/**

* 消息泵，负责取出消息交给Handler来处理。

*/

public class Looper {

static ThreadLocal<Looper> messageThreadLocal = new ThreadLocal<>();

MessageQueue messageQueue;

private Looper() {

messageQueue = new MessageQueue();

}

/**

* 获取Looper的线程副本

*

* @return 返回Looper线程副本

*/

public static Looper getLocalLooper() {

// 返回此线程局部变量的当前线程副本中的值，如果这是线程第一次调用该方法，则创建并初始化此副本。

return messageThreadLocal.get();

}

public static void prepare() {

if (messageThreadLocal.get() != null) {

throw new RuntimeException("Only one Looper may be created per thread");

}

messageThreadLocal.set(new Looper());

}

public static void loop() {

Looper looper = getLocalLooper();

if (looper == null) {

throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");

}

MessageQueue messageQueue = looper.messageQueue;

// 无限循环

for (; ; ) {

Message message = messageQueue.consumeMessage();

if (message == null || message.handler == null) {

continue;

}

message.handler.dispatchMessage(message);

}

}

}