1. UIImageView整体拉伸

图片的几种模式UIImageView-contentMode

  /**
         UIViewContentModeScaleToFill : 图片拉伸至填充整个UIImageView（图片可能会变形）      
         UIViewContentModeScaleAspectFit : 按照图片的原来宽高比进行缩放(一定要看到整张图片)       
         UIViewContentModeScaleAspectFill : 图片拉伸至 图片的宽度等于UIImageView的宽度 或者 图片的高度等于UIImageView的高度 为止    
         UIViewContentModeRedraw : 调用了setNeedsDisplay方法时，就会将图片重新渲染        
         UIViewContentModeCenter : 居中显示
         UIViewContentModeTop,
         UIViewContentModeBottom, 
         UIViewContentModeLeft,    
         UIViewContentModeRight,   
         UIViewContentModeTopLeft,  
         UIViewContentModeTopRight,                    
         UIViewContentModeBottomLeft,
         UIViewContentModeBottomRight,

经验规律：
1.凡是带有Scale单词的，图片都会拉伸
2.凡是带有Aspect单词的，图片都会保持原来的宽高比，图片不会变形

图片讲解：

7.png

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，一条边等于最长边，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;
 }