尝试在自定义视图中绘制缩放的uiimage,但没有任何渲染

我已经创建了一个自定义视图类,现在只想绘制一个缩放到适合视图的图像,给定一个uiimage。我试着画了uiimage.cgimage,但是正如其他人在这个网站(和文档中)所证明的那样,它将图像颠倒了。

所以,在我找到另一个问题的答案的建议下,我试图直接画出来,但是没有任何东西在视图中呈现,我不知道为什么。这是我的绘图代码:

- (void)drawRect:(CGRect)rect {
    // Drawing code
    [super drawRect:rect];

    if (self.originalImage) {
        [self drawImage];
    }
}

- (void) drawImage {
    if (CGSizeEqualToSize(originalImage.size, self.frame.size) == NO) {
        CGFloat scaleFactor = 1.0;
        CGFloat scaledWidth = 0.0;
        CGFloat scaledHeight = 0.0;
        CGPoint thumbPoint = CGPointMake(0.0, 0.0);

        CGFloat widthFactor = self.frame.size.width / originalImage.size.width;
        CGFloat heightFactor = self.frame.size.height / originalImage.size.height;

        if (widthFactor < heightFactor) {
            scaleFactor = widthFactor;
        } else {
            scaleFactor = heightFactor;
        }

        scaledWidth = originalImage.size.width * scaleFactor;
        scaledHeight = originalImage.size.height * scaleFactor;

        if (widthFactor < heightFactor) {
            thumbPoint.y = (self.frame.size.height - scaledHeight) * 0.5;
        } else if (widthFactor > heightFactor) {
            thumbPoint.x = (self.frame.size.width - scaledWidth) * 0.5;
        }

        UIGraphicsBeginImageContext(self.frame.size);
        CGRect thumbRect = CGRectZero;
        thumbRect.origin = thumbPoint;
        thumbRect.size.width = scaledWidth;
        thumbRect.size.height = scaledHeight;

        [originalImage drawInRect:thumbRect];
        self.scaledImage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
    } else {
        self.scaledImage = originalImage;
    }   
}

我的理解是(在稍微研究了一下之后)uigraphicsbeginimagecontext函数创建了一个 屏幕外 对于我来说,现在我该如何在原始上下文的基础上呈现上下文呢?