微信截图_20210303172720.png
在子线程中获取完数据后需要更新ui的时候，往往会创建一个Handler实例，然后调用它的sendEmptyMessage等方法，接着在回调方法handleMessage中进行ui的刷新；
handler.sendEmptyMessage(1000);
private final Handler handler = new Handler(Looper.getMainLooper()){
@Override
public void handleMessage(@NonNull Message msg) {
super.handleMessage(msg);
//进行ui刷新动作
}
};
在Handler源码中可以看到这样的描述：
A Handler allows you to send and process {@link Message} and Runnable
objects associated with a thread's {@link MessageQueue}. Each Handler
instance is associated with a single thread and that thread's message
queue. When you create a new Handler it is bound to a {@link Looper}.
一个消息处理程序允许发送消息实体和消息队列，任何一个消息处理程序都与一个线程和该线程的消息队列关联，创建新的处理程序是，会绑定到消息轮询器上面。这样Handler就和Message、MessageQueue、Looper之间相互关联了。
Handler：消息处理
Message：消息实体
MessageQueue：消息队列
Looper：消息轮询
同时得到一个信息：
Looper和当前线程是唯一的关系，一个线程对应一个Looper，并且Looper所在线程必须和当前线程保持一致。
在上面更新ui的代码中只是通过Looper.getMainLooper()获取了一个Looper实例，并没有任何创建Looper对象，程序并有出现任何错误，这是为什么呢？不是说任何一个消息处理对象都会和一个线程、该线程的消息队列、该线程的Looper相关吗？是的，应用程序在启动的时候会创建一个ActivityThread实例，并调用程序入口main方法，
public static void main(String[] args) {
.......
Looper.prepareMainLooper();
.......
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");
}
在main方法中调用了Looper.prepareMainLooper()，进行了主线程中Looper的创建，
@Deprecated
public static void prepareMainLooper() {
//调用prepare方法进行looper的创建
prepare(false);
synchronized (Looper.class) {
//确保线程和Looper的唯一性 如果当前线程中已经存在有looper了 再调用prepare方法去创建就会报错
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
//对looper进行赋值
sMainLooper = myLooper();
}
}
看的出prepareMainLooper是在住线程中创建对应的looper，其最终looper的创建还是交给了prepare方法去创建，如果要在子线程中创建对应的looper就需要调用prepare方法，
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
//一个looper只能在一个线程中被创建 重复创建会出现错误
throw new RuntimeException("Only one Looper may be created per thread");
}
//其实就是直接通过new的方式创建一个looper实例，并添加到一个静态的ThreadLocal中进行缓存
sThreadLocal.set(new Looper(quitAllowed));
}
sTheadLocal是一个静态的ThreadLocal，将创建好的looper缓存在ThreadLocal中，需要的时候通过myLooper方法去获取，保证looper实例的唯一性，
public static @Nullable Looper myLooper() {
//获取之前创建并缓存的looper实例
return sThreadLocal.get();
}
looper实例已经创建好了，但是对应的MessageQueue还没有创建好呀，MessageQueue的实例是在looper的构造方法中进行实例好的，
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
调用prepare方法，就将looper实例和MessageQueue实例就都创建好了，创建好后就可以调用发送消息实体的方法进行消息的发送了，发送消息的方法有不少呢，
public final boolean sendMessage(@NonNull Message msg);
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis);
public final boolean sendEmptyMessage(int what);
public final boolean sendEmptyMessageDelayed(int what, long delayMillis);
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis);
1和2是直接发送一个Message消息实体，区别是是否延时发送，后面这几种的话发送的是一个int类型的标识，区别也是是否延迟发送，虽然发送的是一个int类型的标识，但是最终发送的时候会调用Message.obtain()获取一个Message实例，并将标识封装进去，最终发送的还是一个Message消息实体，
public static Message obtain() {
synchronized (sPoolSync) {
//sPool 就是一个静态的Message实例 如果为null 下面就直接通过new 创建一个Message
if (sPool != null) {
Message m = sPool;
//这里是单向链表的结构
sPool = m.next;
//获取到后 会将其设置为null
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
//将获取到的message实例返回
return m;
}
}
return new Message();
}
上面那些方法最终都会调用sendMessageAtTime方法进行消息的发送，
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
//mQueue 的赋值在创建Handler时 通过mLooper.mQueue进行赋值了，如果没有创建MessageQueue实例 肯定会报错 并且中断消息的发送
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);
}
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//将消息的发送、处理等放到了MessageQueue中
return queue.enqueueMessage(msg, uptimeMillis);
}
boolean enqueueMessage(Message msg, long when) {
//target就是当前对应的Handler实例 在Handler enqueueMessage方法中 msg.target = this;进行赋值了
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
synchronized (this) {
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
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) {
//满足mMessages为null MessageQueue中没有消息 第一次添加时
//或者不延时发送时 不延时发送放在队头 便于直接发送
//或者当前消息发送的时间小于 MessageQueue中已存消息 队头消息的发送时间
// New head, wake up the event queue if blocked.
//新的队列头
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 (;;) {
//双向链表的结构
prev = p;
p = p.next;
//会一直遍历当前的消息队列
//如果到了链表的尾端或者当前消息发送的时间小于 下一条消息发送的时间就会终止循环
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
//将当前的消息添加到链表中合适的位置
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;
}
消息添加完毕后，肯定就是取消息了，是通过loop方法进行取消息的，但是之前更新ui的代码中并有调用loop方法，一样的，在ActivityThread实例的main方法中就已经调用了loop方法了，
public static void loop() {
//首先会去获取looper实例
final Looper me = myLooper();
if (me == null) {
//如果获取到的looper实例为null，会告诉你必须要先调用prepare方法 在当前线程中创建looper实例
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
if (me.mInLoop) {
Slog.w(TAG, "Loop again would have the queued messages be executed"
+ " before this one completed.");
}
me.mInLoop = true;
//获取对应的MessageQueue实例
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;
//进行遍历循环获取消息实体
for (;;) {
//获取消息队列中的消息实体
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
......
try {
//调用Handler中的方法进行消息的分发
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();
}
}
queue.next()就是通过链表中next的节点的移动去获取message消息实体
@UnsupportedAppUsage
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();
//获取之前链表中的消息实体
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;
}
//获取一条后 链表中就减少一条
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);
}
......
}
}
将遍历获取到的消息再通过dispatchMessage去进行分发
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
到这里就看到前面更新ui时创建了一个Handler实例，并重写了其handlerMessage方法，这样在handlerMessage中进行消息的处理，ui的更新，在消息处理完后，还会对消息进行缓存处理，
@UnsupportedAppUsage
void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = UID_NONE;
workSourceUid = UID_NONE;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
//MAX_POOL_SIZE 是50 就是说最大的缓存数量是50 超过50后就不会进行缓存处理
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
这样一个Handler消息处理流程就走完了。
