = null;
            mCount = 0;
        }
    }

    //...
}