I have a custom view that fills my entire screen. (A piano keyboard)
When a user touches the key, it causes invalidate()
to be called and the whole keyboard gets redrawn to show the new state with a touched key.
Currently the view is very simple, but I plan to add a bit more nice graphics. Since the whole keyboard is dynamically rendered this would make redrawing the entire keyboard more expensive.
So I thought, let's look into partial redrawing. Now I call invalidate(Rect dirty)
with the correct dirty region. I set my onDraw(Canvas canvas)
method to only draw the keys in the dirty region if I do indeed want a partial redraw. This results in those keys being drawn, but the rest of the keyboard is totally black/not drawn at all.
Am I wrong in expecting that calling invalidate(Rect dirty)
would "cache" the current canvas
, and only "allows" drawing in the dirty region?
Is there any way I can achieve what I want? (A way to "cache" the canvas and only redraw the dirty area?"
Invoking invalidate() on a View causes it to draw itself via the View. You should check this out: http://developer.android.com/guide/topics/ui/custom-components.html. A TextView internally invalidates itself when you invoke setText() and redraws itself with the new text set via the setText() call.
Override onDraw() The most important step in drawing a custom view is to override the onDraw() method. The parameter to onDraw() is a Canvas object that the view can use to draw itself.
The Canvas class holds the "draw" calls. To draw something, you need 4 basic components: A Bitmap to hold the pixels, a Canvas to host the draw calls (writing into the bitmap), a drawing primitive (e.g. Rect, Path, text, Bitmap), and a paint (to describe the colors and styles for the drawing).
Current nice workaround is to manually cache the full canvas to a bitmap:
private void onDraw(Canvas canvas)
{
if (!initialDrawingIsPerformed)
{
this.cachedBitmap = Bitmap.createBitmap(getWidth(), getHeight(),
Config.ARGB_8888); //Change to lower bitmap config if possible.
Canvas cacheCanvas = new Canvas(this.cachedBitmap);
doInitialDrawing(cacheCanvas);
canvas.drawBitmap(this.cachedBitmap, 0, 0, new Paint());
initialDrawingIsPerformed = true;
}
else
{
canvas.drawBitmap(this.cachedBitmap, 0, 0, new Paint());
doPartialRedraws(canvas);
}
}
Ofcourse, you need to store the info about what to redraw yourself and preferably not use a new Paint
everytime, but that are details.
Also note: Bitmaps are quite heavy on the memory usage of your app. I had crashes when I cached a View that was used with a scroller and that was like 5 times the height of the device, since it used > 10MB memory!
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