1. Check network status
AndroidManifest.xml
<uses-sdk> ... </>
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<application>
MainActivity.java
public boolean isNetworkAvailable() {
ConnectivityManager connectivityManager
= (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
return activeNetworkInfo != null && activeNetworkInfo.isConnected();
}
2. Extends AsyncTask to download images.
Normally, we create mutl-thread which is used to download images.
AsyncTask enables proper and easy use of the UI thread. This class allows to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers.
AsyncTask is designed to be a helper class around Thread andHandler
and does not constitute a generic threading framework. AsyncTasks should ideally be used for short operations (a few seconds at the most.) If you need to keep threads running for long periods of time, it is highly recommended you use the various APIs provided
by thejava.util.concurrent pacakge such as Executor, ThreadPoolExecutor
andFutureTask.
An asynchronous task is defined by a computation that runs on a background thread and whose result is published on the UI thread. An asynchronous task is defined by 3 generic types, calledParams,Progress
and Result, and 4 steps, called onPreExecute,doInBackground,
onProgressUpdate andonPostExecute.
public class ImageDownload extends AsyncTask<String, Integer, Boolean> {
private ImageView view;
private Bitmap img;
public ImageDownload(ImageView view) {
this.view = view;
}
@Override
protected Boolean doInBackground(String... urls) {
try {
URL imageUrl = new URL(urls[0]);
URLConnection conn = imageUrl.openConnection();
img = BitmapFactory.decodeStream(conn.getInputStream());
} catch (MalformedURLException e) {
e.printStackTrace();
return false;
} catch (IOException e) {
e.printStackTrace();
return false;
}
return true;
}
@Override
protected void onPostExecute(Boolean downloadResult) {
if (downloadResult) {
view.setImageBitmap(img);
}
}
protected void onProgressUpdate(Integer... progress) {
//Set progress
}
}
3. Begin to download images
String[] urls = new String[1];
urls[0] = "http://media.zenfs.com/ko_KR/News/starnn/20130121130545_50fcbe998c3d0_1.jpg";
ImageDownload imageDownload = new ImageDownload(imageDownloadView);
imageDownload.execute(urls);
新增了二個預定義的線程池SERIAL_EXECUTOR和THREAD_POOL_EXECUTOR
其實THREAD_POOL_EXECUTOR並不是新增的，之前的就有，只不過之前(Android 2.3)它是AsyncTask私有的，未公開而已。THREAD_POOL_EXECUTOR是一個corePoolSize爲5的線程池，也就是說最多隻有5個線程同時運行，超過5個的就要等待。所以如果使用executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR)就跟2.3版本的AsyncTask.execute()效果是一樣的。