Handler Looper 源码分析
1.Hander sendMessage 分析(MessageQueue 消息队列)
Handler sendMessage之后的流程
Hander.sendMessage(msg) —>Hander.sendMessageDelayed—>Hander.sendMessageAtTime —> Hander.enqueueMessage —> MessageQueue.enqueueMessage
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
目前为止我们没有看到 handler 去调用 handleMessage() 方法 ,目前也只是看到了 MessageQueue , 把消息给了 mMessage
总结:handler.sendMessage其实只是把我们的Message加入了消息队列,队列采用的是链表的方式,按照时间排序,然后再也没干其他。
2.Loop 消息循环
子线程中使用 Handler 必须先调用 Looper.prepare(); 不然会报错,我们在主线程中并没有调用过 Looper.prepare(),为什么就不报错?
因为在我们应用启动的时候,ActivityThread 的入口函数 main() 方法中已经写了
public static void main(String[] args) {
...
Looper.prepareMainLooper();
...
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
Looper.prepareMainLooper();
进入源码查看主要调用 sThreadLocal.set(new Looper(quitAllowed));
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));
}
sThreadLocal.set(new Looper(quitAllowed));
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
总结:其实就是创建一个 Looper ,并且保证一个 Thread 线程中,只有一个 Looper 对象
Looper.loop(); 循环执行MessageQueue里的message
public static void loop() {
// 获取线程的 Looper 对象
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
// 获取队列
final MessageQueue queue = me.mQueue;
for (;;) {
// 死循环不断的获取 消息队列中的消息
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
try {
// 通过 handler 去执行 Message 这个时候就调用了 handleMessage 方法
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
// 回收消息
msg.recycleUnchecked();
}
}
额外知识
msg.recycleUnchecked(); 回收信息
Message 用到 Message 消息池,最大50个,回收消息之后,消息池空闲Message +1
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
void recycleUnchecked() {
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
Message msg = Message.obtin()与 Message msg1 = new Message() 区别
Message.obtin() 源码,先去消息池获取message(sPool),如果没有sPool 再重新new Message(); 综合利用空闲的Message 。
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}