首先明确这一点： 交互基于消息机制。
消息机制主要包含：MessageQueue，Message,Handler和Looper这四大部分
拿一个典型例子分析消息机制怎么工作的
public class Activity extends android.app.Activity {
private Handler mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
System.out.println(msg.what);
}
};
@Override
public void onCreate(Bundle savedInstanceState, PersistableBundle persistentState) {
super.onCreate(savedInstanceState, persistentState);
setContentView(R.layout.activity_main);
new Thread(new Runnable() {
@Override
public void run() {
...............耗时操作
Message message = Message.obtain();
message.what = 1;
mHandler.sendMessage(message);
}
}).start();
}
}
sendMessage 调用sendMessageDelayed(msg, 0)最后调用sendMessageAtTime
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
这里我有个问题 message是什么时候放到队列里的 是时间结束之后 还是立即放入等待？
继续看代码
boolean enqueueMessage(Message msg, long when) {
......
// 插入队列 按照执行顺序从头到尾
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;
......
// mPtr 标记在取消息的时候会用上
if (needWake) {
nativeWake(mPtr);
}
}
至此知道了消息封装了执行的时间 立即插入了队列（这样也导致了内存泄漏的问题），接着又有问题Looper怎么取出消息传送给handler的handleMessage处理呢？
分析下Looper 大概都知道一个线程对应一个Looper Looper执行loop方法死循环取消息队列的消息
public static void loop() {
final Looper me = myLooper();
....
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// 一般不会退出 调用Looper.quit()可以使next返回null
return;
}
final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
// 发送message 给handler处理
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
...
msg.recycleUnchecked();
}
}
但是这里没有涉及到延迟的处理 在代码里找 next方法怎么取消息 这里有关于延时的处理
还有使死循环为什么不会造成ANR？ 其实线程会在适当的时候阻塞 并不会一直死循环执行，onCreate/onStart/onResume等操作时间过长，会导致掉帧，甚至发生ANR，looper.loop本身不会导致应用卡死，队列有新消息时会唤醒线程，最新的消息在头部，会去执行最头部的消息，当执行到延时消息时又会被阻塞，有新消息又会被唤醒，如此往复，并不是一直死循环执行。
Message next() {
final long ptr = mPtr;
if (ptr == 0) {
// loop退出或者dispose
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
// 有还没到时间的消息没被处理时 调用
Binder.flushPendingCommands();
}
// 这是一个native 方法 大体意思是阻塞nextPollTimeoutMilli时间
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// 在这里如果执行时间没到 是不会返回message的
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// 正确返回message
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// looper quit
if (mQuitting) {
dispose();
return null;
}
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
这样整个流程差不多走完了 其他细节方面需要精读下源码 - -
为什么主线程不会导致应用卡死？参考https://www.zhihu.com/question/34652589
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