ios中图片拉伸的几种方式

1. UIImageView整体拉伸

UIImageView-contentMode

typedef NS_ENUM(NSInteger, UIViewContentMode) {
    UIViewContentModeScaleToFill,         // 默认 拉伸（会变形）
    UIViewContentModeScaleAspectFit,      // 等比例拉伸
    UIViewContentModeScaleAspectFill,     // 等比例填充
    UIViewContentModeRedraw,              // redraw on bounds change (这个不清楚) 
    UIViewContentModeCenter,              // 下面的就是不拉伸按位置显示了
    UIViewContentModeTop,
    UIViewContentModeBottom,
    UIViewContentModeLeft,
    UIViewContentModeRight,
    UIViewContentModeTopLeft,
    UIViewContentModeTopRight,
    UIViewContentModeBottomLeft,
    UIViewContentModeBottomRight,
};

demo：https://github.com/vitoziv/VICMAImageView

2. UIImage局部拉伸

// 按4边间距显示不拉伸的区域
- (UIImage *)resizableImageWithCapInsets:(UIEdgeInsets)capInsets NS_AVAILABLE_IOS(5_0); 
// 按2点拉伸
- (UIImage *)stretchableImageWithLeftCapWidth:(NSInteger)leftCapWidth topCapHeight:(NSInteger)topCapHeight;

- (UIImage *)resizableImageWithCapInsets:(UIEdgeInsets)capInsets resizingMode:(UIImageResizingMode)resizingMode;
// 拉伸模式
typedef NS_ENUM(NSInteger, UIImageResizingMode) {
    UIImageResizingModeTile,//进行区域复制模式拉伸
    UIImageResizingModeStretch,//进行渐变复制模式拉伸
};

3.UIImage修改大小

//内缩放，一条变等于最长边，另外一条小于等于最长边
- (UIImage *)scaleToSize:(CGSize)newSize {
    CGFloat width = self.size.width;
    CGFloat height= self.size.height;
    CGFloat newSizeWidth = newSize.width;
    CGFloat newSizeHeight= newSize.height;
    if (width <= newSizeWidth &&
        height <= newSizeHeight) {
        return self;
    }
    
    if (width == 0 || height == 0 || newSizeHeight == 0 || newSizeWidth == 0) {
        return nil;
    }
    CGSize size;
    if (width / height > newSizeWidth / newSizeHeight) {
        size = CGSizeMake(newSizeWidth, newSizeWidth * height / width);
    } else {
        size = CGSizeMake(newSizeHeight * width / height, newSizeHeight);
    }
    return [self drawImageWithSize:size];
}
- (UIImage *)drawImageWithSize: (CGSize)size {
    CGSize drawSize = CGSizeMake(floor(size.width), floor(size.height));
    UIGraphicsBeginImageContext(drawSize);
    
    [self drawInRect:CGRectMake(0, 0, drawSize.width, drawSize.height)];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
}