ios - CoreGraphics 矩形渐变

Question

有人可以帮我用 CoreGraphics 绘制一个特定的渐变吗？只要给出线索就足够了。示例代码值得我致以最深的敬意。我需要这个形状:

我尝试了以下代码:

-(UIImage *)imageWithGradientBorder
{
    CGRect rect=CGRectMake(0, 0, self.size.width, self.size.height);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
    CGContextRef ctx= CGBitmapContextCreate(NULL, rect.size.width, rect.size.height, 8, rect.size.width, colorSpace, kCGImageAlphaNone);
    CGGradientRef glossGradient;
    size_t num_locations = 4;
    CGFloat locations[4] = { 0.0, 0.1, 0.9, 1.0 };
    CGFloat components[16] = { 0, 0, 0, 1, 1.0, 1.0, 1.0, 1, 1.0, 1.0, 1.0, 1, 0, 0, 0, 1};
    glossGradient = CGGradientCreateWithColorComponents(colorSpace, components, locations, num_locations);
    CGRect currentBounds = rect;

    CGPoint topCenter = CGPointMake(CGRectGetMidX(currentBounds), 0.0f);
    CGPoint midCenter = CGPointMake(CGRectGetMidX(currentBounds), CGRectGetMaxY(currentBounds));
    topCenter = CGPointMake( 0.0f, CGRectGetMidY(currentBounds));
    midCenter = CGPointMake(CGRectGetMaxX(currentBounds), CGRectGetMidY(currentBounds));

    CGContextDrawLinearGradient(ctx, glossGradient, topCenter, midCenter, 0);
    CGImageRef mask = CGBitmapContextCreateImage(ctx);
    CGContextRelease(ctx);
    CGImageRef maskedImageRef = CGImageCreateWithMask([self CGImage], mask);
    CGImageRelease(mask);
    UIImage *maskedImage = [UIImage imageWithCGImage:maskedImageRef];
    CGImageRelease(maskedImageRef);
    return maskedImage;
}

但它只从左侧绘制渐变。

Answer 1

topCenter = CGPointMake( 0.0f, CGRectGetMidY(currentBounds));
midCenter = CGPointMake(CGRectGetMaxX(currentBounds), CGRectGetMidY(currentBounds));

这两个变量的名称具有误导性，它们并不是真正的中上和中中，而是中左和中右。所以当你打电话时:

CGContextDrawLinearGradient(ctx, glossGradient, topCenter, midCenter, 0);

您正在从左到右绘制渐变。

您需要做什么来组织您的点并绘制渐变 4 次(L->R、R->L、T->B、B->T)。根据您希望角落看起来准确的样子，您可能需要在每个渐变绘制之间进行一些 mask ，以防止渐变一直绘制到角落中。