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;
}
/**
* Use the provided queue instead of the default one.
*/
public Handler(Looper looper) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = null;
}
/**
* Use the provided queue instead of the default one and take a callback
* interface in which to handle messages.
*/
public Handler(Looper looper, Callback callback) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
}
在默认构造方法里面，handler是和当前线程的队列关联在一起，如果队列不存在，那么handler就不能接受消息。第二个有参构造方法中，需要传入一个callback接口用于处理handler传递的Message。第三个有参构造函数是传进来一个looper来代替默认的looper。第四个就是传递一个looper和callback。
在70行有个 FIND_POTENTIAL_LEAKS参数：找到潜在的泄露。看下注释：
[plain]
/*
* Set this flag to true to detect anonymous, local or member classes
* that extend this Handler class and that are not static. These kind
* of classes can potentially create leaks.
*/
设置这个标记为true来检测不是静态的匿名,本地或成员类继承Handler类。这些类型的类可以带来潜在的泄漏。在Handler的构造方法里面使用到了这个参数，目的就如上所述：
[java]
if (FIND_POTENTIAL_LEAKS) {
final Class< extends Handler> 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());
}
}
接着下面就是Callback接口：
[java]
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*/
public interface Callback {
public boolean handleMessage(Message msg);
}
当你实例化一个Handler的时候可以使用Callback接口来避免写自定义的Handler子类。这里的机制类似与Thread与runable接口的关系。
在Handler里面，子类必须实现handlerMessage()这个方法，因为在handler里面它是个空方法：
[java]
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
再来看一下：dispatchMessage()这个方法：
[java]
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
用于传递系统消息。当message的callback不为空的时候，调用handleCallback方法，如下：
[java]
private final void handleCallback(Message message) {
message.callback.run();
}
关于调用Message的方法，在这篇文章里面先不谈，解析Message源码的时候再说。
下面是171行的getMessageName()方法：
[java]
/**
* Returns a string representing the name of the specified message.
* The default implementation will either return the class name of the
* message callback if any, or the hexadecimal representation of the
* message "what" field.
*
* @param message The message whose name is