简述
Handler大家不陌生，但是其中的细节还是有不少的，其中有些知识点还是很重要的，我们一起搞一下。
Handler到底是个什么东西
handler是Android消息传递模块中必不缺少的一样，在App启动时，也不乏看到他的身影。比如在ActivityThread中的main方法里就会有主线程Handler的初始化。和主线程looper的初始化。
class ActivityThread {
public static void main(String[] args) {
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
}
Handler 使用的简单流程
当我们使用Handler在不考虑内存泄漏的情况下，一个简单的例子是这样的。
class SecondActivity : AppCompatActivity() {
companion object {
var TAG = SecondActivity::class.java.name
}
var handler = Handler(object:Handler.Callback{
override fun handleMessage(msg: Message): Boolean {
Log.d(TAG,"handle")
return false;
}
})
fun sendMessage () {
var message = Message.obtain()
handler.sendMessage(message)
}
}
一般情况下，handler是处理的是不同线程之间的通信。
Handler Looper MessageQueue 与线程的比例关系
我们知道Handler是我们自己new出来的，所以一个线程里可以有无数个Handler，但是子线程使用Handler的时候要调用一下Looper.prepare()方法，这个东西没有的话，handler是不能正常工作的。
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
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));
}
/**
Handler初始化时候会找Looper，looper没有初始化会报错。
**/
public Handler(@Nullable 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;
}
由此可见Looper与线程的关系是1：1的。而MessageQueue是在Looper里取得，所以Handler Looper MessageQueue 在一个线程中的比例是n:1:1。
Handler 内部消息通信流程
当消息从handler发出的时候有两个方法sendMessage 和post等一系列类似方法，最后都会到
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
boolean enqueueMessage(Message msg, long when) {
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;
// 几种情况下直接插入头
if (p == null || when == 0 || when < p.when) {
// 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;
for (;;) {
prev = p;
p = p.next;
// 寻找适应p.when>when的 message,之后插入
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;
}
仔细看代码就会发现这个方法将message按时间排了个序，也就是说message现在是一个按时间排序的优先级序列 。这块暂时结束了，接着看looper做了什么。
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// Make sure the observer won't change while processing a transaction.
final Observer observer = sObserver;
try {
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
} finally {
ThreadLocalWorkSource.restore(origWorkSource);
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
msg.recycleUnchecked();
}
}
looper就干了一件事情，开启一个死循环，将msg从queue中读出来，然后msg.target.dispatchMessage（msg）这样我们的handler就收到个回调。
关于队列中没有消息的情况下
试想一下，消息不可能一直有的，肯定有时候队列中没有消息，那么这个时候如果还一直循环调用的话，十分的消耗资源，而且完全没用。Android肯定不会这样做的，他在looper中有这么一句注解，在取得queue.next的时候旁边注释已经有提示了might block。
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();
}
//根据nextPollTimeoutMillis 的值判断是否要挂起线程
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 {
// 这里没有message时候，下次进循环就挂起
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
}
}
具体实现就是在nativePollOnce 这个native方法里，这里不展开说了，只需要知道-1的时候这个方法将线程挂起即可，之后有消息之后程序接着运行，和object.wait/ notify类似。
