源码基于:API 32
核心类
- Handler
- Looper
- MessageQueue
- Message
- HandlerCallback
1:Handler机制 是什么?
Handler 是Android提供的消息传递机制,主要用于线程间传递消息
场景:子线程处理数据UI线程更新页面
原因:Android系统中子线程不具有更新UI的能力只能通过子线程通知主线程进行更新
2:核心类
2.1 Handler
Handler 是消息的真正处理者,持有Looper对象,Looper持有MessageQueue,具有发送和接收消息的能力.
UI主线程的Looper 是在ActivityThread的main方法中创建和启动的
public static void main(String[] args) {
...
// 创建UI线程的Looper
Looper.prepareMainLooper();
...
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
//启动UI线程的Looper
Looper.loop();
}
UI线程Looper创建和启动源码分析
public static void prepareMainLooper() {
//准备Looper
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
//将主线程的Looper赋值给sMainLooper
sMainLooper = myLooper();
}
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//ThreadLocal 通过 ThreadLocalMap 将主线程和Looper 绑定在一起
sThreadLocal.set(new Looper(quitAllowed));
}
Handler构造方法
public Handler() {
this(null, false);
}
public Handler(@Nullable Callback callback) {
this(callback, false);
}
public Handler(@NonNull Looper looper) {
this(looper, null, false);
}
public Handler(boolean async) {
this(null, async);
}
public Handler(@Nullable Callback callback, boolean async) {...}
public Handler( Looper looper, Callback callback, boolean async) {... }
Looper构造方法
private Looper(boolean quitAllowed) {
//创建了MessageQueueu 对象
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
总结
Handler持有了Looper对象,Looper对象持有了MessageQueue,Message具有Handler的引用(handler.target 是Handler的引用),这使得Handler拥有了发送和处理消息的能力
2.2 Looper 循环的从MessageQueue中取出Message并将消息传递给Handler
Looper 轮询器 通过loop()方法中的for(;;)死循环达到轮询消息的功能
Looper构造方法
private Looper(boolean quitAllowed) {
//quitAllowed 是否允许退出
//UI线程的Looper不允许退出
//子线程的Looper默认支持退出
//在MessageQueue 的quit()方法中会对这个参数做判断 UI线程Looper退出会抛异常
//创建了MessageQueueu 对象
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
核心方法
- Looper() Looper的构造方法,方法中创建了MessageQueue 队列
- prepare()创建Looper且将Looper和Thread做绑定(ThreadLocal)
- prepareMainLooper() 创建主线程Looper绑定主线程(不允许退出Looper)
- getMainLooper() 获取主线程的Looper
- loop() 开启循环消息
- loopOnce() 轮询消息 返回true继续轮询返回false 就中断此次轮询()
说明:
只有调用quit()和quitSafe()方法之后 MessageQueue 的mQuitting变量未true的时候MessageQueue 的next()方法才会返回null才会执行中断循环的操作
UI的Looper为什么不会中断
因为UI线程的Looper创建的时候是不允许Looper的 所以主线程的的Looper不会执行quit操作 - quit() 退出(清理所有消息)
- quitSafe() 安全退出(时间戳大于当前时间的清理,小于当前的还会执行)
方法说明
1:构造方法:创建MessageQueue
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
2:prepare(): 将线程和Looper绑定在一起
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));
}
3:prepareMainLooper():创建UI线程的Looper ,将UI线程和Looper做绑定且给sMainLooper 赋值.
注意:
UI线程的Looper是不可退出的( prepare(false);)
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
4:loop()
public static void loop() {
//获取looper为null 抛出异常
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//开启死循环处理消息
for (;;) {
//如果轮询不到消息就返回false 停止轮询
if (!loopOnce(me, ident, thresholdOverride)) {
return;
}
}
}
5: loopOnce() 轮询消息 返回true继续轮询返回false 就中断此次轮询
private static boolean loopOnce(final Looper me,
final long ident, final int thresholdOverride) {
Message msg = me.mQueue.next(); // might block
//如果获取未获取到消息返回false
if (msg == null) {
// No message indicates that the message queue is quitting.
return false;
}
....
try {
//处理消息 将消息传递给Handler的回调方法
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
}
...
//销毁已处理的消息
msg.recycleUnchecked();
return true;
}
问题:UI线程的Looper能不能中断循环
说明:loop()只有当loopOnce()返回false的时候才会中断Loop
public static void loop() {
//获取looper为null 抛出异常
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//开启死循环处理消息
for (;;) {
//1:关键代码 如果轮询不到消息就返回false 停止轮询
if (!loopOnce(me, ident, thresholdOverride)) {
return;
}
}
}
private static boolean loopOnce(final Looper me,
final long ident, final int thresholdOverride) {
//2:关键代码 判断是否轮询到消息
Message msg = me.mQueue.next(); // might block
if (msg == null) {
//未轮询到消息
return false;
}
'''
try {
//分发Message target 是handler本身
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);
}
}
'''
//清理已经处理的Message
msg.recycleUnchecked();
return true;
}
MessageQueue next quit和dis代码
Message next() {
//5:核心代码 如果mPtr==0 也返回null 表示退出循环
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
for (;;) {
synchronized (this) {
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
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;
msg.markInUse();
return msg;
}
} else {
nextPollTimeoutMillis = -1;
}
//3:关键代码 判断是否退出
if (mQuitting) {
//退出
dispose();
return null;
}
}
}
private void dispose() {
if (mPtr != 0) {
nativeDestroy(mPtr);
//4:关键代码 赋值 中断
mPtr = 0;
}
}
void quit(boolean safe) {
// UI线程传递进来的是false 子线程默认都是True 所以UI线程不允许退出
//子线程允许退出
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
//5:关键代码是否退出的变量值
mQuitting = true;
//是否安全退出
if (safe) {
//安全清理(不清理所有)
removeAllFutureMessagesLocked();
} else {
//清理所有
removeAllMessagesLocked();
}
nativeWake(mPtr);
}
}
总结
前提: new Looper(quitAllowed)
UI线程创建的时候配置了不允许退出循环所以不能退出循环.
子线程配置了允许退出Looper
Looper 退出流程:
1:Looper 调用quit()和quitSafe()
2:调用到了Messagequeue的quit()和quitSafe()方法,此方法中判断了是否允许退出.UI线程不允许退出 子线程允许退出.
3:允许退出后将mQuitting=true 然后移除Message数据
4:当MessageQueue 的next方法执行的时候判断了mQuitting 如果退出就返回null且mPtr=0
5:当Looper 的loop方法中loopOnce方法获取消息未null就中断了循环
2.3 MessageQueue 以队列形式(先进先出,按时间排序,msg.when)存储消息
消息队列主要用于存储和取出消息,主要用来维护消息
核心方法
- enqueueMessage() 插入消息
- next() next()搭配Looper loop()轮询取出消息
- quit() 退出
- postSyncBarrier() 发送屏障消息
- removeSyncBarrier() 移除屏障消息
- dispose() 如果是退出状态 改变变量
- removeMessages() 移除消息
2.3.1 MessageQueue.enqueueMessage()
插入消息
核心:
- msg.target == null target是handler对象，检查消息是否有线程处理
- msg.isInUse()判断消息是否被使用
- if (mQuitting) {} 判断 looper是否退出
- for(;;)按时间排序
解释说明:
mMessages 表示当前消耗的消息
prev 头消息
next尾消息
链表排序核心是 找到自己前边的消息将前边的消息的next设置成自己,将前边消息的next设置成自己的next
排序流程
for(;;)死循环 ,将mMessages(p) 当前处理的头消息赋值给头变量 将mMessages(p) 变量的next赋值给自身 ,然后这时判断 p.next==null 表示最后一个消息,when<p.when 表示当前需要插入的这个消息在这个消息的前边. 所以这时这个p就表示 插入消息msg的next prev的next是当前消息
boolean enqueueMessage(Message msg, long when) {
// target是handler对象，检查消息是否有线程处理
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
synchronized (this) {
//判断消息是否已经被使用
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
//判断loop是否已退出
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;
}
// 标记消息已经被使用
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
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;
//消息排序(按时间when排序)
for (;;) {
// 当前消息的前一个消息
prev = p;
//核心 1:循环的把p的next赋值给p
p = p.next;
//核心2: 判断p 为null 意味着最后一个消息 when<p.when 表示 当前的时间小于这个消息的时间 这就是现在消息的位置 中断 死循环
if (p == null || when < p.when) {
//获取当前消息的后一个消息
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
//核心3: 给当前消息的后一个消息赋值
msg.next = p; // invariant: p == prev.next
//当前消息前一个消息赋值
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
2.3.2 MessageQueue.next() 获取下一个消息
Message next() {
//mQuitting 退出状态的时候 mPtr=0 所以退出状态直接返回Null loop直接退出循环
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;
// msg.target == null 表示是屏障消息需要提前处理(handler.发送消息的时候在Handler的enqueueMessage中会 msg.target=this 所以target不为空且 )
//返回屏障消息
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 {
//返回一般消息
// 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;
}
// 退出状态的时候直接修改 mPtr=0 直接返回Null loop退出循环
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;
}
}
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