Question

Folks,

While coding up a few dials and sliders (e.g. like a big volume button one can rotate around) – I found that the standard CGContextAddArc() used like:

- (void)drawRect:(CGRect)rect {
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGColorSpaceRef rgbColorspace = CGColorSpaceCreateDeviceRGB();  
    CGContextSetLineWidth(ctx, radius * (KE-KR)+8);
    CGContextSetStrokeColorWithColor(ctx,self.foregroundColor.CGColor);
    .... more some colour/width/etc settings
    ...

    CGContextAddArc(ctx, dx,dy,radius, 0, 2*M_PI, 0);

to be unbelievable slow.

On an iPad – with a handful of filled/stroked circles, less than some 10 clean [self setNeedsDisplay] updates/second during drag. A very quick hack with a hand-drawn circle (shown below) was several orders of magnitude faster. Same applies to the emulator.

Why is this. Seems to be the case for both a normal fill and various gradient fills. What am I doing wrong ?

Dw.

// Stupid replacement for CGContectAddArc() which seems to be very slow.
//
void CGContextAddCirlce(CGContextRef ctx, float ox, float oy, float radius)
{
    double len = 2 * M_PI * radius;
    double step = 1.8 / len; // over the top :)

    // translating/scaling would more efficient, etc..
    //
    float x = ox + radius;
    float y = oy;

    // stupid hack - should just do a quadrant and mirror twice.    
    //
    CGContextMoveToPoint(ctx,x,y);
    for(double a = step; a < 2.0 * M_PI -step; a += step) {
        x = ox + radius * cos(a);
        y = oy + radius * sin(a);
        CGContextAddLineToPoint(ctx, x, y);
    };

    CGContextClosePath(ctx);
};

Answer 1

The vector drawing operations of Quartz 2D can be slow, which is why it is a good idea to redraw only when needed.

In your case, I would suggest drawing your volume button once, then transforming the UIView or CALayer into which you’ve drawn the button using a rotational transform. By simply moving, rotating, or scaling a view, you do not trigger an expensive redraw. The content is already cached as a texture, and the GPU can quickly manipulate and composite this rasterized content on top of your other views.

You’ll find that avoiding redrawing in this manner will yield much improved performance.