Android开发Handler源码分析
2016-05-19 12:48
435 查看
为什么使用Handler
Android中UI控件的访问是线程不安全的,加锁同步访问会影响性能,因此设置只能一个线程更新UI,就是主线程,或者说是UI线程。在UI线程中不能进行耗时的操作,耗时操作需要开启一个新的工作线程,工作线程不能更新UI,因此工作线程通过Handler通知UI线程更新UIHandler构造器分析
创建匿名Handler或者内部Handler的时候,IDE会提示内存泄露的风险,因为内部类持有外部类引用,修改为静态内部类即可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()); } }
Handler内部几个重要的属性
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;
通过构造函数对属性进行赋值。类属性的命名规则以小写字母m开头,命名的规范值得学习。
其他几个重载的构造函数功能类似,对类属性的赋值。
通过Handler发送消息
Handler有多个发送消息的方法,但最终都调用了public boolean sendMessageAtTime(Message msg, long uptimeMillis)
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); }
发送消息就是把消息插入到消息队列,该消息在消息队列的位置由该消息处理的时间when决定
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
在这里将msg的target属性设置为this,也就是当前的Handler,这点很重要,之后的消息处理会用到。最终是调用了MessageQueue类的enqueueMessage方法
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("MessageQueue", 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; }
首先检查了改msg的target属性是不是null,也就是是否设置了处理该msg的Handler,没有设置Handler就抛出异常,同一个msg不能进队列两次,否则抛出异常
通过了上边两项检查后,设置msg已使用,设置msg的处理事件,为msg的when属性赋值
msg.markInUse();
msg.when = when;
然后根据msg的when属性和Message队列的情况,将该msg插入到队列合适的位置
Handler消息的轮询处理
消息通过Handler的post和send方法加入了handler的消息队列MessageQueue,那么消息队列中的消息是如何被依次处理的呢?是通过Looper,Handler有两个重要的属性final MessageQueue mQueue;
final Looper mLooper;
在上边构造函数的分析中,看到mQueue是如何被赋值的
mQueue = mLooper.mQueue;
即Handler的消息队列就是Looper的消息队列,因此消息的轮询处理是交给了Looper。一个Looper和一个线程绑定,要想使一个线程具备Looper的能力(轮询消息队列),需要在线程run方法开头调用
private static void prepare(boolean quitAllowed)
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)); }
在prepare中通过ThreadLocal线程本地变量将Looper绑定到了调用prepare的线程。然后要想使线程具有轮询能力,需要在线程run方法末尾调用
public static void 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 Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } msg.target.dispatchMessage(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); } msg.recycleUnchecked(); } }
可见在loop方法中通过死循环处理消息队列中的消息
for (;;) { Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return; } ... msg.target.dispatchMessage(msg); ... }
从消息队列中依次取出消息,然后调用msg的target属性的dispatchMessage方法。
而target是如何被赋值的呢?是在Handler调用post或者send发送消息的时候被赋值的,被赋值为发送此消息的handler
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; ...
因此消息的处理又回到了Handler线程,通过Handler的dispatchMessage方法回调处理消息
Handler处理消息
在Handler所在线程,通过类似责任链模式的方式,处理该消息/** * 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); } }
相关文章推荐
- 使用C++实现JNI接口需要注意的事项
- Android IPC进程间通讯机制
- Android Manifest 用法
- [转载]Activity中ConfigChanges属性的用法
- Android之获取手机上的图片和视频缩略图thumbnails
- Android之使用Http协议实现文件上传功能
- Android学习笔记(二九):嵌入浏览器
- android string.xml文件中的整型和string型代替
- i-jetty环境搭配与编译
- android之定时器AlarmManager
- android wifi 无线调试
- Android Native 绘图方法
- Android java 与 javascript互访(相互调用)的方法例子
- android 代码实现控件之间的间距
- android FragmentPagerAdapter的“标准”配置
- Android"解决"onTouch和onClick的冲突问题
- android:installLocation简析
- android searchView的关闭事件
- SourceProvider.getJniDirectories