Android笔记（三十六） AsyncTask是如何执行的？

      在上一个例子中，我们是在LoadImage的onPostExecute中修改的UI，不是说只允许在主线程中修改UI吗？我们看一下源代码是如何操作的。

      MainActicity.java

package cn.lixyz.asynctest;

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.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;

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

public class MainActivity extends Activity{

    private Button button;
    private ImageView imageView;
    private ProgressBar progressBar;
    private String URL="http://ww3.sinaimg.cn/bmiddle/612c96afjw1ewwftl3uqkj20u00koq48.jpg";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        //获取组件
        button = (Button) findViewById(R.id.button);
        imageView = (ImageView) findViewById(R.id.imageView);
        progressBar = (ProgressBar) findViewById(R.id.progressBar);

        //给button设置点击事件，点击按钮，启动异步任务
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                new LoadImage().execute(URL);
            }
        });


    }

    class LoadImage extends AsyncTask<String,Void,Bitmap>{


        //开始执行耗时操作，连接网络获取图片，并且将Bitmap返回
        @Override
        protected Bitmap doInBackground(String... params) {
            String url = params[0];
            Bitmap bitmap = null;
            URLConnection connection;
            InputStream is;
            try {
                connection = new URL(url).openConnection();
                Thread.sleep(5000);
                is = connection.getInputStream();
                BufferedInputStream bis = new BufferedInputStream(is);
                bitmap  = BitmapFactory.decodeStream(bis);
                is.close();
                bis.close();
            } catch (IOException e) {
                e.printStackTrace();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return bitmap;
        }

        //onPreExcute方法是在doInBackGround方法前执行，用于做一些初始化操作，这里将progressBar显示出来
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            progressBar.setVisibility(View.VISIBLE);
        }


        //doInBackGround方法执行后，会自动执行该方法，获取到doInBackGround返回的对象，然后更改UI
        @Override
        protected void onPostExecute(Bitmap bitmap) {
            super.onPostExecute(bitmap);
            progressBar.setVisibility(View.GONE);
            imageView.setImageBitmap(bitmap);
        }
    }
}

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