AsyncTask的初步学习

今天太偷懒了，只学了AsyncTask的一点点内容。今晚还要去上课。加油努力吧。

下面这个代码只是用AsyncTask实现一个很简单的事情“从网上下载图片显示到手机上”，但是作为AsyncTask是一个不错的示例。

顺便讲一下，AsyncTask的重要知识点包括他的三个参数（第一个参数：传入doInBackground()方法的参数类型
，第二个参数：传入onProgressUpdate()方法的参数类型 ， 第三个参数：传入onPostExecute()方法的参数类型，也是doInBackground()方法返回的类型。）

另外还有三四个重要的函数，其中三个会在下面的代码里面出现。

吐槽一下：之前错了两三次全是因为图片资源错误了，泪奔。、

package com.example.myasynctask;

import java.io.IOException;

import org.apache.http.HttpEntity;
import org.apache.http.HttpRequest;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
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.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;

public class MainActivity extends Activity {
private Button button;
private ImageView iv;
private String path = "http://cdn.duitang.com/uploads/item/201310/17/20131017194234_eZfFU.thumb.600_0.png";
private ProgressDialog dia;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button = (Button) this.findViewById(R.id.bt);
iv = (ImageView) this.findViewById(R.id.iv);

dia = new ProgressDialog(this);
dia.setTitle("提示信息");
dia.setMessage("正在下载...");

button.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
new myTask().execute(path);
}
});
}

public class myTask extends AsyncTask<String, Void, Bitmap>
{
@Override
protected void onPreExecute() {
// TODO Auto-generated method stub
super.onPreExecute();
dia.show();
}

@Override
protected Bitmap doInBackground(String... params) {
// TODO Auto-generated method stub
HttpClient httpClient = new DefaultHttpClient();
HttpGet httpGet = new HttpGet(params[0]);
Bitmap bitmap = null;
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 (ClientProtocolException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return bitmap;
}

@Override
protected void onPostExecute(Bitmap result) {
// TODO Auto-generated method stub
super.onPostExecute(result);
iv.setImageBitmap(result);
dia.dismiss();
}
}
}


一个简简单单的布局
<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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".MainActivity" >

<Button
android:id="@+id/bt"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:layout_marginBottom="30dp"
android:gravity="center_vertical"
android:text="下載圖片" />

<ImageView
android:id="@+id/iv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="156dp"
android:src="@drawable/ic_launcher" />

</RelativeLayout>