He estado golpeando mi cabeza contra la pared durante mucho tiempo tratando de entender por qué esto no funciona.

Básicamente, estoy tratando de trazar un gráfico (gráfico) con CGPath y luego usarlo para recortar un degradado. El efecto final debería ser como la aplicación Stocks que viene con el iPhone.

El degradado y la ruta se dibujan bien por separado como dos elementos en capas (sin recortar). Pero si comento el CGContextDrawPath, ni la línea ni el degradado se dibujan en la pantalla.

Aquí está mi código drawRect:

CGContextRef context = UIGraphicsGetCurrentContext();

[[UIColor whiteColor] set];
CGContextSetLineWidth(context, 2.0f);
CGPoint lastDrawnPt = [[points objectAtIndex:0] CGPointValue];
CGPoint firstDrawnPt = lastDrawnPt;
//create the path for the gradient
CGMutablePathRef thePath = CGPathCreateMutable();
CGPathMoveToPoint(thePath, NULL, lastDrawnPt.x, self.bounds.size.height); // bottom left
CGPathAddLineToPoint(thePath, NULL, lastDrawnPt.x, lastDrawnPt.y);


for (int i=0; i<(points.count-1); i++) {
    //CGPoint pt1 = [[points objectAtIndex:i] CGPointValue];
    CGPoint pt2 = [[points objectAtIndex:i+1] CGPointValue];
    if (pt2.x > lastDrawnPt.x+2) {
        // only draw if we've moved sunstantially to the right

        //for the gradient
        CGPathMoveToPoint(thePath, NULL, lastDrawnPt.x, lastDrawnPt.y);
        CGPathAddLineToPoint(thePath, NULL, pt2.x, pt2.y);


        lastDrawnPt = pt2;
    }
}

//finish the gradient clipping path
CGPathMoveToPoint(thePath, NULL, lastDrawnPt.x, lastDrawnPt.y);
CGPathAddLineToPoint(thePath, NULL, lastDrawnPt.x, self.bounds.size.height);, // bottom right
CGPathMoveToPoint(thePath, NULL, lastDrawnPt.x, self.bounds.size.height);
CGPathAddLineToPoint(thePath, NULL, firstDrawnPt.x, self.bounds.size.height); // bottom right

CGPathCloseSubpath(thePath);

//add the gradient clipping path to the context
CGContextSaveGState(context);
CGContextAddPath(context, thePath);

//draw the path
float components[4] = {1.0, 1.0, 1.0, 1.0};
CGContextSetStrokeColor(context, components);
CGContextDrawPath(context,kCGPathStroke);

//clip the path
CGContextClip(context);


//Draw Gradient
UIColor *topColor = [UIColor colorWithRed: 1.0 green:1.0 blue:1.0 alpha:1.0];
UIColor *bottomColor = [UIColor colorWithRed:1.0 green:1.0 blue:1.0 alpha:0.0];
CGColorRef colorRef[] = { [topColor CGColor], [bottomColor CGColor] };
CFArrayRef colors = CFArrayCreate(NULL, (const void**)colorRef, sizeof(colorRef) / sizeof(CGColorRef), &kCFTypeArrayCallBacks);

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGGradientRef gradient = CGGradientCreateWithColors(colorSpace, colors, NULL);
CFRelease(colorSpace);
CFRelease(colors);

//  Draw a linear gradient from top to bottom
CGPoint gradStartPoint = CGPointMake(50.0, self.bounds.size.height);
CGPoint gradEndPoint = CGPointMake(50.0, 0.0);
CGContextDrawLinearGradient(context, gradient, gradStartPoint, gradEndPoint, 0);

CFRelease(gradient);

// Cleanup
CGColorSpaceRelease(colorSpace);

CGContextRestoreGState(context);