Decoding gallery of images in separate thread in android

Question

Guys my program takes really long to open my gallery activity. I have gone through my code and I think I have found whats making the the activity slow down. I think its the part where the image gets decoded. It slows down when there are many images in my gallery and it looks like it takes time to decode them all.

This is my code

public View getView(int position, View convertView, ViewGroup parent) {
    ImageView imageView;
    if (convertView == null) {
        imageView = new ImageView(_activity);
    } else {
        imageView = (ImageView) convertView;
    }

    // THIS BELOW IN SEPARATE THREAD

    Bitmap image = decodeFile(_filePaths.get(position), imageWidth, imageWidth);

    // THIS ABOVE IN SEPARATE THREAD

    imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
    imageView.setLayoutParams(new GridView.LayoutParams(imageWidth,
            imageWidth));
    imageView.setImageBitmap(image);

    // image view click listener
    imageView.setOnClickListener(new OnImageClickListener(position));

    return imageView;
}

I think putting the image decoding line in a separate thread might speed up te program. Can anyone tell me how to do it?

Answer 1

You can not use a thread like that. Your whole function will still have to wait for the image to get decoded before returning, because you are returning imageView variable. First of all, why are you casting convertView to an ImageView, I highly doubt its a good idea to do that. You might have to change your xml and add an imageView, if its missing. Then get a reference to that imageView in this function by using something like

final ImageView listImage = (ImageView) convertView.findViewById(R.id.listImage);

Finally, fire off your thread and set the image into this listImage variable, while returning convertView. Use an ExecuterService. Here is a simple example.

private final ExecutorService executorService = Executors.newCachedThreadPool();
private final int layoutResourceId;  //set this

    public View getView(int position, View convertView, ViewGroup parent) {
        if(convertView==null){

            LayoutInflater inflater = (_activity).getLayoutInflater();
            convertView = inflater.inflate(layoutResourceId, parent, false);
        }
        final ImageView listImage = (ImageView) convertView.findViewById(R.id.listImage);
        // THIS BELOW IN SEPARATE THREAD
        executorService.submit(new Runnable() {
            @Override
            public void run() {
                Bitmap image = decodeFile(_filePaths.get(position), imageWidth, imageWidth);
                listImage.setScaleType(ImageView.ScaleType.CENTER_CROP);
                listImage.setLayoutParams(new GridView.LayoutParams(imageWidth, imageWidth));
                listImage.setImageBitmap(image);
                listImage.setOnClickListener(new OnImageClickListener(position));
            }
        });

        return convertView;
    }

I hope you got the general idea.

I highly suggest you use Picasso. Google it. You can do all this without having to implement your own threading, get the advantage of caching and do it in far lesser lines of code. You will run out of memory on low end devices because you are not recycling your bitmap.