如何用渐变填充 UIBezierPath？

我使用 UIBezierPath 绘制了一个图表。我可以用纯色填充图表下方的区域，但我想用渐变而不是纯色填充图表下方的区域。但我不确定如何使渐变仅适用于图表而不适用于整个视图，我已经阅读了一些问题，但没有发现任何适用的内容。

这是主要的绘图代码：

// Draw the graph
UIBezierPath *barGraph = [UIBezierPath bezierPath];
barGraph.lineWidth = 1.0f;
[blueColor setStroke];
TCDataPoint *dataPoint = self.testData[0];
CGFloat x = [self convertTimeToXPoint:dataPoint.time];
CGFloat y = [self convertDataToYPoint:dataPoint.dataUsage];
CGPoint plotPoint = CGPointMake(x,y);
[barGraph moveToPoint:plotPoint];
for (int ii = 1; ii < [self.testData count]; ++ii)
{
    dataPoint = self.testData[ii];
    x = [self convertTimeToXPoint:dataPoint.time];
    y = [self convertDataToYPoint:dataPoint.dataUsage];
    plotPoint = CGPointMake(x, y);
    [barGraph addLineToPoint:plotPoint];
}
[barGraph stroke];

我一直在尝试通过试验如下代码来填充图表，但说实话，尽管阅读了各种教程和文档，但我并不能 %100 确定我在做什么：

[barGraph closePath];

CGFloat colors [] = {
    1.0, 0.0, 0.0, 1.0,
    1.0, 1.0, 1.0, 1.0
};

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

CGPoint startPoint = CGPointMake(CGRectGetMidX(self.bounds), self.topY);
CGPoint endPoint = CGPointMake(CGRectGetMidX(self.bounds), self.bottomY);

CGContextRef context = UIGraphicsGetCurrentContext();
CGContextClip(context);
CGContextDrawLinearGradient(context, gradient, startPoint, endPoint, 0);
CGGradientRelease(gradient);

最简单的方法是使用原始图形贝塞尔曲线路径来帮助您构建蒙版并将该蒙版施加到渐变层上。现在将图形层施加在渐变层的顶部。

因此，例如，制作一个CAShapeLayer，关闭图形贝塞尔曲线路径，将其路径设置为形状图层的路径，并将其填充为黑色。现在您有了一个遮罩，它是图表下方区域的形状。现在创建一个 CAGradientLayer 并将 CAShapeLayer 设为其mask。在其前面放置实际的图表。

例如（EDITED):

这是我用来创建该绘图的代码（我的贝塞尔曲线路径非常简单，只是由三条线连接的四个点，但您可以清楚地看到它下面的区域是渐变）：

CAShapeLayer* shape = [[CAShapeLayer alloc] init];
shape.frame = self.graph.bounds;
CGFloat h = shape.frame.size.height;
CGFloat w = shape.frame.size.width;

NSArray* points = @[
                    [NSValue valueWithCGPoint:CGPointMake(0,h-50)],
                    [NSValue valueWithCGPoint:CGPointMake(70,h-100)],
                    [NSValue valueWithCGPoint:CGPointMake(140,h-75)],
                    [NSValue valueWithCGPoint:CGPointMake(w,h-150)],
                    ];

UIBezierPath* p = [UIBezierPath new];
[p moveToPoint:[points[0] CGPointValue]];
for (NSInteger i = 1; i < points.count; i++)
    [p addLineToPoint:[points[i] CGPointValue]];

shape.path = p.CGPath;
shape.strokeColor = [UIColor blackColor].CGColor;
shape.lineWidth = 2;
shape.fillColor = nil;

CAGradientLayer* grad = [[CAGradientLayer alloc] init];
grad.frame = self.graph.bounds;
grad.colors = @[(id)[UIColor blueColor].CGColor,
                (id)[UIColor yellowColor].CGColor];

CAShapeLayer* mask = [[CAShapeLayer alloc] init];
mask.frame = self.graph.bounds;
[p addLineToPoint:CGPointMake(w,h)];
[p addLineToPoint:CGPointMake(0,h)];
[p closePath];
mask.path = p.CGPath;
mask.fillColor = [UIColor blackColor].CGColor;
grad.mask = mask;

[self.graph.layer addSublayer:grad];
[self.graph.layer addSublayer:shape];