How can I fill a rect with an alpha color using CoreGraphics?

问题

In my drawRect method, I am drawing a PNG image. On top of that, I want to draw a rect with a 20% alpha color, like this:

[[UIColor colorWithWhite:0.0 alpha:0.2] set];
UIRectFill(rect);

The problem is, that the alpha property seems to get ignored. No alpha is applied at all, just a black rectangle is drawn. How can I fix this? Thanks in advance!

回答1:

Use CGContextSetBlendMode() before you draw the rect.

回答2:

Set your view background color to transparent… It should work.

来源：https://stackoverflow.com/questions/2935608/how-can-i-fill-a-rect-with-an-alpha-color-using-coregraphics