Handler消息机制
Message.obtain()
Message msg = new Message()
new Handler(){
handlerMessage(Message msg){
// 处理消息
}
}
public Handler() {
...
// 获取looper
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 = null;
}
public static final void main(String[] args) {
....
// 1.主线程创建Looper
Looper.prepareMainLooper();
if (sMainThreadHandler == null) {
sMainThreadHandler = new Handler();
}
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
public static final void prepare() {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
// 3、在主线程中设置Looper， new Looper()里面创建了一个MessageQueue
sThreadLocal.set(new Looper());
public static final void prepareMainLooper() {
// 2、调用prepare
prepare();
setMainLooper(myLooper());
if (Process.supportsProcesses()) {
myLooper().mQueue.mQuitAllowed = false;
}
}
public static final void loop() {
...
while (true) {
// 取消息，如果没有消息，就阻塞
Message msg = queue.next(); // might block
...
msg.target.dispatchMessage(msg);
...
}
}
public boolean sendMessageAtTime(Message msg, long uptimeMillis)
{
....
// 把Message的target置为当前发送的Handler，以便Looper取到message后根据target把message分发给正确的Handler
msg.target = this;
// 往队列里面添加Message
sent = queue.enqueueMessage(msg, uptimeMillis);
....
}
final boolean enqueueMessage(Message msg, long when) {
...
Message p = mMessages;
if (p == null || when == 0 || when < p.when) {
// 当前发送的message需要马上被处理调，needWake唤醒状态置true
msg.next = p;
mMessages = msg;
needWake = mBlocked; // new head, might need to wake up
} else {
// 当前发送的message被排队到其他message的后面，needWake唤醒状态置false
Message prev = null;
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
msg.next = prev.next;
prev.next = msg;
needWake = false; // still waiting on head, no need to wake up
}
}
// 是否唤醒主线程
if (needWake) {
nativeWake(mPtr);
}
return true;
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
// 把Message交给Handler处理
handleMessage(msg);
}
}