首先展示一个主线程向子线程发消息的案例。通过这个案例来了解下线程
//创建子线程
class MyThread extends Thread{
private Looper looper;//取出该子线程的Looper
public void run() {
Looper.prepare();//创建该子线程的Looper
looper = Looper.myLooper();//取出该子线程的Looper
Looper.loop();//只要调用了该方法才能不断循环取出消息
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);
myThread = new MyThread("handler thread");
myThread.start();
mHandler = new Handler(myThread.getLooper()) {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Log.d("SecondActivity", "handleMessage: " + Thread.currentThread());
}
};
mHandler.sendEmptyMessage(1);
}
- 在消息接收线程MyThread中创建调用 Looper.prepare() 创建looper。
- Looper.myLooper() 保存当前的looper对象
- Looper.loop() 不断轮询MessegeQueue，获取到msg。然后调用msg.target.dispatchMessage(msg);msg.target实际上就是handler。dispatchMessage会调用handleMessage(msg)。这样handler就收到消息了。
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();
}
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;
//...
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 {
...
}
...
}
- 消息接收线程创建handler，并且获取MyThead线程的looper对象及looper中的messgeQueue对象
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
- 通过调用sendEmptyMessage向messageQueue中添加msg。这里源码就不粘贴了，就是msg插入到链表Message mMessages中
这样就完成了通信，总的来说就是一方通过looper轮询，另一方拿到looper向looper的messageQueue中发送消息。
但是要注意的一点，以上代码实际运行会报looper的messageQueue为空，解决方法就是使用HandlerThread。因为在主线程发消息时，子线程looper可能还没创建好。继承HandlerThread就好了
class MyThread extends HandlerThread {
public MyThread(String name) {
super(name);
}
@Override
public void run() {
super.run();
}
}
最后放张关系流程图片
handler.png
