Finger drawing application with unlimited scroll

Question

I want to make finger-draw app with vertical scroll and possibly unlimited space to draw. Scroll can be locked/unlocked.
I know 2 ways to do that:

• Make a bitmap with given width and height, draw points/lines in onTouchEvent(MotionEvent) via Canvas(Bitmap) and draw that bitmap in onDraw(Canvas).
• Save touch points to array and draw them directly in onDraw(Canvas).

But both ways have disadvantages:

• OutOfMemoryException with big bitmap.
• Very slow with numerous points while scrolling and OOM possible too.

I have an idea to use window and read/write regions of bitmap on demand. So there is a problem: how to write/append bitmap to another bitmap?

I cant find an appropriate way to append 200x200px bitmap to 8000x8000px bitmap for example. Is there a way to do that without native libs?

Or am I totally wrong and there is an easier method to do what I need to?

Answer 1

What about using a grid of small bitmaps, without appending them to a larger bitmap at all? When one of your bitmaps goes out of the screen you save it to the disk and destroy it or recycle it, a bit like how GridView recycles cells (it reuses the same 10 cells or so to display a potentially infinite grid of cells). Saving the whole drawing would just mean saving all bitmaps to the disk.

The main concern with this approach is the time it takes to save/load chunks from disk while the user scrolls through the view, which may cause the app to freeze for a short time, but there are ways to make it smooth (for example by preloading bitmaps in a background thread).

Storing touch points in a list is also completely possible, there are many ways to speed up the drawing part, for example by caching what's on the screen and drawing only what's missing when the user scrolls. Which one is best depends on what you will draw on your view.