在处理图片的过程中，我们常常也会对其大小进行处理，有压缩啊，有直接画啊，可回看下iOS 中压缩图片和截图。

此处我们先看YYImageView是如何处理的：

- (UIImage *)yy_imageByResizeToSize:(CGSize)size contentMode:(UIViewContentMode)contentMode

简单的直接看实现

- (UIImage *)yy_imageByResizeToSize:(CGSize)size {
    if (size.width <= 0 || size.height <= 0) return nil;
    UIGraphicsBeginImageContextWithOptions(size, NO, self.scale);
    [self drawInRect:CGRectMake(0, 0, size.width, size.height)];
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

再加上考虑UIViewContentMode的情况下

- (UIImage *)yy_imageByResizeToSize:(CGSize)size contentMode:(UIViewContentMode)contentMode {
    if (size.width <= 0 || size.height <= 0) return nil;
    UIGraphicsBeginImageContextWithOptions(size, NO, self.scale);
    [self yy_drawInRect:CGRectMake(0, 0, size.width, size.height) withContentMode:contentMode clipsToBounds:NO];
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}
- (void)yy_drawInRect:(CGRect)rect withContentMode:(UIViewContentMode)contentMode clipsToBounds:(BOOL)clips{
    CGRect drawRect = _YYCGRectFitWithContentMode(rect, self.size, contentMode);
    if (drawRect.size.width == 0 || drawRect.size.height == 0) return;
    if (clips) {
        CGContextRef context = UIGraphicsGetCurrentContext();
        if (context) {
            CGContextSaveGState(context);
            CGContextAddRect(context, rect);
            CGContextClip(context);
            [self drawInRect:drawRect];
            CGContextRestoreGState(context);
        }
    } else {
        [self drawInRect:drawRect];
    }
}

其中根据尺寸化出来，是核心的，也不难。
但我们可以了解下 _YYCGRectFitWithContentMode中 mode 的处理

static CGRect _YYCGRectFitWithContentMode(CGRect rect, CGSize size, UIViewContentMode mode) {
    rect = CGRectStandardize(rect);
    size.width = size.width < 0 ? -size.width : size.width;
    size.height = size.height < 0 ? -size.height : size.height;
    CGPoint center = CGPointMake(CGRectGetMidX(rect), CGRectGetMidY(rect));
    switch (mode) {
        case UIViewContentModeScaleAspectFit:
        case UIViewContentModeScaleAspectFill: {
            if (rect.size.width < 0.01 || rect.size.height < 0.01 ||
                size.width < 0.01 || size.height < 0.01) {
                rect.origin = center;
                rect.size = CGSizeZero;
            } else {
                CGFloat scale;
                if (mode == UIViewContentModeScaleAspectFit) {
                    if (size.width / size.height < rect.size.width / rect.size.height) {
                        scale = rect.size.height / size.height;
                    } else {
                        scale = rect.size.width / size.width;
                    }
                } else {
                    if (size.width / size.height < rect.size.width / rect.size.height) {
                        scale = rect.size.width / size.width;
                    } else {
                        scale = rect.size.height / size.height;
                    }
                }
                size.width *= scale;
                size.height *= scale;
                rect.size = size;
                rect.origin = CGPointMake(center.x - size.width * 0.5, center.y - size.height * 0.5);
            }
        } break;
        case UIViewContentModeCenter: {
            rect.size = size;
            rect.origin = CGPointMake(center.x - size.width * 0.5, center.y - size.height * 0.5);
        } break;
        case UIViewContentModeTop: {
            rect.origin.x = center.x - size.width * 0.5;
            rect.size = size;
        } break;
        case UIViewContentModeBottom: {
            rect.origin.x = center.x - size.width * 0.5;
            rect.origin.y += rect.size.height - size.height;
            rect.size = size;
        } break;
        case UIViewContentModeLeft: {
            rect.origin.y = center.y - size.height * 0.5;
            rect.size = size;
        } break;
        case UIViewContentModeRight: {
            rect.origin.y = center.y - size.height * 0.5;
            rect.origin.x += rect.size.width - size.width;
            rect.size = size;
        } break;
        case UIViewContentModeTopLeft: {
            rect.size = size;
        } break;
        case UIViewContentModeTopRight: {
            rect.origin.x += rect.size.width - size.width;
            rect.size = size;
        } break;
        case UIViewContentModeBottomLeft: {
            rect.origin.y += rect.size.height - size.height;
            rect.size = size;
        } break;
        case UIViewContentModeBottomRight: {
            rect.origin.x += rect.size.width - size.width;
            rect.origin.y += rect.size.height - size.height;
            rect.size = size;
        } break;
        case UIViewContentModeScaleToFill:
        case UIViewContentModeRedraw:
        default: {
            rect = rect;
        }
    }
    return rect;
}

同时也将其 mode 了解了一次

typedef NS_ENUM(NSInteger, UIViewContentMode) {
    UIViewContentModeScaleToFill, // 拉伸自适应填满整个视图
    UIViewContentModeScaleAspectFit,  // 自适应比例大小显示
    UIViewContentModeScaleAspectFill,    // 原始大小显示
    UIViewContentModeRedraw,     // 尺寸改变时重绘      
    UIViewContentModeCenter,              
    UIViewContentModeTop,
    UIViewContentModeBottom,
    UIViewContentModeLeft,   //中间贴左
    UIViewContentModeRight,  //中间贴右
    UIViewContentModeTopLeft,  // 贴左上
    UIViewContentModeTopRight, // 贴右上
    UIViewContentModeBottomLeft,  // 贴左下
    UIViewContentModeBottomRight,   // 贴右下
};

此处图文并茂，可以更好的理解，看看这个 UIViewContentMode各模式的含义和效果
我们在处理图片的时候,可能会出现拉伸变形的情况,根据UIViewContentMode的属性,来控制图片也是很必要的。