问题
My whole app uses touchesMoved for drawing (Coregraphics). Should I be calling [self setNeedsDisplay] from within touchesMoved instead of performing the drawing code in there? Would that stop some lag that I am experiencing? The only reason I didn't try this before posting is because it is going to take me a few hours to convert all drawing to within drawRect and I wanted to know if it would be more would efficient than drawing in touchesMoved?
回答1:
Yes. You should always keep your drawing in drawRect and use setNeedsDisplay to trigger the actual drawing. This is key to the rendering of views in Cocoa Touch.
For example, if you didn't use this method, you could potentially have drawing code scattered around your class in multiple methods. In addition, this also ensures that your drawing code executes only once during a render cycle. Essentially, calling setNeedsDisplay triggers an invalidation flag that lets your UIView know to redraw itself. Without this, you could be performing extra drawing operations that aren't actually needed due to the render cycle.
回答2:
You should not do drawing inside a UI callback. It will slow down or freeze the UI responsiveness, and you won't have the proper drawing context if you want to draw directly into a UIView.
Instead save, schedule or queue the information required for drawing for later, either into your own bitmap drawing context, or during the drawRect callback for that view.
Depending on the frame rate required, you many not even need to do a setNeedsDisplay on every touchesMoved event, but can queue the data and set a flag for an animation rate timer or CADisplayLink callback to do the setNeedsDisplay at a more constant rate (say only 15 or 30 frames per second). This will also help eliminate extra drawing operations.
来源：https://stackoverflow.com/questions/7621229/is-drawrect-more-efficient-for-coregraphcs-drawing-than-using-core-graphics-in-t