持续创作，加速成长！这是我参与「掘金日新计划 · 10 月更文挑战」的第4天，点击查看活动详情
一、前言
如果你使用handler创建一个消息使用new Message()进行创建的,那么不妨看下这篇文章吧~
二、Message
Android官方更推荐我们使用这两种方式创建message
Message msg1 = Message.obtain();
Message msg2 = handler.obtainMessage();
2.1 Message.obtain()
先来看下Message.obtain()方法吧
public static Message obtain() {
//加锁
synchronized (sPoolSync) {
//消息回收池不为Null
if (sPool != null) {
//获取链表头节点
Message m = sPool;
//获取第一个Message
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
//消息回收池长度 - 1
sPoolSize--;
//返回此消息
return m;
}
}
//如果是空链表,就新建Message
return new Message();
}
从上述代码可以看出,obtain()方法会先判断回收池是否为Null,如果为Null,就会new Message().
如果不为Null,就从回收池获取一个旧的Message,然后将msg的flags置为0,返回该消息.
sPool是这个复用的核心.
2.2 handler.obtainMessage()
public final Message obtainMessage()
{
return Message.obtain(this);
}
handler这里最终还是调用Message内部的obtain(handler)
public static Message obtain(Handler h) {
Message m = obtain();
m.target = h;
return m;
}
2.3 message.recycle()
我们来找一下sPool的复用逻辑吧
public void recycle() {
//是否正在被使用
if (isInUse()) {
//是否能够回收消息
if (gCheckRecycle) {
//抛出异常,消息正在被消费,不能够被回收
throw new IllegalStateException("This message cannot be recycled because it "
+ "is still in use.");
}
return;
}
//消息已经被消费过了,可以被回收了
recycleUnchecked();
}
void recycleUnchecked() {
//将Message回复到初始状态,
//flags置为FLAG_IN_USE,不能重复入队
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;
//将消息添加到回收池,可见这个回收池也是一个链表
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
至此咱们Handler系列的源码解读即将走向终结,正好把几个核心的源码再读一遍吧~
2.4 Message 成员变量
//用于标记传递执行什么操作,通过用于用户逻辑判断
public int what;
//如果传递数据是整数类型就可以使用arg1和arg2变量
public int arg1;
public int arg2;
//在进程间通信时,传递数据时需要实现Android方式的序列化
public Object obj;
//在进程间通信时,可以通过replyTo给服务端传递一个客户端的Messager的对象,
//此时服务端就可以使用Messager发送消息给客户端
public Messenger replyTo;
public static final int UID_NONE = -1;
public int sendingUid = UID_NONE;
public int workSourceUid = UID_NONE;
/*package*/ static final int FLAG_IN_USE = 1 << 0;
/** If set message is asynchronous */
/*package*/ static final int FLAG_ASYNCHRONOUS = 1 << 1;
/** Flags to clear in the copyFrom method */
/*package*/ static final int FLAGS_TO_CLEAR_ON_COPY_FROM = FLAG_IN_USE;
//标记位,标记Message是否正在被使用
/*package*/ int flags;
//消息什么时候被执行
public long when;
//传递数据,封装成Bundle对象
/*package*/ Bundle data;
//新建此消息的handler对象
/*package*/ Handler target;
//handler.postMessage的runnable对象
/*package*/ Runnable callback;
// sometimes we store linked lists of these things
@UnsupportedAppUsage
/*package*/ Message next;
public static final Object sPoolSync = new Object();
//回收的消息池
private static Message sPool;
//回收消息池此时的长度
private static int sPoolSize = 0;
//回收消息池的最大长度
private static final int MAX_POOL_SIZE = 50;
//消息是否能够被回收
private static boolean gCheckRecycle = true;
-
从上面可以看出,在平常创建Message时,我们一般使用的when,target,obj这些变量.
-
flags这个变量作为标志位,标记此时Message的状态,FLAG_IN_USE表示此时正在被使用
-
sPool是一个单向链表,内部存储着回收消息
2.5 总结
Message就是消息传递的介质,内部的重点就是flags标记状态,sPool是消息回收池,target绑定了handler对象.
三、MessageQueue
MessageQueue是存储消息的一个队列
3.1 构造方法
MessageQueue(boolean quitAllowed) {
//队列是否能够退出
mQuitAllowed = quitAllowed;
//native的MessageQueue引用
mPtr = nativeInit();
}
3.2 初始化
可以看到MessageQueue的构造方法并没有被开放出来
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
在Looper的构造方法中被实例化,所以Looper是和MessageQueue绑定的,只要创建一个Looper就会创建一个MessageQueue
3.3 MessageQueue成员变量
private static final String TAG = "MessageQueue";
private static final boolean DEBUG = false;
// 是否能够退出
@UnsupportedAppUsage
private final boolean mQuitAllowed;
@UnsupportedAppUsage
@SuppressWarnings("unused")
private long mPtr; // used by native code
@UnsupportedAppUsage
Message mMessages;//链表的头节点
@UnsupportedAppUsage
private final ArrayList<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();
private SparseArray<FileDescriptorRecord> mFileDescriptorRecords;
private IdleHandler[] mPendingIdleHandlers;
private boolean mQuitting;
// 是否阻塞
private boolean mBlocked;
// The next barrier token.
// Barriers are indicated by messages with a null target whose arg1 field carries the token.
@UnsupportedAppUsage
private int mNextBarrierToken;
private native static long nativeInit();
private native static void nativeDestroy(long ptr);
@UnsupportedAppUsage
private native void nativePollOnce(long ptr, int timeoutMillis); /*non-static for callbacks*/
private native static void nativeWake(long ptr);
private native static boolean nativeIsPolling(long ptr);
private native static void nativeSetFileDescriptorEvents(long ptr, int fd, int events);
3.4 enqueueMessage消息入队
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
//判断消息是否持有发送消息的handler对象
throw new IllegalArgumentException("Message must have a target.");
}
//加锁,单任务
synchronized (this) {
if (msg.isInUse()) {
//判断消息是否正在被使用--之前提到的标志位
throw new IllegalStateException(msg + " This message is already in use.");
}
//消息队列是否退出了
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
//消息回收
msg.recycle();
return false;
}
//以上条件都不成立,说明消息可以入队
//将消息置为正在被使用状态
msg.markInUse();
//消息的延迟执行时间
msg.when = when;
//链表头节点
Message p = mMessages;
//是否需要被唤醒
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// 如果阻塞,唤醒事件队列
//消息直接入队
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
//插入队列中间,通常不必唤醒队列
//因为之前已经有消息在处理,已经处于消费状态
needWake = mBlocked && p.target == null && msg.isAsynchronous();
//链表的插入操作
Message prev;
//死循环
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);
}
}
//入队成功
return true;
}
可以看到,消息入队前会判断
- 1、消息是否持有发送消息的handler
- 2、消息是否正在被使用
- 3、消息队列是否退出了,如果退出了,那么回收消息
3.5 next消息出队
@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.
//如果消息循环已经退出并被释放,则返回这里
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
//死循环
for (;;) {
if (nextPollTimeoutMillis != 0) {
//如果有消息过段时间再处理,就执行如下方法
//Binder管道连接,等待到手消费消息
Binder.flushPendingCommands();
}
//Epoll机制的阻塞式等待
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
//加锁
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
//遍历消息链表,从链表中取出一个msg
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
//判断消息是否为null并且是否拥有发送消息的handler
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
//消息不为Null
if (now < msg.when) {
//将来执行,现在暂时不执行消息
// Next message is not ready. Set a timeout to wake up when it is ready.
//下一个消息尚未准备好,设置一个超时,以便
//在准备就绪时唤醒,
//设置一个超时时间,到这个时间点自动唤醒执行消息
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
//消息可以立即处理
//链表的删除操作
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.
//没有消息可以执行了
nextPollTimeoutMillis = -1;
}
//现在处理退出消息,处理所有挂起的消息
//消息队列是否需要退出,处理所有的消息,或消费或回收
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
//调用的底层C代码,处理还没处理的消息
//准备退出队列了,此方法是安全的退出方式,
//默认也是安全的退出
dispose();
//返回null,looper退出的判断条件
return null;
}
//接下来的就是处理将要执行的消息了
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;
}
}
可以看出消息出队的基本步骤
- 1、消息队列是否已经退出
- 2、如果没有退出,进入死循环,遍历链表,从链表中取出一个消息,如果消息是Null,就表明没有消息可以处理了
- 3、如果Msg不为Null,判断消息是否可以立即执行,不过不可以,计算将要执行的时间点
- 4、如果消息可以立即执行,接下来进行链表的删除操作
可以看到消息的入队和出队操作,其实是在MessageQueue内部维护了一个消息链表,本质上是链表的插入和删除操作.
3.6 quit退出
void quit(boolean safe) {
if (!mQuitAllowed) {
//队列是否允许退出,在构造队列时传入
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
//队列已经退出
return;
}
mQuitting = true;
if (safe) {
//是否安全地退出
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
3.7 removeAllFutureMessagesLocked()
private void removeAllFutureMessagesLocked() {
final long now = SystemClock.uptimeMillis();
Message p = mMessages;
if (p != null) {
if (p.when > now) {
//如果链表头节点是延迟消息,那么直接回收掉所有的消息
removeAllMessagesLocked();
} else {
Message n;
for (;;) {
n = p.next;
if (n == null) {
//如果消息链表是Null,返回
return;
}
if (n.when > now) {
//找出链表中的第一个延迟消息
break;
}
p = n;
}
p.next = null;
do {
//循环删除第一个延迟消息的所有
p = n;
n = p.next;
p.recycleUnchecked();
} while (n != null);
}
}
}
3.8 removeAllMessagesLocked
private void removeAllMessagesLocked() {
Message p = mMessages;
//删除所有的消息
while (p != null) {
Message n = p.next;
p.recycleUnchecked();
p = n;
}
mMessages = null;
}
可以看到
- 不安全的退出:删除所有的消息
- 安全地退出:删除第一个延迟消息之后的所有消息(包括第一个延迟消息)
四、Looper
4.1 Looper成员变量
//存储不同线程的数据,使用ThreadLocal实现数据隔离
@UnsupportedAppUsage
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
//UI主线程关联的mainLooper
@UnsupportedAppUsage
private static Looper sMainLooper; // guarded by Looper.class
private static Observer sObserver;
//关联的消息队列
@UnsupportedAppUsage
final MessageQueue mQueue;
//该Looper对应的线程
final Thread mThread;
private boolean mInLoop;
@UnsupportedAppUsage
private Printer mLogging;
从成员变量中可以看出
-
Looper已经关联了当前线程的MessageQueue,并且创建了当前线程对象,同时自带主线程的Looper
-
使用了ThreadLocal来实现线程间的数据隔离
4.2 Looper的创建
public static void prepare() {
//是否能够出队列
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
//当前线程下是否已经存储过Looper对象
//如果已经存在,抛出异常
throw new RuntimeException("Only one Looper may be created per thread");
}
//新建Looper对象,并且将该线程的Looper对象存储到ThreadLocal中
sThreadLocal.set(new Looper(quitAllowed));
}
@Deprecated
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
private Looper(boolean quitAllowed) {
//新建MessageQueue
mQueue = new MessageQueue(quitAllowed);
//持有当前线程的线程对象
mThread = Thread.currentThread();
}
创建Looper主要有这几个步骤
- 1、创建当前线程的MessageQueue
- 2、将Looper对象添加到ThreadLocal中
- 3、持有当前Looper的线程对象
另外值得注意的是,一个线程只允许存在一个Looper,否则会抛异常
4.3 Looper.loop()
public static void loop() {
//返回当前线程关联的looper对象
final Looper me = myLooper();
if (me == null) {
//表明当前线程不存在looper,没有实例化looper
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
...
//死循环
for (;;) {
if (!loopOnce(me, ident, thresholdOverride)) {
return;
}
}
}
private static boolean loopOnce(final Looper me,
final long ident, final int thresholdOverride) {
//获取当前线程中Looper对象对应的MessageQueue对象
//从MessageQueue中取出消息,有就取出消息,没有消息,就阻塞
Message msg = me.mQueue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
//消息是否为Null,消息队列退出之后会返回null
return false;
}
// This must be in a local variable, in case a UI event sets the logger
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " "
+ msg.callback + ": " + msg.what);
}
// Make sure the observer won't change while processing a transaction.
final Observer observer = sObserver;
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
Object token = null;
if (observer != null) {
token = observer.messageDispatchStarting();
}
long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
try {
//回调给发送这个消息的handler处理
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);
}
}
if (logSlowDelivery) {
if (me.mSlowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
me.mSlowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
me.mSlowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
}
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}
//消息处理完毕,消息回收
msg.recycleUnchecked();
return true;
}
从上述代码可以看出,loop方法内部其实是个死循环,通过轮询MessageQueue的next()方法获取消息,有消息就处理,没有消息时就阻塞等待.
4.4 跳出循环
if (msg == null) {
// No message indicates that the message queue is quitting.
return false;
}
当msg为null时这个循环才可以跳出
什么时候在Looper轮询时MessageQueue的next()方法返回null呢
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
mQuitting为true时会返回null
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
messageQueue调用quit时会结束looper的轮询
4.5 Looper的退出
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
从上面可以看出,MessageQueue的quit(safe)方法是在looper中调用的.
- 不安全退出:直接将所有的消息回收掉
- 安全退出:退出之前,会先将所有可以立即处理的消息发送给相应的handler处理,然后再回收掉,但是延迟消息是会被直接回收
五、Handler
5.1 成员变量
//当前线程的looper
@UnsupportedAppUsage
final Looper mLooper;
//消息队列
final MessageQueue mQueue;
@UnsupportedAppUsage
//回调,构造函数的参数
final Callback mCallback;
//消息是否是异步的
final boolean mAsynchronous;
@UnsupportedAppUsage
//跨进程间通信时使用
IMessenger mMessenger;
可以看到,handler持有Looper对象和MessageQueue
5.2 handler的创建
@Deprecated
public Handler() {
this(null, false);
}
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());
}
}
//获取一个looper对象
mLooper = Looper.myLooper();
if (mLooper == null) {
//判断当前线程的looper对象是否为Null
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
//获取MessageQueue
mQueue = mLooper.mQueue;
//callback回调
mCallback = callback;
//是否异步
mAsynchronous = async;
}
@UnsupportedAppUsage
public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
可以看到,handler有两种创建方式
- 1、直接创建handler,new Hander()
- 2、创建handler,并实现callback接口
在handler的构造函数中我们也可以看到mQueue和mLooper都是直接获取的,所以必须先创建Looper,再创建handler
5.3 handler发送消息
public final boolean post(@NonNull Runnable r) {
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
public final boolean sendMessage(@NonNull Message msg) {
return sendMessageDelayed(msg, 0);
}
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);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
其实最终都是sendMessageDelayed的调用. 因为需要计算这个消息需要延迟执行的时间
- post():参数实现callback的接口类,在Runnable的run()方法中处理消息
- sendMessage():复写handleMessage()方法
5.4 接收消息
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
//实现的Runnable接口是否为Null,如果不是null,就执行
//handleCallback方法
handleCallback(msg);
} else {
if (mCallback != null) {
//判断mCallback是否为Null
if (mCallback.handleMessage(msg)) {
return;
}
}
//以上判断都不成立,handleMessage方法
handleMessage(msg);
}
}
- 首先是否实现了Runnable接口,如果已实现,回调给run()方法,就此结束,不再走步骤2,否则步骤2
- 是否实现了Callback接口,如果实现了,回调给callBack内的handleMessage方法,此时handleMessage返回true,就结束,不再走步骤3,否则步骤3
- new Handler(handleMessage(msg))内的方法执行
六、总结
至此,我们再来总结一下handler的使用方法和流程
- 1、实例化handler之前必须确保Looper.prepare()已经被调用
- 2、确保looper和messageQueue已经创建
- 3、handler通过sendXXX或者postXXX发送消息
- 4、发送消息的本质就是调用MessageQueue的enqueuMessage方法(消息入队列)
- 5、Looper的loop,进行消息的轮询,其实是阻塞式地拿到MessageQueue.next()消息
- 6、Looper取到消息后根据msg.target将消息分发给关联的handler
- 7、handler内部处理消息通过dispatchMessage(Msg),回调给开发者
