Handler学习笔记

使用经典示例代码:

public class MainActivity extends AppCompatActivity {
    private TextView tv;
    private Handler mHandler;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv = (TextView) findViewById(R.id.tv);
        mHandler = new Handler(){
            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
                String msgStr = (String) msg.obj;
                tv.setText(msgStr);
            }
        };
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                    Message message = Message.obtain();
                    message.obj = "message 1";
                    mHandler.sendMessage(message);
                    Thread.sleep(1000);
                    message.obj = "message 2";
             mHandler.sendMessageDelayed(message,1000);
                    Thread.sleep(1000);
                    message.obj = "message 3";
                    mHandler.sendMessageAtTime(message,300);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }
}

mHandler.sendMessageXXX的核心逻辑代码

mHandler.sendMessage 最终会调用MessageQueue中的enqueueMessage(Message msg, long when)方法，其核心代码如下

boolean enqueueMessage(Message msg, long when) {
        //省略部分代码后的代码
        synchronized (this) {
            msg.when = when;
            Message p = mMessages;
            if (p == null || when == 0 || when < p.when) {
                //链表的下一个赋值
                msg.next = p;
                mMessages = msg;
            } else {
                Message prev;
                for (;;) {
                    //链表前一个
                    prev = p;
                    //当前
                    p = p.next;
                    //按照when排序
                    if (p == null || when < p.when) {
                        break;
                    }
                }
                //链表的下一个
                msg.next = p; 
                //前一个的下一个是当前
                prev.next = msg;
            }
        }
        return true;
    }

sendMessageXXX总结

enqueueMessage的核心操作是将Message对象按照when从小到大顺序插入到消息队列链表，并没有其它操作。例如示例中的Message顺序接口为
msg1 msg1.next->msg3
msg3 msg3.next->msg2
msg2 msg2.next->null

为啥MessageQueue采用链表结构存储

链表的插入和删除快，特别是在中间插入数据时。但是链表结构不适合查询操作

Looper的相关问题分析

1、 子线程中创建Handler直接new会出异常，原因如下

mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }

2、为啥主线程中不会报错，因为在ActivityThread的main方法中有如下代码

//prepareMainLooper最终调用了Looper的prepare()方法会自动为主线程创建应的Looper
Looper.prepareMainLooper();
//能让消息循环起来
Looper.loop();

3、 Looper.prepare()的源码

private static void prepare(boolean quitAllowed) {
        //sThreadLocal 能保证一个线程对应一个对象
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        //当前线程绑定Looper对象
        sThreadLocal.set(new Looper(quitAllowed));
    }

4、 Looper.loop源码分析

public static void loop() {
        //得到当前Looper
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        //Looper关联的详细队列
        final MessageQueue queue = me.mQueue;

        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();
        //死循环
        for (;;) {
            Message msg = queue.next(); // might block
            //没有消息直接返回
            if (msg == null) {
                return;
            }
            try {
                //msg.target是在sendMessage时指定为当前的
                //Handler，这里会调用handlerMessage回调
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            //消息回收
            msg.recycleUnchecked();
        }
    }