Canvas lose previous drawed data

[sinnuswong]

I am developing wear os watch faces,
my watch face will refresh screen 16fps a second.
In order to save power , I didn't draw all the canvas area in every frame
I draw no animated background once at some area,
and then draw the animated area every fps ,
but the background area sometimes is black and sometimes it doesn‘t，one by one display.
so is it a problem or normal?
I just don't want to draw all the canvas area everytime.