Android(安卓)异步更新UI----handler+thread
android应用是单线程模式的。
单线程模式需要记住两条:
一、防止UI线程阻塞
二、确保只在UI线程中访问Android UI工具包
在开发Android应用时必须遵守单线程模型的原则:Android UI操作并不是线程安全的并且这些操作必须在UI线程中执行。每个Android应用程序都运行在一个dalvik虚拟机进程中,进程开始的时候会启动一个主线程(MainThread),主线程负责处理和ui相关的事件,因此主线程通常又叫UI线程。而由于Android采用UI单线程模型,所以只能在主线程中对UI元素进行操作。
开一个线程或者在后台线程中来执行耗时的操作,如下面的例子:
publicvoidonClick(Viewv){
newThread(newRunnable(){
publicvoidrun(){
Bitmapb=loadImageFromNetwork(); //从网络上下载图片
mImageView.setImageBitmap(b); //把图片设置给ImageView
}
}).start()
}
上面的代码会报错,你可能会说逻辑很正确啊,但是它违背了Android单线程模型:Android UI操作并不是线程安全的并且这些操作必须在UI线程中执行.
例如:如果在非UI线程直接对UI进行了操作,则会报错:
CalledFromWrongThreadException:only the original thread that created a view hierarchy can touch its views
Android为我息循环们提供了消的机制,我们可以利用这个机制来实现线程间的通信。那么,我们就可以在非UI线程发送消息到UI线程,最终让Ui线程来进行ui的操作。
Andriod提供了几种在其他线程中访问UI线程的方法:
Activity.runOnUiThread(Runnable)
View.post(Runnable)
View.postDelayed(Runnable,long)
Hanlder
对于运算量较大的操作和IO操作,我们需要新开线程来处理这些繁重的工作,以免阻塞ui线程。
例子:下面我们以获取CSDN logo的例子,演示如何使用Thread+Handler的方式实现在非UI线程发送消息通知UI线程更新界面
ThradHandlerActivity.java:package com.example.thread;import org.apache.http.HttpResponse;import org.apache.http.client.HttpClient;import org.apache.http.client.methods.HttpGet;import org.apache.http.impl.client.DefaultHttpClient;import com.example.test.R;import android.annotation.SuppressLint;import android.app.Activity;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.Bundle;import android.os.Handler;import android.os.Message;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;import android.widget.ImageView;import android.widget.Toast;public class ThreadHandlerActivity extends Activity{private static final int MSG_SUCCESS = 0;private static final int MSG_FAILURE = 1;private ImageView mImageView;private Button mButton;private Thread mThread;@SuppressLint("HandlerLeak")private Handler mHandler = new Handler(){@Overridepublic void handleMessage(Message msg) {switch (msg.what) {case MSG_SUCCESS:mImageView.setImageBitmap((Bitmap)msg.obj);Toast.makeText(getApplication(), "成功获取图片", Toast.LENGTH_LONG).show(); break; case MSG_FAILURE: Toast.makeText(getApplication(), "获取图片失败", Toast.LENGTH_LONG).show(); break;}super.handleMessage(msg);}};@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.thread_layout);mImageView= (ImageView) findViewById(R.id.logo);//显示图片的ImageView mButton = (Button) findViewById(R.id.click); mButton.setOnClickListener(new OnClickListener() {@Overridepublic void onClick(View v) {if (mThread == null) {mThread = new Thread(runnable);mThread.start();}else { Toast.makeText(getApplication(), "线程已经运行", Toast.LENGTH_LONG).show(); } }});}Runnable runnable = new Runnable() {@Overridepublic void run() {HttpClient hc = new DefaultHttpClient();HttpGet hg = new HttpGet("http://csdnimg.cn/www/images/csdnindex_logo.gif");final Bitmap bm;try {HttpResponse hr = hc.execute(hg);bm = BitmapFactory.decodeStream(hr.getEntity().getContent());} catch (Exception e) {e.printStackTrace();mHandler.obtainMessage(MSG_FAILURE).sendToTarget();return;}mHandler.obtainMessage(MSG_SUCCESS, bm).sendToTarget();//mImageView.setImageBitmap(bm); //出错!不能在非ui线程操作ui元素// mImageView.post(new Runnable() {//另外一种更简洁的发送消息给ui线程的方法。 // @Override // public void run() {//run()方法会在ui线程执行 // mImageView.setImageBitmap(bm); // } // }); }};}
对于上面的方法,我们使用的是handler+Thread来实现更新UI,在里面也有一条注意的就是
mImageView.setImageBitmap(bm); //出错!不能在非ui线程操作ui元素
其实我们上面提到一个方法Activity.runOnUiThread(Runnable),将这个Runnable以UI线程的方式启动/** * Runs the specified action on the UI thread. If the current thread is the UI * thread, then the action is executed immediately. If the current thread is * not the UI thread, the action is posted to the event queue of the UI thread. * * @param action the action to run on the UI thread */ public final void runOnUiThread(Runnable action) { if (Thread.currentThread() != mUiThread) { mHandler.post(action); } else { action.run(); } }
上面Activity的runOnUiThread(Runnable)方法实现。
利用Activity.runOnUiThread(Runnable)把更新ui的代码创建在Runnable中,然后在需要更新ui时,把这个Runnable对象传给Activity.runOnUiThread(Runnable)。 这样Runnable对像就能在ui程序中被调用。如果当前线程是UI线程,那么行动是立即执行。如果当前线程不是UI线程,操作是发布到事件队列的UI线程。
使用示例:
current_activity.this. runOnUiThread(new Runnable() @Override public void run() { // refresh ui 的操作代码 }});
这里需要注意的是runOnUiThread是Activity中的方法,在线程中我们需要告诉系统是哪个activity调用,所以前面显示的指明了activity. 所以我们修改一下上面的代码:
package com.example.thread;import org.apache.http.HttpResponse;import org.apache.http.client.HttpClient;import org.apache.http.client.methods.HttpGet;import org.apache.http.impl.client.DefaultHttpClient;import com.example.test.R;import android.annotation.SuppressLint;import android.app.Activity;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.Bundle;import android.os.Handler;import android.os.Message;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;import android.widget.ImageView;import android.widget.Toast;public class ThreadHandlerActivity extends Activity{private static final int MSG_SUCCESS = 0;private static final int MSG_FAILURE = 1;private ImageView mImageView;private Button mButton;@SuppressLint("HandlerLeak")private Handler mHandler = new Handler(){@Overridepublic void handleMessage(Message msg) {switch (msg.what) {case MSG_SUCCESS:mImageView.setImageBitmap((Bitmap)msg.obj);Toast.makeText(getApplication(), "成功获取图片", Toast.LENGTH_LONG).show(); break; case MSG_FAILURE: Toast.makeText(getApplication(), "获取图片失败", Toast.LENGTH_LONG).show(); break;}super.handleMessage(msg);}};@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.thread_layout);mImageView= (ImageView) findViewById(R.id.logo);//显示图片的ImageView mButton = (Button) findViewById(R.id.click); mButton.setOnClickListener(new OnClickListener() {@Overridepublic void onClick(View v) {ThreadHandlerActivity.this.runOnUiThread(runnable);}});}Runnable runnable = new Runnable() {@Overridepublic void run() {HttpClient hc = new DefaultHttpClient();HttpGet hg = new HttpGet("http://csdnimg.cn/www/images/csdnindex_logo.gif");final Bitmap bm;try {HttpResponse hr = hc.execute(hg);bm = BitmapFactory.decodeStream(hr.getEntity().getContent());} catch (Exception e) {e.printStackTrace();mHandler.obtainMessage(MSG_FAILURE).sendToTarget();return;}mImageView.setImageBitmap(bm);}};}
也可以在线程里面直接更新UI。有人会说我传递一个当前的Activity到一个线程中,然后实现UI更新,那我就是调用的当前的Activity的内容,其实这个也是不对的也会提示
android.view.ViewRoot$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
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