根据我的理解，你只需要一个绘制渐变的图层， CGContextDrawRadialGradient 可以完美地满足这种需要。并重申你所说的， CAGradientLayer 不支持径向渐变，我们无能为力，除了可以用 CALayer 子类。

From what I understood, you just need a layer that draws a gradient, and CGContextDrawRadialGradient works perfectly for that need. And to reiterate on what you said, CAGradientLayer doesn't support radial gradients, and nothing we can do about that, except unnecessary swizzling that can be done cleanly with a CALayer subclass.

（注意：渐变绘图代码从这里开始。这不是这个答案的内容。）

(note: the gradient drawing code was taken from here. It isn't what this answer is about.)

viewDidLoad ：

GradientLayer *gradientLayer = [[GradientLayer alloc] init];
gradientLayer.frame = self.view.bounds;

[self.view.layer addSublayer:gradientLayer];

CALayer 子类：

- (instancetype)init
{
    self = [super init];
    if (self) {
        [self setNeedsDisplay];
    }
    return self;
}

- (void)drawInContext:(CGContextRef)ctx
{

    size_t gradLocationsNum = 2;
    CGFloat gradLocations[2] = {0.0f, 1.0f};
    CGFloat gradColors[8] = {0.0f,0.0f,0.0f,0.0f,0.0f,0.0f,0.0f,0.5f};
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGGradientRef gradient = CGGradientCreateWithColorComponents(colorSpace, gradColors, gradLocations, gradLocationsNum);
    CGColorSpaceRelease(colorSpace);

    CGPoint gradCenter= CGPointMake(CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds));
    CGFloat gradRadius = MIN(self.bounds.size.width , self.bounds.size.height) ;

    CGContextDrawRadialGradient (ctx, gradient, gradCenter, 0, gradCenter, gradRadius, kCGGradientDrawsAfterEndLocation);


    CGGradientRelease(gradient);
}