Images in ListView search results

Question

I have a ListActivity that launches a task to hit a web service and display the results in a ListView. Each one of the results has an image ID attached to it.

I wrote a method that will get the image IDs of the rows displayed on screen (firstVisiblePosition() to lastVisiblePosition()) and launch a task to query another web service to get the images to display for those items. I call this method when the list's scroll state becomes SCROLL_STATE_IDLE. This makes it so the user can scroll and the task to get the images for the visible rows does not execute until the scrolling stops, preventing it from looking up images for off-screen rows.

My issue is that when the results initially show in the ListView, I can't find a good way to call my method to look up which image IDs to query for. Apparently, calling this method right after calling setAdapter does not work (I'm guessing because some of the ListView's work happens asynchronously). I am using multiple Adapter's (for reasons not pertinent to this post), so I need a good way of waiting for the list items to show before I call my method to get the IDs.

Any ideas?

score 1 · Accepted Answer · answered Jul 07 '11 at 19:50

1

After you've set the adapter or called notifyDatasetChanged() on the adapter, add your "load images" code to the list's post queue as a Runnable:

list.post( new Runnable() {
    @Override
    public void run() {
    //do stuff
    }
  });

answered Jul 07 '11 at 19:50

dmon

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If you're interested, I did the same thing you are doing right now, and I found the `SCROLL_STATE_IDLE` to not be as precise as I would like, so I implemented something fancier. I posted this as an answer previously [here](http://stackoverflow.com/questions/6358428/implementation-of-onscrolllistener-to-detect-the-end-of-scrolling-in-a-listview/6358818#6358818). – dmon Jul 07 '11 at 21:05

score 0 · Answer 2 · answered Jul 07 '11 at 19:55

If I'm understanding your question right, you're having trouble loading images over the net and performance issues; if so,

I would create a simple image cache in my Adapter such as a local but global HashMap:

private HashMap&ltString, Drawable&gt imgCache = new HashMap&ltString, Drawable&gt();

then in my getView() method, I would asynchronously (using a Thread and a Handler) load the images and save loaded images in my imgCache by assigning position as the key and loaded images as Drawables.

final Handler h = new Handler() {
    public void handleMessage(Message msg) {
        if(msg.obj != null) {
            Drawable drawable = (Drawable)msg.obj;
            image.setImageDrawable(drawable);
            image.postInvalidate();
            image.requestLayout();
            imgCache.put(cacheKey, drawable);
        }
    }
};
loadImage(myImageView, imageURL, h); // threaded method which loads the images from net

also, in my getView() method I would first ask imgCache to see if the image already exist before loadImage is called.

This should optimize your list and rescue you from using multiple Adapter etc.

Hope this helps,

-serkan

I've done all of this. The problem is using getView() will load ALL of the images for the list if you scroll from the top to the bottom. This is a waste of time and memory. It should only attempt looking for the images the user can see. So, I moved the code from getView to onScrollChanged. — Andrew, Jul 07 '11 at 20:27

Images in ListView search results

2 Answers2