Android异步处理四：AsyncTask的达成原理

发布时间：2021-12-22 12:47:12 所属栏目：PHP教程来源：互联网

导读：AsyncTask.java package com.zhuozhuo; 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 android.app.Act

AsyncTask.java

package com.zhuozhuo;


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 android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;
import android.widget.Toast;

public class AsyncTaskActivity extends Activity {

    private ImageView mImageView;
    private Button mButton;
    private ProgressBar mProgressBar;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        mImageView= (ImageView) findViewById(R.id.imageView);
        mButton = (Button) findViewById(R.id.button);
        mProgressBar = (ProgressBar) findViewById(R.id.progressBar);
        mButton.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                GetCSDNLogoTask task = new GetCSDNLogoTask();
                task.execute("http://www.linuxidc.com/pic/logo.gif");
            }
        });
    }

    class GetCSDNLogoTask extends AsyncTask<String,Integer,Bitmap> {//继承AsyncTask

        @Override
        protected Bitmap doInBackground(String... params) {//处理后台执行的任务，在后台线程执行
            publishProgress(0);//将会调用onProgressUpdate(Integer... progress)方法
            HttpClient hc = new DefaultHttpClient();
            publishProgress(30);
            HttpGet hg = new HttpGet(params[0]);//获取csdn的logo
            final Bitmap bm;
            try {
                HttpResponse hr = hc.execute(hg);
                bm = BitmapFactory.decodeStream(hr.getEntity().getContent());
            } catch (Exception e) {

                return null;
            }
            publishProgress(100);
            //mImageView.setImageBitmap(result); 不能在后台线程操作ui
            return bm;
        }

        protected void onProgressUpdate(Integer... progress) {//在调用publishProgress之后被调用，在ui线程执行
            mProgressBar.setProgress(progress[0]);//更新进度条的进度
         }

         protected void onPostExecute(Bitmap result) {//后台任务执行完之后被调用，在ui线程执行
             if(result != null) {
                 Toast.makeText(AsyncTaskActivity.this, "成功获取图片", Toast.LENGTH_LONG).show();
                 mImageView.setImageBitmap(result);
             }else {
                 Toast.makeText(AsyncTaskActivity.this, "获取图片失败", Toast.LENGTH_LONG).show();
             }
         }

         protected void onPreExecute () {//在 doInBackground(Params...)之前被调用，在ui线程执行
             mImageView.setImageBitmap(null);
             mProgressBar.setProgress(0);//进度条复位
         }

         protected void onCancelled () {//在ui线程执行
             mProgressBar.setProgress(0);//进度条复位
         }

    }


}
main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <ProgressBar android:layout_width="fill_parent" android:layout_height="wrap_content" android:id="@+id/progressBar" style="?android:attr/progressBarStyleHorizontal"></ProgressBar>
    <Button android:id="@+id/button" android:text="下载图片" android:layout_width="wrap_content" android:layout_height="wrap_content"></Button>
    <ImageView android:id="@+id/imageView" android:layout_height="wrap_content"
        android:layout_width="wrap_content" />
</LinearLayout>
mainifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.zhuozhuo"
      android:versionCode="1"
      android:versionName="1.0">
    <uses-sdk android:minSdkVersion="10" />
<uses-permission android:name="android.permission.INTERNET"></uses-permission>
    <application android:icon="@drawable/icon" android:label="@string/app_name">
        <activity android:name=".AsyncTaskActivity"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

    </application>
</manifest>

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