Android面试准备-handler

基本信息

1.Handler-控制器

2.Looper-轮询

3.Message-消息对象

4.MessageQueue-单链表结构的消息队列

一个消息的前生今世

一般情形下,涉及到异步的操作才需要引入handler来接收异步操作的结果,来更新UI信息.所以常规操作是这样

图片引用自

https://zhuanlan.zhihu.com/p/25222485

private Handler handler = new Handler(){
    @Override
    public void handleMessage(Message msg) {
         super.handleMessage(msg);
         textView.setText("对UI进行操作");
    }
};
@Override
protected void onCreate(Bundle savedInstanceState){
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);
       textView = (TextView) findViewById(R.id.mytv);
       new Thread(new Runnable() {
           @Override
           public void run() {
               //模拟耗时操作
               SystemClock.sleep(3000);
               handler.sendMessage(new Message());
           }
       }).start();

   }

 首先创建一个handler

 第二创建一个Thread

 Thread处理完某项操作以后,通过handler.sendMessage(msg)把异步的结果发送到消息队列MessageQueue中,

 然后我们通过复写handlerMessage()完成该次操作.

 问题

  Looper呢?MessageQueue呢?

  这TM到底怎么关联起来的.

  图同样取自https://zhuanlan.zhihu.com/p/25222485

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);
}

 获取MessageQueue 判断是否为空,不为空

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

 重头戏

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;
    }

 Message类中几个有趣的东西

 Handler target;
 Message next;
    public static Message obtain(Handler h) {
        Message m = obtain();
        m.target = h;

        return m;
    }

重点标注target,你会发现尼玛:Message在创建的时候就关联了handler.

关于Looper

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));
    }

重点注意ThreadLocal,

这是个什么类呢?

static final ThreadLocal sThreadLocal = new ThreadLocal();

ThreadLocal用于保存某个线程变量：对于同一个static ThreadLocal，不同线程只能从中get，set，remove自己的变量，而不会影响其他线程的变量。

1、ThreadLocal.get: 获取ThreadLocal中当前线程变量的值。

2、ThreadLocal.set: 设置ThreadLocal中当前线程变量的值。

3、ThreadLocal.remove: 移除ThreadLocal中当前线程变量的值。

4、ThreadLocal.initialValue: ThreadLocal没有被当前线程赋值时或当前线程刚调用remove方法后调用get方法，返回此方法值。

这个太重要了.这是个啥子勒.

ThreadLocal保存保存了各自线程的Looper,Looper又包含MessageQueue,Looper的构造器
    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

sendMsg发送一次,ThreadLocal就set一次.

Looper中loop方法是个死循环(在主线程的死循环为啥没引起ANR呢?)

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);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            //......
            msg.recycleUnchecked();
        }

呀哈,谁污染,谁治理,谁发送,谁处理 

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

结束:handler收到了消息,最后过上了没羞没糙的生活