学android的都知道，handler的作用，就是线程间通讯，其中最经常做的，就是在其他线程发个通知，给UI线程来更新界面。这里就来分析一下是如何做到了。
怎么用handler
class HandlerDemo:AppCompatActivity(){
private val innerHandler = InnerHandler(this)
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
innerHandler.sendEmptyMessageDelayed(1,3000)
}
private class InnerHandler(activity: Activity) : Handler() {
private val activityWeakReference: WeakReference<Activity>?
init {
activityWeakReference = WeakReference(activity)
}
override fun handleMessage(msg: Message) {
super.handleMessage(msg)
if (null != activityWeakReference) {
val activity = activityWeakReference.get()
when(msg.what){
1->{
Toast.makeText(activity,"handler!",Toast.LENGTH_SHORT).show()
}
}
}
}
}
}
完整的用法就是这样，发送和接收message。
先来看看系统是怎么发的，发的方法有很多种，sendMessage、sendEmptyMessage、sendEmptyMessageDelayed、sendEmptyMessageAtTime等，先查看代码：
public final boolean sendMessage(@NonNull Message msg) {
return sendMessageDelayed(msg, 0);
}
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull 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);
}
可以发现，其实所有的方法，到最后，都是调用sendMessageAtTime，在sendMessageAtTime中，进行enqueueMessage(queue, msg, uptimeMillis)，就是将我们创建的message，放到一个类型为MessageQueue的对象中。先继续看enqueueMessage(queue, msg, uptimeMillis)
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;//使得message持有handler的引用
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
这里重要的有2个地方，msg.target = this和queue.enqueueMessage(msg, uptimeMillis)，第一个的作用就是使得每一个我们创建的message都持有handler的引用，用于处理message时，找回对应的handler。而enqueueMessage方法，就是把message放到MessageQueue链表中。
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;//设置message中的延迟时间
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {//新的message触发时间，在MessageQueue中是最快的
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
//根据延迟时间遍历链表，将新的message，按照触发时间的顺序排列好
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;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
到这里发送的全部逻辑就完成了。接下来看接收处理的逻辑。但是接收处理的代码，在上面的例子中，是我们自己实现的handleMessage方法，这已经得到了message，并不是我们要找的原理部分，那实际的逻辑在哪里？是在Looper中。
public static void loop() {
final Looper me = myLooper();
...
final MessageQueue queue = me.mQueue;
...
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
try {
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
...
msg.recycleUnchecked();
}
}
这里的loop方法，就是Looper的无限循环，不停从MessageQueue中获取写一个message（queue.next）,那我们接着看MessageQueue的next()方法。
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
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) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a 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;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
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;
}
}
代码很多，其实这里主要搞清楚的就是，next方法就是从MessageQueue中取出下一个message，如果已经没有消息的，就进行睡眠。
发送和接收的过程和原理都清楚了，但是上面提到的Looper和MessageQueue我们还不清楚
这两个东西，在整个handler的使用过程中，我们都没有创建过，但是很明显这两个都很重要，那我们先来看它们是啥时候创建的。
public Handler(Callback callback, boolean async) {
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());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
接着看
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
完了？原来就是ThreadLocal.get()就能得到了？那ThreadLocal又是啥时候新建的。这就要看Looper的prepare方法了。
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));
}
能看到，prepare的时候，new了一个Looper，然后放到ThreadLocal中，而ThreadLocal里有一个HashMap。
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
每一个线程都有一个ThreadLocalMap，拿出来，然后把Looper对象set进去。而且key还是每个线程的本身，所以，每个线程只能对应一个Looper。那MessageQueue呢？
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper的构造方法，就new了一个，所以每个Looper也只有一个MessageQueue。
面试时，handler会问什么？
1、一个线程或者一个activity可以有多少个handler？
无数个，从handler的构造方法可以得知，无论创建多少次，Looper和 MessageQueue都只有一份。
2、一个线程可以有多少个Looper？
一个，Looper是由ThreadLocal创建的，并且在prepare的方法中，如果Looper已经存在，会直接报错，至于为什么能保持一个，因为ThreadLocal用一个HashMap来存储，而key是这个线程的本身，达到了唯一性。
3、handler造成内存泄漏，怎么优化？
文章最开头的demo代码就是优化后的了，为什么会造成内存泄漏，可以看sendMessage的逻辑，也就是message加入MessageQueue链表的过程，每个message的target都会持有handler的引用，而handler又持有activity的引用，如果activity已经被用户离开了，但是handler里还有无数的message处理，那就会导致activity的引用一直无法被释放，造成了内存泄漏。优化方案就是使用嵌套类（java叫静态内部类），使得嵌套类不默认持有外部类的引用，并且在activity的onDestroy中移除handler的所有message。
4、handler如何做到的线程安全？
在加入链表和从链表中取出，都有同步加锁的代码逻辑，从而达到线程安全。
5、Looper中的loop方法，是一个死循环，为什么不会卡界面？
首先概念需要弄清楚，卡界面其实说的就是ANR，而ANR其实是事件没有得到响应，例如点击事件等，5秒内没有响应，就会ANR。service是10秒。
而Looper中的loop，里面是一个死循环，当没有消息需要处理的时候，是在休眠，并没有所谓的没有响应事件，所以肯定是不会产生ANR的，所以也不存在卡界面呢。
6、为什么主线程不需要调用Looper的prepare方法
因为在APP启动的时候，ActivityThread的main方法已经帮我们做了。
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
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