iphone - 如何在drawRect中用渐变填充路径:?

Question

用纯色填充路径非常简单:

CGPoint aPoint;
for (id pointValue in points)
{
    aPoint = [pointValue CGPointValue];
    CGContextAddLineToPoint(context, aPoint.x, aPoint.y);
}
[[UIColor redColor] setFill];
[[UIColor blackColor] setStroke];
CGContextDrawPath(context, kCGPathFillStroke);

我想绘制渐变而不是纯红色，但我遇到了麻烦。我已经尝试过问题/答案中列出的代码:Gradients on UIView and UILabels On iPhone

这是:

CAGradientLayer *gradient = [CAGradientLayer layer];
[gradient setFrame:rect];
[gradient setColors:[NSArray arrayWithObjects:(id)[[UIColor blueColor] CGColor],
                (id)[[UIColor whiteColor] CGColor], nil]];
[[self layer] setMasksToBounds:YES];

[[self layer] insertSublayer:gradient atIndex:0];

但是，这会用渐变绘制整个 View ，覆盖我原来的路径。

Answer 1

我会剪辑到您想要填充的路径，并使用CGContextDrawLinearGradient。下面以一个drawRect的简单实现为例:

- (void) drawRect:(CGRect)rect
{
    // Create a gradient from white to red
    CGFloat colors [] = { 
        1.0, 1.0, 1.0, 1.0, 
        1.0, 0.0, 0.0, 1.0
    };

    CGColorSpaceRef baseSpace = CGColorSpaceCreateDeviceRGB();
    CGGradientRef gradient = CGGradientCreateWithColorComponents(baseSpace, colors, NULL, 2);
    CGColorSpaceRelease(baseSpace), baseSpace = NULL;

    CGContextRef context = UIGraphicsGetCurrentContext();

    CGContextSaveGState(context);
    CGContextAddEllipseInRect(context, rect);
    CGContextClip(context);

    CGPoint startPoint = CGPointMake(CGRectGetMidX(rect), CGRectGetMinY(rect));
    CGPoint endPoint = CGPointMake(CGRectGetMidX(rect), CGRectGetMaxY(rect));

    CGContextDrawLinearGradient(context, gradient, startPoint, endPoint, 0);
    CGGradientRelease(gradient), gradient = NULL;

    CGContextRestoreGState(context);

    CGContextAddEllipseInRect(context, rect);
    CGContextDrawPath(context, kCGPathStroke);
}