(每次学习一点xamarin就做个学习笔记和视频来加深记忆巩固知识)
如有不正确的地方,请帮我指正。
安卓异步消息处理简介
有时候需要执行一些耗时的操作,例如从远程服务器读取数据,读取的时间的长度由很多因素决定,我们不希望主线程被阻塞程序无法进行其它工作,而且Android中只能在主线程进行UI操作,不能在子线程操作,如果要根据子线程执行的结果来更新UI时,这时就需要用到安卓异步消息处理机制。
异步消息原理
安卓异步消息处理主要有四个类:Message、Handler、Looper和MessageQueue,
我们直接打交道的是前两个类,后面两个类在内部自动处理的。
类 | 说明 |
---|---|
Message | 用于携带数据 |
MessageQueue | 是一个存放消息的队列 |
Looper | 用于不断的从队列中取消息,交给消息关联的Handler对象去处理 |
Handler | 用于处理消息 |
如图所示,首先在主线程中创建一个Handler实例对象并重写了HandlerMessage方法,然后在子线程中执行run方法并创建Message对象携带好数据,通过消息对象关联的hander对象的SendMessage方法将Message对象发送到消息队列,Looper会自动去从队列中取消息,取到后自动交给Handler的HandlerMessage方法处理。
下面我提取了Android中这四个类的源码的主要属性和方法,配上个人加的注释(如有注释不对的地方,请大家指正)
Message类
//用于携带数据
public final class Message implements Parcelable {
public int what;//一个整型标识
public int arg1;//可保存一个整数
public int arg2;//可保存一个整数
public Object obj;//可保存任意数据
long when;//消息应该被处理的时间
Handler target;//保存与它关联的Handlder对象的引用
Runnable callback;//保存处理消息的回调方法的引用
Message next;//指向下一个消息,因为消息队列是一个单链表的数据结构
private static final Object sPoolSync = new Object();//用于同步锁的对象
private static Message sPool;//缓存对象
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
public Message() { }
public static Message obtain() {
//比上面构造方法效率高,因为用了缓存对象
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
public long getWhen() {
return when;}
public void setTarget(Handler target) {
this.target = target;}
public Handler getTarget() {
return target;}
public Runnable getCallback() {
return callback;}
public void sendToTarget() {target.sendMessage(this);}
}
MessageQueue类
//消息队列(以Message的when值来排序的链表结构)
public final class MessageQueue {
private final boolean mQuitAllowed;
Message mMessages;//链表中最前头的Message对象(when值最小)
private boolean mQuitting;
// Indicates whether next() is blocked waiting in pollOnce() with a non-zero timeout.
private boolean mBlocked;
//一些本地方法
private native static long nativeInit();
private native static void nativeDestroy(long ptr);
private native void nativePollOnce(long ptr, int timeoutMillis);
private native static void nativeWake(long ptr);
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
}
//从队列中取消息
Message next() {
final long ptr = mPtr;
if (ptr == 0) {
return null;}
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
//以同步的方式取,确保同一时刻只有一个线程访问
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
//如果消息关联的Handlder对象没有
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());//那么只取异步消息
}
if (msg != null) {
if (now < msg.when) {
//如果没到消息应该被处理的时间
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);// 计算一个超时时间来等待
} else {
//到达消息应该被处理的时间就返回消息
mBlocked = false;
if (prevMsg != null) {prevMsg.next = msg.next;}//取出一个消息后整理好链表
else {mMessages = msg.next;}
msg.next = null;
msg.markInUse();
return msg;
}
} else {
nextPollTimeoutMillis = -1;
}
}
nextPollTimeoutMillis = 0;
}
}
//添加一个新的消息到队列中
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) {
//以同步方式添加消息
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
//如果链表为空或者新来消息比队列中所有消息的优先级更高
msg.next = p;//把原先链表中最前头的消息放到新的消息后面
mMessages = msg;//设置新消息为链表中最前头的消息
needWake = mBlocked;
} else {
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;//从链表第二个开始找合适的位置(第一个已经在上面的when < p.when判断过了)
if (p == null || when < p.when) {
break;//找到合适链表位置p后退出循环
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p;
prev.next = msg;//这两行代码是把新消息插到prev与p之间
}
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
}
Handler类
//用于处理消息
public class Handler {
final Looper mLooper; //保存着Looper对象的引用
final MessageQueue mQueue; //保存着Looper对象中MessageQueue对象的引用
final Handler.Callback mCallback; //保存着构造方法传入的回调方法的引用
public interface Callback {
//一个接口,规定了消息处理的方法的规范
public boolean handleMessage(Message msg);
}
public Handler() {
this(null, false);
} //构造方法
public Handler(Handler.Callback callback, boolean async) {
//构造方法
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;
}
public void handleMessage(Message msg)
{
//如果dispatchMessage方法中的msg.callback==null 或 mCallback==null
//那么子类应该重写此方法来处理消息
}
public void dispatchMessage(Message msg) {
//分发消息给回调方法处理
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
public final boolean sendMessage(Message msg) { //发送一个消息
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)//以指定的延迟时间发送消息
{
if (delayMillis < 0) {delayMillis = 0;}
//当前时间加上延迟时间就是消息应该被处理的时间
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
//在指定的时间发送消息
MessageQueue queue = mQueue;
if (queue == null) { return false;}
return enqueueMessage(queue, msg, uptimeMillis);
}
//将消息发送到消息队列中
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
//通过MessageQueue对象的enqueueMessage方法把消息添加到消息队列
return queue.enqueueMessage(msg, uptimeMillis);
}
}
Looper类
//用于不断的从队列中取消息,交给消息关联的Handler对象去处理
public final class Looper {
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();//以键值对形式存储(key:线程 value:此线程的Looper对象)
private static Looper sMainLooper;
final MessageQueue mQueue;//自己持有的消息队列
final Thread mThread;//自己所在的线程
private Looper(boolean quitAllowed) {
//私有构造方法
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("每个线程只能有一个Looper");
}
sThreadLocal.set(new Looper(quitAllowed));
}
public static void prepareMainLooper() {
//准备好主Looper对象
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
//开启循环
public static void loop() {
final Looper me = myLooper();
final MessageQueue queue = me.mQueue;
for (;;) {
Message msg = queue.next(); //从消息队列中取出一个消息
if (msg == null) {
return;
}
try {
msg.target.dispatchMessage(msg);//将消息交给关联的Handlder对象去处理
} finally {
}
msg.recycleUnchecked();
}
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}//取出Looper
public static @NonNull MessageQueue myQueue() {
return myLooper().mQueue;
}
public boolean isCurrentThread() {
return Thread.currentThread() == mThread;}
public void quit() {mQueue.quit(false);}
public @NonNull Thread getThread() {
return mThread;}
public @NonNull MessageQueue getQueue() {
return mQueue;}
}
我们直接打交道的是Message类和Handlder类,那么MessageQueue和Looper类是怎么来的,这就要看ActivityThread类程序的入口方法main,贴上部分代码如下:
public final class ActivityThread {
//..........省略其它代码
public static void main(String[] args) {
//..........省略其它代码
Looper.prepareMainLooper();//创建好Looper对象,它构造方法里同时也创建了MessageQueue对象
ActivityThread thread = new ActivityThread();//创建好主线程
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
Looper.loop(); //开启无限循环,不断从MessageQueue中取消息交给消息关联的Handler处理
throw new RuntimeException("Main thread loop unexpectedly exited");
}
}
大概了解了这四大类和它们的机制后,接下来就开始做个简单的异步消息处理程序。
//处理消息的类
public class MyHandler : Handler
{
private Action<Message> _action;//指向一个回调方法的委托
public MyHandler(Action<Message> action)
{
_action = action;//构造方法中传入处理消息的回调方法
}
//重写父类的HandleMessage方法
public override void HandleMessage(Message msg)
{
//if (_action != null)
//{
// _action.Invoke(msg);
//}
_action?.Invoke(msg); //C#6.0新增判断空值的方式
}
}
//模拟从服务器读取数据(实现Java.Lang.IRunnable接口可让线程使用)
public class ReadDataTask : Java.Lang.Object, Java.Lang.IRunnable
{
private Handler _handler;
public ReadDataTask(Handler handler)
{
_handler = handler;
}
public void Run()//子线程要运行的任务
{
try
{
Java.Lang.Thread.Sleep(4000);// 模拟任务,比如从远程服务器读取数据
Message msg = _handler.ObtainMessage();//创建一个消息对象
msg.What = 123; //设置消息对象的标识
msg.Obj = "已读取到数据";//消息对象携带一个字符串数据
_handler.SendMessage(msg);// 将消息对象发送到主线程的消息队列(MessageQueue)中
}
catch (Java.Lang.InterruptedException e)
{
Log.Debug("ReadInfoTask", e.Message);
}
}
}
上面两个类准备好后,接下来就在Activity中使用了
public class MainActivity : AppCompatActivity,IOnClickListener
{
private MyHandler _myHandler;
private EditText _editText;
public void OnClick(View v)
{
switch (v.Id)
{
case Resource.Id.button1:
Java.Lang.Thread thread = new Java.Lang.Thread(new ReadDataTask(_myHandler));
thread.Start();//启动一个子线程去执行耗时任务
break;
}
}
protected override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);
SetContentView(Resource.Layout.Main);
Button btn1 = this.FindViewById<Button>(Resource.Id.button1);
btn1.SetOnClickListener(this);
_editText = this.FindViewById<EditText>(Resource.Id.editText1);
_myHandler = new MyHandler(RefreshUI); //在主线程实例化一个Handler对象
}
private void RefreshUI(Message msg)//在主线程更新UI
{
switch (msg.What)
{
case 123:
string s = Convert.ToString(msg.Obj);
_editText.Text = s;
break;
case 456:
//……………………
break;
}
}
}
代码和视频在我上传的CSDN资源中http://download.csdn.net/download/junshangshui/10022514