AsyncTask异步1

AsyncTask异步一

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:text="下载网络图片" />

    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:src="@drawable/ic_launcher" />

</RelativeLayout>

 

添加网络访问权限

 <uses-permission android:name="android.permission.INTERNET" />

MainActivity

package com.example.android_asytask_download;

import org.apache.http.HttpEntity;
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 org.apache.http.util.EntityUtils;

import android.os.AsyncTask;
import android.os.Bundle;
import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Entity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;

public class MainActivity extends Activity {
	private Button button;
	private ImageView imageView;
	private ProgressDialog progressDialog;
	private String image_path = "http://a.hiphotos.baidu.com/image/w%3D2048/sign=7610c70c5143fbf2c52ca1238446cb80/d4628535e5dde71120a3a1f2a5efce1b9d166145.jpg";

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);

		button = (Button) this.findViewById(R.id.button1);
		imageView = (ImageView) this.findViewById(R.id.imageView1);
		progressDialog=new ProgressDialog(this);
		progressDialog.setTitle("提示");
		progressDialog.setMessage("正在下载，请稍后...");

		button.setOnClickListener(new View.OnClickListener() {

			@Override
			public void onClick(View arg0) {
				// TODO Auto-generated method stub
				new myTask().execute(image_path);//执行异步任务操作
			}
		});
	}

	/**
	 * 使用异步任务规则 1.声明一个类继承AsyncTask类
	 * 2.第一个参数表示要执行的任务通常是网络的路径，第二参数表示刻度，第三个参数表示执行返回的结果
	 * */
	public class myTask extends AsyncTask<String, Void, Bitmap> {
		// 表示任务之前操作
		@Override
		protected void onPreExecute() {
			// TODO Auto-generated method stub
			progressDialog.show();
			super.onPreExecute();
		}

		// 表示耗时操作
		@Override
		protected Bitmap doInBackground(String... params) {
			// TODO Auto-generated method stub
			Bitmap bitmap=null;
			HttpClient httpClient=new DefaultHttpClient();
			HttpGet httpGet=new HttpGet(params[0]);
			try {
				HttpResponse httpResponse=httpClient.execute(httpGet);
				if(httpResponse.getStatusLine().getStatusCode()==200)
				{
					HttpEntity httpEntity=httpResponse.getEntity();
					byte[]data=EntityUtils.toByteArray(httpEntity);
					bitmap=BitmapFactory.decodeByteArray(data, 0, data.length);
				}
			} catch (Exception e) {
				// TODO: handle exception
			};
			return bitmap;
		}

		// 表示更新ui操作
		@Override
		protected void onPostExecute(Bitmap result) {
			// TODO Auto-generated method stub
			imageView.setImageBitmap(result);
			progressDialog.dismiss();
			super.onPostExecute(result);
		}
	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}

}

1.异步任务相当于一个线程的框架，内部封装了Thread,Handler，所以使用异步任务代码中不需要开启一个线程，一般较短的耗时时候使用异步任务。

2.Ui主线程不能直接访问网络，否则会出android.os.NetworkOnMainThreadException异常。