android_86_AsyncTask

AsyncTask的作用:

1.子线程中更新UI

2.封装异步任务







效果:



代码:

MainActivity:

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

// 只能调用一次excute
SGAsyncTask task = new SGAsyncTask();
task.execute();
}

自定义的SGAsyncTask

package com.sg31.asynctask;

import android.os.AsyncTask;

public class SGAsyncTask extends AsyncTask {
@Override
protected void onPreExecute() {
super.onPreExecute();
System.out.println("sg__on Pre Execute");
}

@Override
protected Object doInBackground(Object... params) {
System.out.println("sg__do in background");

publishProgress(params);
return null;
}

// 必须在doInBackground方法中,手动调用publishProgress,才会触发本方法onProgressUpdate
@Override
protected void onProgressUpdate(Object... values) {
super.onProgressUpdate(values);
System.out.println("sg__on Progress Update");
}

@Override
protected void onPostExecute(Object result) {
super.onPostExecute(result);
System.out.println("sg__on Post Execute");
}
}

================================================================================================================

下面演示异步下载图片:

效果:



布局:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:padding="10dp"
>
<ImageView
android:id="@+id/imgView"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#1f00"

/>
<ProgressBar
android:id="@+id/progressBar"
android:layout_centerInParent="true"
android:visibility="invisible"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>

</RelativeLayout>

代码:

package com.sg31.asynctask;

import java.io.IOException;
import java.io.InputStream;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.opengl.Visibility;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.View;
import android.widget.ImageView;
import android.widget.ProgressBar;

public class ImageDownloadActivity extends ActionBarActivity {

private ImageView imgView;
private ProgressBar progressBar;
private static String imgURLString = "http://sg31.com/sgry.png";

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_imgdownload);
imgView = (ImageView) findViewById(R.id.imgView);
progressBar = (ProgressBar) findViewById(R.id.progressBar);
new SGImageDownloadAsyncTask().execute(imgURLString);
}

class SGImageDownloadAsyncTask extends AsyncTask<String, Void, Bitmap>{
@Override
protected void onPreExecute() {
super.onPreExecute();
// 异步下载前, 让进度条显示
progressBar.setVisibility(View.VISIBLE);
}
@Override
protected void onPostExecute(Bitmap result) {
super.onPostExecute(result);
// 下载完成后,让进度条隐藏;并设置图片
progressBar.setVisibility(View.INVISIBLE);
imgView.setImageBitmap(result);
}

// 唯一,必须实现的,在后台运行的方法
@Override
protected Bitmap doInBackground(String... params) {
String imgUrl = params[0];
Bitmap bitmap = null;
URLConnection connection = null;
InputStream is = null;
try {
connection = new URL(imgUrl).openConnection();
is = connection.getInputStream();
Thread.sleep(2000);
bitmap = BitmapFactory.decodeStream(is);
// 千万注意关流
is.close();
} catch (Exception e) {
e.printStackTrace();
}
return bitmap;
}
}

}

================================================================================================================

下面演示异步更新进度条:

效果图:



布局:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:padding="20dp"
>

<ProgressBar
style="?android:attr/progressBarStyleHorizontal"
android:id="@+id/progressBar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:layout_centerVertical="true" />

</RelativeLayout>

代码:

package com.sg31.asynctask;

import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.widget.ProgressBar;

public class ProgressBarUpdateActivity extends ActionBarActivity {

private ProgressBar progressBar;
private ProgressBarUpdateAsyncTask asyncTask;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_updateprogressbar);
progressBar = (ProgressBar) findViewById(R.id.progressBar);
// 使用成员变量,目的是为了能够取消
asyncTask = new ProgressBarUpdateAsyncTask();
// 只能执行一次,否则出错
asyncTask.execute();
}

@Override
protected void onPause() {
// TODO Auto-generated method stub
super.onPause();

// 设置停止的标记
if (asyncTask != null && asyncTask.getStatus() == AsyncTask.Status.RUNNING) {
asyncTask.cancel(true);
}
}

class ProgressBarUpdateAsyncTask extends AsyncTask<Void, Integer, Void> {

@Override
protected Void doInBackground(Void... params) {
for (int i = 0; i < 100; i++) {

// 停止,其实是通过标记,手动退出循环
if (isCancelled()) {
break;
}

// 必须通过此方法,激活onProgressUpdate方法;传递过去的参数就是当前进度
publishProgress(i);
try {
Thread.sleep(50);
} catch (Exception e) {
e.printStackTrace();
}
}
return null;
}

@Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
// 这儿可更新UI
progressBar.setProgress(values[0]);
}

@Override
protected void onPostExecute(Void result) {
super.onPostExecute(result);
finish();
}

}
}