使用异步任务加载网络图片

1.新建一个Activity，布局文件如下

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

   <ImageView
       android:id="@+id/imageView"
       android:layout_width="match_parent"
       android:layout_height="wrap_content" />

    <Button
        android:id="@+id/button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="获取图片"/>
</LinearLayout>

Activity的java代码如下:

一个显示图片的ImageView,一个用户触发操作的Button，urlString是网络图片的地址

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private ImageView mImageView;
    private Button mButton;
    private String urlString = "http://image.tianjimedia.com/uploadImages/2012/231/59/W19D0E6GL776.jpg";

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

        mImageView = (ImageView)findViewById(R.id.imageView);
        mButton = (Button)findViewById(R.id.button);

        mButton.setOnClickListener(this);

    }

    @Override
    public void onClick(View v) {
        switch (v.getId()){
            case R.id.button:
                MyAsyncTask myAsyncTask = new MyAsyncTask(mImageView);
                myAsyncTask.execute(urlString);
                break;
            default:
                break;
        }
    }
}

2.自定义的异步任务MyAsyncTask

传入的参数为String，也就是网络图片的地址，返回值为一个Bitmap，用来显示在ImageView中，同时MyAsyncTask持有一个主Activity中ImageView的引用

通过网络图片地址构建一个URL，设置超时时间以及请求方法，如果返回值为200，代表成功，然后获取输入流，通过BitmapFactory构建Bitmap

public class MyAsyncTask extends AsyncTask<String,Void,Bitmap> {
    public static  final String TAG = "MyAsyncTask";
    private ImageView mImageView;

    public MyAsyncTask(ImageView imageView){
        this.mImageView = imageView;
    }

    @Override
    protected Bitmap doInBackground(String... params) {

        String urlString = params[0];
        try{
            URL url = new URL(urlString);
            HttpURLConnection conn = (HttpURLConnection)url.openConnection();
            conn.setConnectTimeout(5000);
            conn.setRequestMethod("GET");
            if(conn.getResponseCode() == 200){
                InputStream inputStream = conn.getInputStream();
                Bitmap bitmap = BitmapFactory.decodeStream(inputStream);
                return bitmap;
            }
        }catch (Exception ex){
            ex.printStackTrace();
        }
        return null;
    }

    @Override
    protected void onPostExecute(Bitmap bitmap) {
        if(bitmap != null){
            mImageView.setImageBitmap(bitmap);
        }
    }
}