iOS绘图弧无法按预期工作

Question

I'm trying to draw a simple arc with Quartz Core but I'm not getting the expected result.

My arc is a basic 0 degrees to 90 degree arc (counter-clockwise direction).

I have this code here:

- (void)drawRect:(CGRect)rect {

    CGContextRef ctx = UIGraphicsGetCurrentContext();

    CGContextSetStrokeColorWithColor(ctx, self.strokeColor.CGColor);

    CGContextMoveToPoint(ctx, self.center.x, self.center.y);

    CGFloat radius = self.bounds.size.width / 2.0;

    CGContextAddArc(ctx, self.center.x, self.center.y, radius, [MathTools degreesToRadians:0], [MathTools degreesToRadians:90], 0);

    CGContextStrokePath(ctx);

}

Note: MathTools is just a convenient class I create for converting degrees to radians and vice versa, the implementation for degreesToRadians: is:

+(CGFloat)degreesToRadians:(CGFloat)degrees
{
    return degrees * M_PI / 180.0;
}

But instead of seeing a white arc inside my purple circle, all I see is white dash:

I'm trying to get it to look like this:

Edit

Based on answer given by rmaddy, my new code looks like this:

- (void)drawRect:(CGRect)rect
{
    CGContextRef ctx = UIGraphicsGetCurrentContext();

    CGContextSetStrokeColorWithColor(ctx, self.strokeColor.CGColor);

    CGContextSetLineWidth(ctx, self.strokeWidth);

    CGFloat radius = self.bounds.size.width / 2.0 - self.strokeWidth;

    CGContextAddArc(ctx, CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds), radius, [MathTools degreesToRadians:0], [MathTools degreesToRadians:-90], 1);

    CGContextStrokePath(ctx);
}

Not sure if this helps anyone else but based on what I see here, it seems Apple's angle positive and negative direction is not the same as mathematics. From memory, mathematic +90 degrees is anti-clockwise, but Apple's +90 degrees seems like clockwise.

Answer 1

One critical issue I see is your use of self.center . Your intent is to move to the center of the view but self.center is relative to the view's frame, not its bounds.

Everything in drawRect: needs to be relative to the view's bounds, not its frame.

Change this line:

CGContextMoveToPoint(ctx, self.center.x, self.center.y);

to:

CGContextMoveToPoint(ctx, CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds));

Make a similar change to the call to CGContextAddArc .

BTW - the only time self.center would work for this case is if the view's origin is at 0, 0 . With any other origin, self.center will give you a problem for this case.