Android面试常考点整理

Handler机制，子线程为什么不能更新UI？

Android UI操作并不是线程安全的，并且这些操作必须在UI线程执行。

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。

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1.得到了looper对象并且2.looper在实例化的时候同时获取到当前线程的引用，还会3.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。

/**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        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();

        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

            // 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);
            }

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            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);
            }

            msg.recycleUnchecked();
        }
    }

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：

/**
 * 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。
 * 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。
 * 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。
 * queue.  When you create a new Handler, it is bound to the thread /
 * 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。
 * it will deliver messages and runnables to that message queue and execute
 * them as they come out of the message queue.
 * 
 * <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；
 * 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）
 * an action to be performed on a different thread than your own.
 * 
 * <p>Scheduling messages is accomplished with the　　　　　　　　　　　　　　　　　　借助post和send系列方法，调度消息得到完成。
 * {@link #post}, {@link #postAtTime(Runnable, long)},
 * {@link #postDelayed}, {@link #sendEmptyMessage},
 * {@link #sendMessage}, {@link #sendMessageAtTime}, and
 * {@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；
 * 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。
 * they are received; the <em>sendMessage</em> versions allow you to enqueue
 * a {@link Message} object containing a bundle of data that will be
 * processed by the Handler's {@link #handleMessage} method (requiring that
 * you implement a subclass of Handler).
 * 
 * <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。
 * allow the item to be processed as soon as the message queue is ready
 * to do so, or specify a delay before it gets processed or absolute time for
 * it to be processed.  The latter two allow you to implement timeouts,
 * ticks, and other timing-based behavior.
 * 
 * <p>When a
 * 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。
 * 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。
 * 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。
 * they create.  You can create your own threads, and communicate back with
 * the main application thread through a Handler.  This is done by calling
 * the same <em>post</em> or <em>sendMessage</em> methods as before, but from
 * your new thread.  The given Runnable or Message will then be scheduled
 * in the Handler's message queue and processed when appropriate.
 */

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():

/**
     * Enqueue a message into the message queue after all pending messages
     * before (current time + delayMillis). You will receive it in
     * {@link #handleMessage}, in the thread attached to this handler.
     *  
     * @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 final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

    /**
     * Enqueue a message into the message queue after all pending messages
     * before the absolute time (in milliseconds) <var>uptimeMillis</var>.
     * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
     * 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);
    }


 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

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：

/**
     * Subclasses must implement this to receive messages.
     */
    public void handleMessage(Message msg) {
    }
    
    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

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。