径向渐变：

- (void)drawRect:(CGRect)rect
{
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    
    //创建一个RGB的颜色空间
    CGColorSpaceRef rgb = CGColorSpaceCreateDeviceRGB();
    //定义渐变颜色数组
    CGFloat colors[] =
    {
        0.0f, 0.0f, 0.0f, 0.3f,
        0.0f, 0.0f, 0.0f, 0.1f,
        0.0f, 0.0f, 0.0f, 0.3f
    };
    
    const CGFloat locations[3] = { 0.0f, 0.5f, 1.0f };
    
    //创建一个渐变的色值 1:颜色空间 2:渐变的色数组 3:位置数组,如果为NULL,则为平均渐变,否则颜色和位置一一对应 4:位置的个数
    CGGradientRef _gradient = CGGradientCreateWithColorComponents(rgb, colors, locations, sizeof(colors)/(sizeof(colors[0])*4));
    CGColorSpaceRelease(rgb);
    
    //获得一个CGRect
    //CGRect clip = CGRectInset(CGContextGetClipBoundingBox(ctx), 20.0, 20.0);
    //剪切到合适的大小
    //CGContextClipToRect(ctx, clip);
    
    //定义起始点和终止点
    CGPoint start = CGPointMake(CGRectGetMinX(self.bounds), CGRectGetMinY(self.bounds));
    CGPoint end = CGPointMake(CGRectGetMinX(self.bounds), CGRectGetMaxY(self.bounds));
    
    //绘制渐变, 颜色的0对应start点,颜色的1对应end点,第四个参数是定义渐变是否超越起始点和终止点
    CGContextDrawLinearGradient(ctx, _gradient, start, end, kCGGradientDrawsBeforeStartLocation);
}

圆形辐射渐变

- (void)drawRect:(CGRect)rect
{
    const CGFloat components[8] = { 0.0f, 0.0f, 0.0f, 0.3f, 0.0f, 0.0f, 0.0f, 0.7f };
    const CGFloat locations[2] = { 0.0f, 1.0f };
    
    CGColorSpaceRef space = CGColorSpaceCreateDeviceRGB();
    CGGradientRef gradient = CGGradientCreateWithColorComponents(space, components, locations, 2);
    CGColorSpaceRelease(space);
    
    CGFloat x = CGRectGetMidX(self.bounds);
    CGFloat y = CGRectGetMidY(self.bounds);
    CGPoint point = CGPointMake(x, y);
    CGFloat radius = MAX(x, y);
    
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextDrawRadialGradient(context, gradient, point, 0, point, radius, kCGGradientDrawsAfterEndLocation);
    CGGradientRelease(gradient);
}