I am setting up a game loop. The loop is set to update at 50 FPS. Currently the only action the app does is draw screen coordinates on response to touch events. Originally I set up the draw function to only draw new touch events. This caused the text to blink. Multiple touch events blink at different times, which leads me to think that Canvas uses multiple pages behind the scenes. In an effort to get around this I made a list of every touch event that happened and on every draw call I redraw the entire list.
Please correct me if I am wrong, the lockCanvas()/unlockCanvasAndPost() calls are in essence a backbuffer swap. Reading the documentation on locking and unlocking it sounds like it is necessary to redraw the entire scene between calls unless you use lockCanvas(Rect dirty). In this case the dirty rect area needs to be redraw while the outside area is preserved.
The content of the Surface is never preserved between unlockCanvas()
and lockCanvas(), for this reason, every pixel within the Surface area
must be written. The only exception to this rule is when a dirty
rectangle is specified, in which case, non-dirty pixels will be
preserved.
I know it sounds like I’ve answered my own question, but according to the documentation the Surface is never preserved between calls. However this does not explain the behavior of my first implementation, which was I would tap the screen and the text would start blinking. Since I only drew the text one time, this would mean that the blinking is from swapping to the “backbuffer” which didn’t get the drawText() call, and the original Surface which did not get destroyed but perhaps it is to be considered unreliable.
So, the question: Do I need to redraw all objects on each draw call? And if so, do I need to “clear” the canvas, or at the least redraw the background image also?
yes, unless you’re doing a dirty redraw where you define the region that you are redrawing, every point on your canvas gets destroyed.