Handler机制从入门到放弃(二)
从注释看起
Hander的源码只有不到800行，而且大多数代码相对来说还是比较好理解的，尤其是相对于其他更加接近底层的代码来说，在看源码时候有一点挺重要的就是不要忽略注释的作用，Handler类开头有这么几行注释：
There are two main uses for a Handler: (1) to schedule messages and
runnables to be executed as some point in the future; and (2) to enqueue
an action to be performed on a different thread than your own.
归纳一下就是：
- 安排消息和任务在将来的某一个点执行
- 使一个动作进入队列为了能够在另一个线程中执行
回顾一下我们为什么要用Handler：
在Android中，当要更新UI的时候，我们必须要在主线程中进行更新，原因时当主线程被阻塞了5s以上就会出现ANR异常，会导致程序崩溃。所以一些耗时的操作必须要放在子线程中，但是在子线程中又不能做更新UI的操作，所以为了解决这个问题，Android设计了handler机制。
这么一对比，很容易的印证了这段话：使一个动作进入队列在另一个线程中执行：这不就是异步执行耗时任务么；安排消息和任务在将来的某一个点执行：联想一下postDelayed之类的延时操作的方法，或者给出一个很常见的例子，比如说引导页延时启动：
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
Log.d("ThreadName 1",Thread.currentThread().getName());
//第一次登陆扫描本地音乐
if (SPUtils.getValue(SplashActivity.this, "isFirst", "First", true)) {
new Thread(new Runnable() {
@Override
public void run() {
//耗时操作
//清空表
Log.d("ThreadName 2",Thread.currentThread().getName());
DataSupport.deleteAll(MusicInfoDetail.class);
MusicUtils.scanMusic(SplashActivity.this, musicInfo);
DataSupport.saveAll(musicInfo);
SPUtils.putValue(SplashActivity.this, "isFirst", "First", false);
}
}).start();
}, 2000); 这里是我自己的Demo里面的一部分代码，这里使用postDelayed延时2s启动，然后在子线程执行更新数据库的操作，很好的印证了上面两点。
创建Handler
在上一篇文章Handler机制从入门到放弃(一)里面我们已经演示了两种创建Handler的方法并且给出了部分实际操作的代码，但是都是在主线程也就是UI线程创建的，我们可以尝试一下在子线程中创建Handler：
public class MainActivity extends AppCompatActivity {
private Handler mainHandler;
private Handler childHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mainHandler=new Handler();
new Thread(new Runnable() {
@Override
public void run() {
childHandler=new Handler();
}
}).start();
}
}运行一下，果不其然代码蹦了：
报错信息：
java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()告诉我们说在Thread里面创建Handler需要调用Looper.prepare( )，那把这一句加上试试：
public class MainActivity extends AppCompatActivity {
private Handler mainHandler;
private Handler childHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mainHandler=new Handler();
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
childHandler=new Handler();
}
}).start();
}
}果然很成功的运行了，但是这是为什么，来看一下Handler的源码：
这里提供一个简便的方法，为了快速找到原因可以在打开的源码(我这里使用sublimeText查看)里使用ctrl+f快捷键搜索Looper.prepare( )出现的地方：
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class 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 that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}这是Handler的其中一个构造方法，看到这么一段：
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}在构造方法里通过Looper.myLooper()获取到一个Looper对象mLooper，如果为空则报错，找到Looper.myLooper()方法：
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}注释给出的解释是这个方法回返回跟当前线程相关联的Looper对象，如果没有则返回空，还是没找到答案，接着找Looper类里面对sThreadLocal的定义：
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal sThreadLocal = new ThreadLocal(); 又是注释里面告诉了我们重要信息，这里告诉我们只有你调用了Looper.prepare()方法sThreadLocal.get()才不会返回空，那么说来说去还是要看Looper.prepare()的代码：
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
prepare(true);
}
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));
}看下面那个，Looper.prepare()调用了prepare()的重载方法prepare(boolean quitAllowed)并且传入了true参数，这个方法判断sThreadLocal.get()是否会返回一个Looper对象，如果没有的话就set一个新的Looper进去，如果已经有了再调用prepare()方法的话就会报错，不信邪的可以在mainHandler创建之前也调用一个Looper.prepare()，控制台就会出现这个错误：
那么问题来了，为什么我们在主线程创建Handler不需要调用Looper.prepare()，而在子线程中需要呢，可以合理的猜想是不是系统给我们主动调用了，毕竟我们大部分的操作还是在主线程上，每次都要那么Looper.prepare()来一次多麻烦，有了猜想还要去源码寻求验证，主线程是ActivityThread，从ActivityThread类里搜索相关信息，用跟上面一样的方法：
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
SamplingProfilerIntegration.start();
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("" );
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
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");
}抓重点：
Looper.prepareMainLooper();
找到Looper类中关于这个方法的定义：
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}这里又会主动调用prepare(boolean quitAllowed)方法，通过注释也了解到我们不需要主动去创建UI线程的looper，系统自动会给我们创建好了，这里印证了前面的猜想。
这里得出一个结论：
在主线程中可以直接创建Handler对象，而在子线程中需要先调用Looper.prepare()才能创建Handler对象。
这里先不管Looper是什么，暂时知道有这个东西，下面可以看一下如何发送消息。
如何发送消息
这里就用到了第二种创建Handler的方法：
Handler myHandler = new Handler() {
public void handleMessage(Message msg) {
switch (msg.what) {
//根据参数进行操作
break;
}
super.handleMessage(msg);
}
};
//其他地方调用
myHandler.sendMessage(xxx);这里的其他地方调用指的就是在子线程里面，当我们在子线程里面执行完耗时操作之后如果需要传递一些数据给主线程，比如通知主线程更新UI之类的，就可以这么做：
final Handler myHandler = new Handler() {
public void handleMessage(Message msg) {
switch (msg.what) {
//根据参数进行操作
}
}
};
new Thread(new Runnable() {
@Override
public void run() {
Message msg=new Message();
//what是用户自定义的识别码
msg.what=1;
//通过arg1和arg2可以给Message传递简单的int型数据
msg.arg1=123;
msg.arg2=456;
//通过给obj赋值Object类型传递向Message传入任意数据
msg.obj=null;
myHandler.sendMessage(msg);
}
}).start();当然除了传递这些简单数据之外Message类还能以setData方式携带Bundle数据：
Bundle bundle = new Bundle();
bundle.putString("data", "data");
message.setData(bundle); 我们看到这里是在子线程中调用了sendMessage(msg)方法，然而我们却在主线程中使用handleMessage(Message msg)接受消息，这之间一定发生了一些不可描述的事情，让我们来找找看，当然除了sendMessage(msg)方法Message类还有许多其他发送消息的方法：
public final boolean sendMessage(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);
}
.....巧的是，这些方法无论转折多少次都走向了同一个方法：
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) uptimeMillis.
* The time-base is {@link android.os.SystemClock#uptimeMillis}.
* Time spent in deep sleep will add an additional delay to execution.
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public boolean sendMessageAtTime(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);
}这个方法接受两个参数，msg和uptimeMillis，msg就是我们需要传递的消息，uptimeMillis则是发送消息时候的绝对时刻，它的值等于自系统开机到当前时间的毫秒数再加上延迟时间，这个延迟时间就是我们调用sendxxxDelayed里面传入的时间参数，这个方法会把一个消息放入消息队列(message queue)，然后把这个方法的两个参数加上新建的MessageQueue 对象传入enqueueMessage(queue, msg, uptimeMillis)方法里，从字面上理解MessageQueue 是一个消息队列，那么队列就会有入队和出队的方法，这个enqueueMessage(queue, msg, uptimeMillis)应该就是入队的方法：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}没有发现什么，这里又调用了enqueueMessage(msg, uptimeMillis)方法，这个方法在MessageQueue类里面：
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;
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;
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;
}代码有点长，一步一步看，先看前面一部分：
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.");
}这里判断了一下msg.target对象是否为空，还记得之前的enqueueMessage(queue, msg, uptimeMillis)方法吗：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}这个方法是在Handler中执行的，这里把一个this对象赋值给msg.target，那么从Message类找一下这个target到底是什么，找到这个：
/*package*/ Handler target;这样脉络就很清晰了，这里是把Handler跟Message对象绑定起来，接着往下看：
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;
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);
}
}这个msg.when就是用传入的uptimeMillis参数赋值，表示入队时间，看到这个if判断：
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 {
...
}有人可能会好奇这个when怎么会为0呢，这里提一嘴，Handler除了有正常的sendMessage之流的方法还有一个比较特殊的方法：
public final boolean sendMessageAtFrontOfQueue(Message msg) {
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, 0);
}这个方法特殊在什么地方呢，打个比方，如果说我们正常的sendMessage之流的方法是一群正常排队的人，按照来的时间先后有序排队，但是sendMessageAtFrontOfQueue就是那种个别不老实的，它能直接插队到最前面，然后他传递的uptimeMillis为0，这也是唯一一个特殊的发送消息的方法。
这个判断语句成立的条件有三点：p == null || when == 0 || when < p.when
- p == null说明当前looper处于空闲状态，也就是没有什么消息需要处理
- when == 0说明有消息插队插到了MessageQueue最前面
- when < p.when指的是新入队的消息队列需要排队的时间比正在执行的消息排队的时间短
综合来说就是，如果这时候新进来一个消息，这时候消息队列里面没有需要执行的消息，或者新进来的这个消息是通过sendMessageAtFrontOfQueue(Message msg)方法传进来的，或者说新进来的这个消息需要等待的时间比之前在等待的消息等待的时间短，那么这个消息队列就要开始有一个新的消息头，此时系统会唤醒这个消息队列无论队列是否堵塞。
// Got a message.
mBlocked = false;这一行代码说明只要消息队列有消息，这个队列就不阻塞，然后把这个布尔值传递给
boolean needWake
needWake = mBlocked;那么这一块代码就打通了