浅谈 -Android-Handler，h5 移动端开发面试题

Values?values?=?values(currentThread);if?(values?!=?null)?{Object[]?table?=?values.table;int?index?=?hash?&?values.mask;if?(this.reference?==?table[index])?{return?(T)?table[index?+?1];}}?else?{values?=?initializeValues(currentThread);}

return?(T)?values.getAfterMiss(this);}

get 方法就是取出当前线程对应的 looper，也就是说 ThreadLocal 是负责 thread 和 looper 之间的关系的

下面看一下 Looper.prepare()方法

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));}

默认情况下 ThreadLocal 是没有存储的，所以要创建一个新的 looper

private?Looper(boolean?quitAllowed)?{mQueue?=?new?MessageQueue(quitAllowed);mThread?=?Thread.currentThread();}

默认情况下 ThreadLocal 是没有存储的，所以要创建一个新的 looper

private?Looper(boolean?quitAllowed)?{mQueue?=?new?MessageQueue(quitAllowed);mThread?=?Thread.currentThread();}

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从 looper 方法中，创建了一个 MessageQueue，在 looper 中维护着一个消息队列

知道了 looper 和 MessageQueue 之后，究竟 handler 跟这两者有什么关系呢，继续看源码

public?Handler(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());}}

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;}

首先调用 Looper.myLooper()

public?static?@Nullable?Looper?myLooper()?{return?sThreadLocal.get();}

获得当前的 looper 对象，通过 looper 拿到 MessageQueue，就完成了 handler 和 looper 之间的关联

下面继续看 handler 的消息发送

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);}

先获得当前的消息队列，如果队列为空就抛出异常，不为空，向消息队列中插入消息

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

插入消息之前就指定消息发送给谁（msg.target），默认情况下发送给自己的 handler，然后把消息放入队列中，handler 就完成了发送 message 到 MessageQueue 的过程

那么消息又是如何轮询的呢？

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?blockif?(msg?==?null)?{//?No?message?indicates?that?the?message?queue?is?quitting.return;}