开发Android这么久了，总会听到有人说:主线程不能访问网络，子线程不能更新UI。Android的主线程的确不能长时间阻塞，但是子线程为什么不能更新UI呢？今天把这些东西整理，顺便在子线程更新UI。
首先写了一个handler在子线程更新主线程UI，在子线程做了一个耗时操作：从网络下载了一个图片并利用handler发送到handleMessage（）的回调中，并更新到主线程的bitmap。图片显示成功，没有问题。接下来在子线程中更新onCreate（）中实例化的textview，报错：
android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.只有创建了这个view的thread才能操纵这个view。Android加载view有两种方式，一是setContentView，二是inflater.inflate()。所以第三步，在子线程中用WindowManager.add()展示了view，run()代码如下：
LayoutInflater layoutInflater = LayoutInflater.from(getApplicationContext());
View view = layoutInflater.inflate(R.layout.test, null);
TextView textView = ((TextView) view.findViewById(R.id.test_tv));
textView.setText("十年一剑");
textView.setTextColor(getResources().getColor(R.color.colorAccent));
WindowManager windowManager = getWindowManager();
WindowManager.LayoutParams layoutParams = new WindowManager.LayoutParams();
layoutParams.height = 600;
layoutParams.width = 400;
layoutParams.flags = 2;
layoutParams.format = 1;
windowManager.addView(view, layoutParams);
报错： java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare().看他这意思，还需要looper。加上后，果然子线程内view显示出来了。尴尬的是，这个WindowManager.LayoutParams的flags和format等属性还没掌握。而且在代码前加上sleep方法view仍可以显示。那么，在onCreate()中的子线程中更新主线程UI呢？
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ButterKnife.bind(this);
new Thread() {
@Override
public void run() {
super.run();
textview.setText("heun3540");
}
}.start();
}
没有问题，子线程更新了主线程的UI，没有报错。但在加了一句Thread.sleep(1000)后，出现了android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.此外在点击事件中new Thread()更新主线程UI，点击后可以看到textview的文本已经改变，随后应用崩掉。
对以上几个例子总结一下：利用handler可以从子线程发送消息到主线程达到更新UI的目的；在子线程里可以更新在子线程中加载的view（需要looper）；在主线程的onCreate()中创建的子线程也可以更新主线程的UI，前提是不做其他耗时操作；除外，在onCreate()的子线程做耗时后更新UI报错；子线程没有looper想更新自己的UI也报错；点击事件（可以看做耗时事件）中的子线程更新主线程UI报错。由此可以看出，持有view的线程都可以更改自己的view，主线程默认looper不需要手动添加。一般的更新其他线程的UI需要handler即线程间的通信但handler只是线程间传递数据，更新操作还是要rootview来完成。那为什么在onCreate()的子线程更新主线程UI没有报错呢？而稍一耗时就报错了呢？必然是因为更新UI快于异常线程检测以至UI更新已经完了可能ViewRootImpl才刚刚初始化完成，但这样是不安全的，大家都不推荐这种方式。
妈的，以为下午换办公室，直接电源断了，一上午写的全没了，日。
我们都非常熟悉的生命周期，为什么按照onCreate，onStart这样的顺序执行的呢，可能是在源码中某个方法确定了调用顺序，也可能跟堆栈，硬件交互，全局变量有关。
关于Java中的接口回调，在Android的点击事件和网络请求等有大量的应用。比如项目中有大量重复的工作需要抽取到工具类中，但具体的业务逻辑灵活多变，后者业务后期的维护升级这些都要求代码无法在工具类中一步到位。这时就可以用回调解决，封装的时候调用接口的抽象方法，在业务实现的地方添加相应的代码。那么关于接口的理解，就不再仅限于初学时的定义了。
接口的出现，一是解决了Java类只能继承一个父类而导致父类过于庞杂，抽象程度不高的问题，二是对于日后代码维护十分的方便。比如说一个项目里有动物，鸟，人，猴子，海豚，飞机这些东西，动物作为抽象类，具体的动物作为实现类，动物有吃，睡觉，走抽象方法，鸟是啄，人是用碗吃。至于鸟和飞机，他们都可以飞，飞就可以设计成接口。
再举个例子，鸟，喜鹊，鸵鸟，孔雀，飞机，战斗机。在这个例子中，鸟是需要抽象的，那么到底抽象成接口还是抽象类呢？假如是接口的话，走，叫，吃这些可以有，可是鸵鸟不会飞，只有孔雀可以开屏，那就需要再单独设计两个接口，一个包含fly()，一个包含开屏()，三个接口分别选择实现，而飞机只要实现飞这个接口就可以了。假如鸟抽象成抽象类，同样走吃叫设计为抽象方法，剩下的一样，这样看起来接口和抽象类对于鸟来说没太大区别。这个倒很好解释，因为鸟的几个方法都设计成了抽象方法，假如吃这个方法已经定下来了，显然抽象类更合适。
此外，按照java的规则，一切皆对象，而类是对象的抽象。那么孔雀是鸟，所以鸟也应该设计成类更合适。套用一句大神的话：类定义了是不是；接口定义了有没有。孔雀继承了鸟，那他就是鸟的一种，而飞机实现了飞，则是具备了飞的能力。需要提到一点是：飞设计成接口，是因为这个行为与吃，叫不是同一类行为，假若飞和吃放在一起，那飞机岂不是得会吃才行。所以也可以看出来，两种不同属性的行为是不能写到一起的。
而Handler更新主线程的UI也是在主线程中进行的，只不过通过handler对象将子线程等耗时操作中得到的数据利用message传到了主线程。关于handler的原理，老生常谈。温故而知新。今天试着解释一下相关的源码，6.0以上的。
Looper是final修饰，不可继承。
Class used to run a message loop for a thread.
Threads by default do not have a message loop associated with them; to create one ,call #prepare in the thread that is to run the loop,and then #loop to have it process messages until the loop is stopped.
Most interaction with a message loop is through the #Handler class.
Looper类的解释告诉我们两个主要方法，prepare和loop。主线程不需要显示调用Looper的两个方法。但在子线程中，则需要显式调用。几个全局变量：
private static final String TAG = "Looper";
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper; // guarded by Looper.class
final MessageQueue mQueue;
final Thread mThread;
ThreadLocal在这里理解为将Looper对象与当前线程绑定，在同一个线程作用域内可见，是一个Java工具类。一个静态的looper引用，一个messageQueue引用，一个线程引用。
/** 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));
}
prepare方法中传入的布尔值最终传给了MessageQueue的构造方法中，它代表了 True if the message queue can be quit。prepare方法得到了looper对象并且looper在实例化的时候同时获取到当前线程的引用，还会实例化一个成员变量MessageQueue。
Looper中的构造方法是私有的：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper的loop方法，首先会拿到looper和消息队列实例，接着在无限循环中调用queue.next()取出队列的消息，交给msg.target.dispatchMessage(msg)处理。这其中消息的发送正是由handler.sendMessageAtTime()来做。
1 /**
2 * Run the message queue in this thread. Be sure to call
3 * {@link #quit()} to end the loop.
4 */
5 public static void loop() {
6 final Looper me = myLooper();
7 if (me == null) {
8 throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
9 }
10 final MessageQueue queue = me.mQueue;
11
12 // Make sure the identity of this thread is that of the local process,
13 // and keep track of what that identity token actually is.
14 Binder.clearCallingIdentity();
15 final long ident = Binder.clearCallingIdentity();
16
17 for (;;) {
18 Message msg = queue.next(); // might block
19 if (msg == null) {
20 // No message indicates that the message queue is quitting.
21 return;
22 }
23
24 // This must be in a local variable, in case a UI event sets the logger
25 final Printer logging = me.mLogging;
26 if (logging != null) {
27 logging.println(">>>>> Dispatching to " + msg.target + " " +
28 msg.callback + ": " + msg.what);
29 }
30
31 final long traceTag = me.mTraceTag;
32 if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
33 Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
34 }
35 try {
36 msg.target.dispatchMessage(msg);
37 } finally {
38 if (traceTag != 0) {
39 Trace.traceEnd(traceTag);
40 }
41 }
42
43 if (logging != null) {
44 logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
45 }
46
47 // Make sure that during the course of dispatching the
48 // identity of the thread wasn't corrupted.
49 final long newIdent = Binder.clearCallingIdentity();
50 if (ident != newIdent) {
51 Log.wtf(TAG, "Thread identity changed from 0x"
52 + Long.toHexString(ident) + " to 0x"
53 + Long.toHexString(newIdent) + " while dispatching to "
54 + msg.target.getClass().getName() + " "
55 + msg.callback + " what=" + msg.what);
56 }
57
58 msg.recycleUnchecked();
59 }
60 }
在第6行，调用myLooper方法返回了ThreadLocal保存的looper对象：
/**
* 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();
}
第十行将looper实例化时的messageQueue传给了新的引用MessageQueue，十七到二十行是无限循环，queue.next()取出消息。msg.target.dispatchMessage(msg)处理消息，msg.recycleUnchecked()回收资源。到此，消息队列和轮询已经建立，下面应该是发送消息了。那就来看一下Handler的代码：
1 /**
2 * A Handler allows you to send and process {@link Message} and Runnable Handler对象允许发送处理和一个线程的消息队列相关联的message和runnable对象。
3 * objects associated with a thread's {@link MessageQueue}. Each Handler 每一个Handler实例都与一个单独的线程和它的消息队列关联。
4 * instance is associated with a single thread and that thread's message 当创建一个Handler对象时，它就与创建它的线程和线程的消息队列绑定了。
5 * queue. When you create a new Handler, it is bound to the thread /
6 * message queue of the thread that is creating it -- from that point on, 至此，它就会分发消息和runnable对象到绑定的消息队列，并且在它们从消息队列取出时执行。
7 * it will deliver messages and runnables to that message queue and execute
8 * them as they come out of the message queue.
9 *
10 * <p>There are two main uses for a Handler: (1) to schedule messages and handler主要有两个用处：一是安排messages和runnable对象在未来的某一刻执行；
11 * runnables to be executed as some point in the future; and (2) to enqueue 二是为在其他线程执行的动作排队（此处翻译不当）
12 * an action to be performed on a different thread than your own.
13 *
14 * <p>Scheduling messages is accomplished with the 借助post和send系列方法，调度消息得到完成。
15 * {@link #post}, {@link #postAtTime(Runnable, long)},
16 * {@link #postDelayed}, {@link #sendEmptyMessage},
17 * {@link #sendMessage}, {@link #sendMessageAtTime}, and
18 * {@link #sendMessageDelayed} methods. The <em>post</em> versions allow post允许当Runnable对象被接收且将要被messagequeue调用时为它们排队；
19 * you to enqueue Runnable objects to be called by the message queue when sendMessage允许为一个包含了数据集且将会被handler的handleMessage方法（需要自己重写）处理的消息对象排队。
20 * they are received; the <em>sendMessage</em> versions allow you to enqueue
21 * a {@link Message} object containing a bundle of data that will be
22 * processed by the Handler's {@link #handleMessage} method (requiring that
23 * you implement a subclass of Handler).
24 *
25 * <p>When posting or sending to a Handler, you can either 当用post或者send向handler发消息时，可以在消息队列就绪时立即处理也可以指定延迟做延时处理。后者需要实现超时等时间行为。
26 * allow the item to be processed as soon as the message queue is ready
27 * to do so, or specify a delay before it gets processed or absolute time for
28 * it to be processed. The latter two allow you to implement timeouts,
29 * ticks, and other timing-based behavior.
30 *
31 * <p>When a
32 * process is created for your application, its main thread is dedicated to 当应用中的进程创建时，主线程致力于运行消息队列。队列着重于顶层的应用组件如活动，广播接收者等和任何这些组件创建的window。
33 * running a message queue that takes care of managing the top-level 可以创建子线程并且通过handler与主线程通信。和以前一样，是靠调用post或者sendMessage来实现，当然，是在子线程中调用。
34 * application objects (activities, broadcast receivers, etc) and any windows 发出的Runnable对象或者消息就会调度到handler的消息队列中并在恰当时处理。
35 * they create. You can create your own threads, and communicate back with
36 * the main application thread through a Handler. This is done by calling
37 * the same <em>post</em> or <em>sendMessage</em> methods as before, but from
38 * your new thread. The given Runnable or Message will then be scheduled
39 * in the Handler's message queue and processed when appropriate.
40 */
说来惭愧，这一段类注释翻译花了好长时间，好歹六级也过了好多年了。从类的注释中得知，handler主要用send和post发送消息，在重写的handleMessage方法处理消息。
所有的send方法底层都是通过sendMessageAtTime实现的，其中在sendMessageDelayed方法中调用sendMessageAtTime时传入了SystemClock.uptimeMills():
1 /**
2 * Enqueue a message into the message queue after all pending messages
3 * before (current time + delayMillis). You will receive it in
4 * {@link #handleMessage}, in the thread attached to this handler.
5 *
6 * @return Returns true if the message was successfully placed in to the
7 * message queue. Returns false on failure, usually because the
8 * looper processing the message queue is exiting. Note that a
9 * result of true does not mean the message will be processed -- if
10 * the looper is quit before the delivery time of the message
11 * occurs then the message will be dropped.
12 */
13 public final boolean sendMessageDelayed(Message msg, long delayMillis)
14 {
15 if (delayMillis < 0) {
16 delayMillis = 0;
17 }
18 return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
19 }
20
21 /**
22 * Enqueue a message into the message queue after all pending messages
23 * before the absolute time (in milliseconds) <var>uptimeMillis</var>.
24 * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
25 * Time spent in deep sleep will add an additional delay to execution.
26 * You will receive it in {@link #handleMessage}, in the thread attached
27 * to this handler.
28 *
29 * @param uptimeMillis The absolute time at which the message should be
30 * delivered, using the
31 * {@link android.os.SystemClock#uptimeMillis} time-base.
32 *
33 * @return Returns true if the message was successfully placed in to the
34 * message queue. Returns false on failure, usually because the
35 * looper processing the message queue is exiting. Note that a
36 * result of true does not mean the message will be processed -- if
37 * the looper is quit before the delivery time of the message
38 * occurs then the message will be dropped.
39 */
40 public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
41 MessageQueue queue = mQueue;
42 if (queue == null) {
43 RuntimeException e = new RuntimeException(
44 this + " sendMessageAtTime() called with no mQueue");
45 Log.w("Looper", e.getMessage(), e);
46 return false;
47 }
48 return enqueueMessage(queue, msg, uptimeMillis);
49 }
50
51
52 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
53 msg.target = this;
54 if (mAsynchronous) {
55 msg.setAsynchronous(true);
56 }
57 return queue.enqueueMessage(msg, uptimeMillis);
58 }
handler的enqueueMessage方法中将handler对象赋给了msg的target属性，接着调用了MessageQueue的enqueueMessage方法。MessageQueue也是final类，算是这几个类中比较native的，很多都是与底层交互的方法。在它的enqueueMessage方法中将message压入消息队列，接着loop()方法中msg.target.dispatchMessage(msg)，上文已经提到了。Message也是final类。所以最后是调用handler的dispatchMessage方法：
1 /**
2 * Subclasses must implement this to receive messages.
3 */
4 public void handleMessage(Message msg) {
5 }
6
7 /**
8 * Handle system messages here.
9 */
10 public void dispatchMessage(Message msg) {
11 if (msg.callback != null) {
12 handleCallback(msg);
13 } else {
14 if (mCallback != null) {
15 if (mCallback.handleMessage(msg)) {
16 return;
17 }
18 }
19 handleMessage(msg);
20 }
21 }
所以看到消息最后的处理正是在我们实例化handler时覆写的HandlerMessage方法，至此，消息发送处理机制走完了。
那么总结一下：消息机制的过程是，Looper.prepare()实例化looper对象和消息队列，handler实例化获得上一步的looper对象和消息队列的引用，handler.sendMessageAtTime()发送消息到消息队列（这其中包括了给message的target赋值，将message压入到消息队列），Looper.loop()轮询队列取出消息交给message.target.dispatchMessage()处理，实质上是调用了我们自己重写的handleMessage()。而Android为我们做了大量的封装工作。开发人员只需要构造message并发送，自定义消息处理逻辑就可以了。
在研究源码时，首先看类注释，接着明确自己的需求，再去找关键方法，千万莫要在庞杂的代码中迷失。
在探寻源码的过程中，发现了下一次博客的内容，就是WindowManager.LayoutParams，SystemClock，ThreadLocal，AtomicInteger。
水往低处流，人往高处走。
