了解Handler,Looper,MessageQueue，Message的工作流程

Handler的作用

异步通信，消息传递

Handler的基本用法

Handler的用法，示例1、（子线程向主线程发送消息）

public class HandlerActivity extends AppCompatActivity {
Handler handler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
        Toast.makeText(HandlerActivity.this, "接收到了消息", Toast.LENGTH_SHORT).show();
    }
};
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_handler);

}

public void onClickView(View v) {
    switch (v.getId()) {
        case R.id.button: {
            Thread thread = new Thread() {
                @Override
                public void run() {
                    Message msg = Message.obtain();
                    handler.sendMessage(msg);
                }
            };
            thread.start();
            break;
        }
    }
}
}

这个界面只有一个Button, 点击事件是：启动一个线程，在这个线程里使用handler，发送一个消息；这个消息不带任何数据。
Message: 这个可以使用new来生成，但是最好还是使用Message.obtain()来获取一个实例。这样便于消息池的管理。
handler初始化的时候，我们复写了handleMessage方法，这个方法用来接收，子线程中发过来的消息。

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示例2、主线程向子线程发送消息

public class HandlerActivity extends AppCompatActivity {
Handler handler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
        Log.d("child", "----" + Thread.currentThread().getName());
    }
};
Handler childHandler = null;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_handler);
    Thread child = new Thread("child thread") {
        @Override
        public void run() {
            Looper.prepare();
            childHandler = new Handler(Looper.myLooper()) {
                @Override
                public void handleMessage(Message msg) {
                    Log.d("child", "----" + Thread.currentThread().getName() + ",  " + msg.obj);
                }
            };
            Looper.loop();
        }
    };
    child.start();
}

public void onClickView(View v) {
    switch (v.getId()) {
        case R.id.button: {
            Thread thread = new Thread() {
                @Override
                public void run() {
                    Message msg = Message.obtain();
                    handler.sendMessage(msg);
                }
            };
            thread.start();
            break;
        }
        case R.id.button1: {
            if (childHandler != null) {
                Message msg = Message.obtain(childHandler);
                msg.obj = "123---" + SystemClock.uptimeMillis();
                childHandler.sendMessage(msg);

            } else {
                Log.d("-----", "onClickView: child Handler is null");
            }

            break;
        }
    }
}
}

主线程向子线程发送消息时，我们需要使用的是handler,但是这个handler是需要在子线程中实例化，否则子线程无法接收到消息。
在child thread中准备的工作：

• 1. Looper.prepare();
• 2. 实例化childHandler的时候出入了一个Looper实例。

• 3. Looper.loop()调用
     假如我们直接在子线程中直接实例化一个handler，而不传入Looper实例。程序会直接抛出异常：
     java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
     这个异常是在hanlder源码：

     public Handler(Callback callback, boolean async) {
     if (FIND_POTENTIAL_LEAKS) {
     final Class 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();
     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;
     }
     出现的，从这里可知，在线程中实例化一个handler需要一个Looper实例。

     如果其他线程的实例出来的Looper会怎么样？

     首先看Looper源码中是怎么写的。Looper的构造方法是私有的，这样我们只能通过Looper的静态方法来实例化一个Looper.在Looper类中还有一个ThreadLocal sThreadLocal 变量，而在上面的代码中使用了Looper.myLooper()方法来给handler一个Looper实例。

     * Looper.myLooper()

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

     这个方法返回了一个存放在ThreadLocal中的Looper实例。

     为什么在调用Looper.myLooper()方法之前需要先调用 Looper.prepare()呢？

     Looper.prepare源码：

  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));
  }
  方法很简单，就是检查sThreadLocal里面是否有有值，然后将Looper的实例存放到ThreadLocal，如果不为空，直接就是 RuntimeException 异常。这也保证了一个线程中只能有一个Looper实例。因此，Looper.prepare方法只能调用一次。

  那么ThreadLocal的作用是什么呢？

  在这个程序中，可以大概理解成线程间数据的隔离。意思就是我存放在ThreadLocal中的数据，只能在我本线程中可以获得到值，在其他线程中获取不到（其他线程中获取的是null）。
  这样就能得出，其他线程中的Looper，在本线程中通过Looper.myLooper()获取不到数据。

  那么为什么在主线程中不需要传入Looper实例呢？

  通过查找Android源码，可以知道在ActivityThread中main方法里面，UI线程已经初始化了Looper.prepareMainLooper();这样就在UI线程中有Looper实例了。当然在main方法的下面，也调用了Looper.loop()方法。

  Looper.loop()方法是做什么的

  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
      final Printer logging = me.mLogging;
      if (logging != null) {
          logging.println(">>>>> Dispatching to " + msg.target + " " +
                  msg.callback + ": " + msg.what);
      }
  
      final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
  
      final long traceTag = me.mTraceTag;
      if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
          Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
      }
      final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
      final long end;
      try {
          msg.target.dispatchMessage(msg);
          end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
      } finally {
          if (traceTag != 0) {
              Trace.traceEnd(traceTag);
          }
      }
      if (slowDispatchThresholdMs > 0) {
          final long time = end - start;
          if (time > slowDispatchThresholdMs) {
              Slog.w(TAG, "Dispatch took " + time + "ms on "
                      + Thread.currentThread().getName() + ", h=" +
                      msg.target + " cb=" + msg.callback + " msg=" + msg.what);
          }
      }
  
      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();
  **}**

  }

这个方法里面，我们需要注意到：

1. Looper me这个不允许为空
2. for (;;)循环
3. Message msg = queue.next()

4. msg.target.dispatchMessage(msg);

5. 在looper.loop方法里面，检查了本线程中的Looper实例，也对应了在调用looper.loop方法之前必须先调用looper.prepare方法。

6. for循环是一个死循环，这个需要一直取出MessageQueue队列中的数据，也就是刚才所列的第三个 queue.next方法。这个方法会阻塞，直到有消息从消息队列中取出来。
   next方法源码：

   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;
   }

   }

7. queue.next方法，返回一个Message，而这个message会被传入到msg.target的dispatchMessage方法中。msg.target是一个handler，就是发送消息的实例。
   dispatchMessage(Message)源码：
   public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

   Handler初始化的时候，我们并没有传入msg.callback和mCallback这两个回调。所以这个方法最终执行的是handleMessage方法。
   上面我们分析了ThreadLocal,Looper,Message,MessageQueue,下面开始分析handler发送消息的方式。

   Handler发送消息

   Handler 通过sendMessage方法发送消息。这个方法最终调用的是

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

参数：MessageQueue queue则是Handler中mQueue变量，这个变量在Handler(Callback callback, boolean async)初始化完成。它是Looper中一个final MessageQueue的变量，在初始化Looper的时候，就开始初始化MessageQueue。这也是一个线程中只有一个MessageQueue的原因.
Message msg 是封装的消息。
long uptimeMillis 延迟发送时间。
之前分析looper.loop方法的时候，说了msg.target.dispatchMessage, 这个target就是在这个方法里面赋值的。
从这个方法里面，可以知道handler的sendMessage，只是把消息(Message实例)添加到了queue队列中。

    boolean enqueueMessage(Message msg, long when) {
    if (msg.target == null) {
        throw new IllegalArgumentException("Message must have a target.");
    }
    if (msg.isInUse()) {
        throw new IllegalStateException(msg + " This message is already in use.");
    }
    synchronized (this) {
        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;
        }

        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;
}

Handler, Looper, Message, MessageQueue之间的关系

通过handler发送Message，将Message压入MessageQueue队列中；而Looper.loop方法又在不停的循环这个消息队列，取出压入MessageQueue的Message， 然后dispatchMessage分发，最终会调用handler.handleMessage方法来处理发送过来的Message.