AsyncTask的用法

onPreExcute 执行后台耗时操作前被调用，通常用户完成一些初始化动作

doInBackground 必须重写，异步执行后台线程将要完成的任务,不能在这方法中更新UI

onPostExecute 当doInBackground()完成后，系统会自动调用onPostExecute()方法，并将doInBackground()方法返回的值传递给该方法

onProgressUpdate 在doInBackground()方法中调用publishProgress()方法，更新任务的执行进度后，就会触发该方法(更新进度用publishProgress()方法)

注:执行异步操作在doInBackground中完成，获取网络图片

AsyncTask执行任务时是单线程，会造成一个任务正在执行时退出，之后在进入时要把先前的任务执行完成时才能执行新任务，解决办法是把Activity和AsyncTask同步，在onPause中标记AsyncTask取消为真（只是标记取消为真，Android中不能粗暴停止任务），之后在doInBackground和onProgressUpdate中判断（以进度条为例）

　　　　　　　　　　urlConnection=new URL(url).openConnection();/*通过openConnection()获取URLConnection对象*/
                in=urlConnection.getInputStream();/*得到输入流对象*/
                Thread.sleep(3000);
                BufferedInputStream buf=new BufferedInputStream(in);/*转换为缓冲输入流对象*/
                bitmap= BitmapFactory.decodeStream(buf);/*通过BitmapFactory.decodeStream(buf)方法，得到bitmap对象*/
                in.close();
                buf.close();

接收网络图片

ImageActivity类

 1 package asysnctaskdemo.example.administrator.asynctaskdemo;
 2 
 3 import android.app.Activity;
 4 import android.graphics.Bitmap;
 5 import android.graphics.BitmapFactory;
 6 import android.os.AsyncTask;
 7 import android.os.Bundle;
 8 import android.view.View;
 9 import android.widget.ImageView;
10 import android.widget.ProgressBar;
11 
12 import java.io.BufferedInputStream;
13 import java.io.IOException;
14 import java.io.InputStream;
15 import java.net.URL;
16 import java.net.URLConnection;
17 
18 /**
19  * Created by Administrator on 2016/5/11.
20  */
21 public class ImageActivity extends Activity {
22     private ImageView mImageView;
23     private ProgressBar mProgressBar;
24     private static String Url="http://pic3.zhongsou.com/image/38063b6d7defc892894.jpg";
25     @Override
26     protected void onCreate(Bundle savedInstanceState) {
27         super.onCreate(savedInstanceState);
28         setContentView(R.layout.image);
29         mImageView= (ImageView) findViewById(R.id.image);
30         mProgressBar= (ProgressBar) findViewById(R.id.progressbar);
31         new MyAsyncTask().execute(Url);
32     }
33     class MyAsyncTask extends AsyncTask<String,Void,Bitmap>{/*接收三个参数*/
34         InputStream in;
35         URLConnection urlConnection;
36         Bitmap bitmap;
37 
38         @Override
39         protected void onPostExecute(Bitmap bitmap) {
40             super.onPostExecute(bitmap);
41             mProgressBar.setVisibility(View.GONE);
42             mImageView.setImageBitmap(bitmap);
43         }
44 
45         @Override
46         protected void onPreExecute() {
47             super.onPreExecute();
48             mProgressBar.setVisibility(View.VISIBLE);
49         }
50 
51         @Override
52         protected Bitmap doInBackground(String... params) {
53            String url=params[0];/*获取输入对象*/
54             try {
55                 urlConnection=new URL(url).openConnection();/*通过openConnection()获取URLConnection对象*/
56                 in=urlConnection.getInputStream();/*得到输入流对象*/
57                 Thread.sleep(3000);
58                 BufferedInputStream buf=new BufferedInputStream(in);/*转换为缓冲输入流对象*/
59                 bitmap= BitmapFactory.decodeStream(buf);/*通过BitmapFactory.decodeStream(buf)方法，得到bitmap对象*/
60                 in.close();
61                 buf.close();
62 
63 
64             } catch (IOException e) {
65                 e.printStackTrace();
66             } catch (InterruptedException e) {
67                 e.printStackTrace();
68             }
69             return bitmap;
70         }
71     }
72 }

image.xml

 1 <?xml version="1.0" encoding="utf-8"?>
 2 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
 3     android:layout_width="match_parent"
 4     android:layout_height="match_parent"
 5     android:padding="10dp">
 6     <ImageView
 7         android:layout_width="match_parent"
 8         android:layout_height="match_parent"
 9         android:id="@+id/image"/>
10     <ProgressBar
11         android:layout_centerInParent="true"
12         android:visibility="gone"
13         android:layout_width="wrap_content"
14         android:layout_height="wrap_content"
15         android:id="@+id/progressbar"/>
16 
17 </RelativeLayout>

activity_main.xml

 1 <?xml version="1.0" encoding="utf-8"?>
 2 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 3     xmlns:tools="http://schemas.android.com/tools"
 4     android:layout_width="match_parent"
 5     android:layout_height="match_parent"
 6     android:orientation="vertical"
 7    >
 8 
 9 
10     <Button
11         android:id="@+id/button2"
12         android:layout_width="match_parent"
13         android:layout_height="wrap_content"
14         android:text="更新进度"
15         android:onClick="loadImage2"/>
16     <Button
17         android:id="@+id/button"
18         android:layout_width="match_parent"
19         android:layout_height="wrap_content"
20         android:text="点击"
21         android:onClick="loadImage"/>
22 
23 </LinearLayout>

MainActivity代码

 1 package asysnctaskdemo.example.administrator.asynctaskdemo;
 2 
 3 import android.content.Intent;
 4 import android.support.v7.app.AppCompatActivity;
 5 import android.os.Bundle;
 6 import android.view.View;
 7 import android.widget.Button;
 8 
 9 public class MainActivity extends AppCompatActivity {
10     private Button button;
11     private Button button2;
12     @Override
13     protected void onCreate(Bundle savedInstanceState) {
14         super.onCreate(savedInstanceState);
15         setContentView(R.layout.activity_main);
16         MyAsyncTask myAsyncTask = new MyAsyncTask();
17         myAsyncTask.execute();
18         button= (Button) findViewById(R.id.button);
19         button2= (Button) findViewById(R.id.button2);
20         button2.setOnClickListener(new View.OnClickListener() {
21             @Override
22             public void onClick(View v) {
23                 Intent intent2=new Intent(MainActivity.this,ProgressBarDemo.class);
24                 startActivity(intent2);
25             }
26         });
27         button.setOnClickListener(new View.OnClickListener() {
28             @Override
29             public void onClick(View v) {
30                 Intent intent=new Intent(MainActivity.this,ImageActivity.class);
31                 startActivity(intent);
32 
33             }
34         });
35     }
36    /* public void loadImage(View view) {
37         startActivity(new Intent(this, ImageActivity.class));
38     }*/
39 }

更新进度条

ProgressBarDemo类

 1 import android.app.Activity;
 2 import android.os.AsyncTask;
 3 import android.os.Bundle;
 4 import android.widget.ProgressBar;
 5 
 6 /**
 7  * Created by Administrator on 2016/5/11.
 8  */
 9 public class ProgressBarDemo extends Activity {
10     ProgressBar mProgressBar;
11     MyAsyncTask myTask=new MyAsyncTask();
12     @Override
13     protected void onCreate(Bundle savedInstanceState) {
14         super.onCreate(savedInstanceState);
15         setContentView(R.layout.progress);
16         mProgressBar= (ProgressBar) findViewById(R.id.progressbar2);
17         myTask=new MyAsyncTask();
18         myTask.execute();
19     }
20 
21     @Override
22     protected void onPause() {
23         super.onPause();
24         if(myTask!=null&&myTask.getStatus()==AsyncTask.Status.RUNNING){
25             myTask.cancel(true);/*只是把myTask标记为取消状态，没有真正取消*/
26         }
27     }
28 
29     class MyAsyncTask extends AsyncTask<Void,Integer,Void>{
30         @Override
31         protected Void doInBackground(Void... params) {
32             for (int i=0;i<100;i++){
33                 if(isCancelled()){
34                     break;
35                 }
36                 publishProgress(i);
37                 try {
38                     Thread.sleep(300);
39                 } catch (InterruptedException e) {
40                     e.printStackTrace();
41                 }
42             }
43             return null;
44         }
45 
46         @Override
47         protected void onProgressUpdate(Integer... values) {
48             mProgressBar.setProgress(values[0]);
49             if (isCancelled()){
50                 return;
51             }
52             super.onProgressUpdate(values);
53         }
54 
55 
56     }
57 }

progress.xml

 1 <?xml version="1.0" encoding="utf-8"?>
 2 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 3     android:orientation="vertical" android:layout_width="match_parent"
 4     android:layout_height="match_parent">
 5     <ProgressBar
 6         android:layout_gravity="center_horizontal"
 7         style="?android:progressBarStyleHorizontal"
 8         android:layout_width="match_parent"
 9         android:layout_height="wrap_content"
10         android:id="@+id/progressbar2"/>
11 </LinearLayout>

posted @ 2016-05-11 19:20 成功人土阅读(257) 评论(0) 编辑收藏举报

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AsyncTask的用法

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