Question

I am writing a custom event and would like some help please. Most of what I am about to talk about is based on the help provided at Custom event listener on Android app

So here is my issue. I am writing an app that needs to download updated images from the web, store the images on the phone, then later display those images. Basically, I download any needed images during a splash screen. Then when the images are downloaded and stored, the splash screen clears and any necessary (newly downloaded) images are displayed on the screen. Here is the problem: the download process is done via an asynctask so the part where the images are loaded on to the screen can’t be done inside the asynctask. It has to be done on the main UI thread. I would like to create an event and a custom event listener for the main thread to listen for that basically tells the main UI thread that it is safe to start loading the downloaded images from memory.

According to the discussion from the link above, I came up with this so far… a download listener interace

public interface DataDownloadListener {
 void onDownloadStarted();
 void onDownloadFinished();
}

an event class…

 public class DataDownloadEvent {
      ArrayList<DataDownloadListener> listeners = new ArrayList<DataDownloadListener>();
      public void setOnDownload(DataDownloadListener listener){
           this.listeners.add(listener);
      }
 }

My problem is that I don’t understand where to put the last two steps in those instructions. I thought I would have to put the listener and event inside the class that actually initiates the downloads. But where? Here is my function that initiates the download and saves it to the device:

 public String download(String sourceLocation) {
    String filename = "";
    String path = "";
    try {
        File externalStorageDirectory = Environment
                .getExternalStorageDirectory();
        URL urlTmp = new URL(sourceLocation);
        filename = urlTmp.getFile()
                .substring(filename.lastIndexOf("/") + 1);
        path = externalStorageDirectory + PATH;

        // check if the path exists
        File f = new File(path);
        if (!f.exists()) {
            f.mkdirs();
        }

        filename = path + filename;
        f = new File(filename);

        //only perform the download if the file doesn't already exist
        if (!f.exists()) {

            Bitmap bitmap = BitmapFactory.decodeStream(urlTmp.openStream());
            FileOutputStream fileOutputStream = new FileOutputStream(
                    filename);
            if (bitmap != null) {
                bitmap.compress(getFormat(filename), 50, fileOutputStream);
                Log.d(TAG, "Saved image " + filename);
                return filename;
            }
        }
        else{
            Log.d(TAG, "Image already exists: " + filename + " Not re-downloading file.");
        }
    } catch (MalformedURLException e) {
        //bad url
    } catch (IOException e) {
        //save error
    }

    return null;
}

And the last step about registering the listener, where do I put that? The instructions say to put that somewhere during initialization. Does that mean in the onCreate method of my main activity? outside the class in the import section of the main activity? Never done a custom event before, so any help would be appreciated.

Answer 1

According to the discussion from the link above, I came up with this so far… a download listener interace

public interface DataDownloadListener {
void onDownloadStarted();
void onDownloadFinished();
}

an event class…

public class DataDownloadEvent {
      ArrayList<DataDownloadListener> listeners = new ArrayList<DataDownloadListener>();
      public void setOnDownload(DataDownloadListener listener){
           this.listeners.add(listener);
      }
 }

Ok…

Now in your download procedure, at the start of the download, cycle all the elements on the listeners ArrayList and invoke the onDownloadStarted event to inform all your listeners that the download is just started (in this event i presume you’ll need to open the splashscreen).

Always in your download procedure, at the and of the download, cycle all the elements on the listeners ArrayList and invoke the onDownloadFinished event to inform all your listeners that the download is finished (now close the splashscreen).

How to cycle listeners on download completed

  foreach(DataDownloadListener downloadListener: listeners){
        downloadListener.onDownloadFinished();
  }

How to cycle listeners on download started

  foreach(DataDownloadListener downloadListener: listeners){
        downloadListener.onDownloadStarted();
  }