Scrollview Optimizations

Question

I have a scrollview that contains a custom view. The custom view is bigger than the area of the screen and draws properly.

The scrollview however tends to call onDraw() non-stop when scrolling, and I can't seem to make it smooth.

I used ScrollView.getDrawingRect() to calculate the visible portion of the screen and only draw to that, but it still returns the entire viewport (so it's optimized to not draw offscreen areas), and not the delta between the last position and the current one. Ideally I'd want to draw only the delta, and not the entire visible window.

If anyone can point me to more information about how to use the drawing caches, and if that will help optimize scrolling, I'd love to implement it, or any other possible solutions it would be greatly appreciated.

score 5 · Accepted Answer · edited Apr 06 '11 at 21:51

5

When content is scrolled, the entire viewport needs to be redrawn because all of the content has moved. I don't think there's anything that needs to be done to optimize a ScrollView - if scrolling is slow then it's the drawing method of your custom view that is too slow.

Try to avoid object creation in your draw methods which is usually the main culprit for poor drawing performance.

Edit: Also the scrollview could blit the old content up or down quickly that is still drawn on the screen, and then request a redraw of only the "new" portion of the screen. (only applies to opaque views).

edited Apr 06 '11 at 21:51

HaMMeReD

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answered Apr 06 '11 at 21:36

Joseph Earl

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There are better solutions, I could implement my own cache by drawing my view to a bitmap, and then drawing that bitmap onDraw instead of recalculating my view. The android view caching should be able to do something similar, and the documentation hints at it, but I've never seen something clearly explaining view caching. – HaMMeReD Apr 06 '11 at 21:40
Have you taken a look at http://groups.google.com/group/android-developers/browse_thread/thread/224457b8cb4e039c?pli=1. But I agree, the drawing cache topic is not well discussed. – Joseph Earl Apr 06 '11 at 21:53
For now I've just implemented the bitmap route. In my onDraw I see if there is a cache bitmap, create it to be the same size as the view if it does exist, than call my old onDraw with a canvas pointing to the bitmap. Than in regular onDraw it just draws the bitmap. It's a memory/performance tradeoff, but I'm ok with that here, since these views shouldn't get to big. If it can't allocate the cache, I can fall back to drawing straight to the view. – HaMMeReD Apr 06 '11 at 22:06

score 2 · Answer 2 · answered Mar 04 '13 at 12:56

I encountered the same problem. I solved it by using the function setDrawingCacheEnabled(true). By enabling this setting, your canvas view will be cached as bitmap, so you don't have to call canvas' draw method each time onDraw() is called.

In your custom view's constructor, you will need something like this:

public CustomView(Context context) {
    setDrawingCacheEnabled(true);
    drawnFlag = false;
}

In your onDraw method, you will need something like this:

public void onDraw(Canvas canvas) {
    if (! drawnFlag) {
        canvas.drawPath(...);
        canvas.drawPath(...);
        drawnFlag = true;
    }
}

Now, scrolling on this custom view should be smooth since we only call the drawing methods once.

I don't know why no one upvoted your answer. It works! @Andree — zbz.lvlv, Dec 11 '15 at 11:52

score -1 · Answer 3 · answered Nov 03 '12 at 14:26

-1

Afaik the ScrollView sets a proper clip rect on the canvas your view gets in onDraw so you only need to draw what's inside that rect. You could also implement cache bitmaps based on the clip rect's size.

answered Nov 03 '12 at 14:26

Oderik

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Scrollview Optimizations

3 Answers3