iOS 动态对UIImageView加载到的图片进行偏移裁剪处理

关键点：图片偏移重绘、图片重绘时机处理

图片偏移重绘问题

原因：UIImageView的现有填充模式已经不能满足应用需求，需自定义偏移量结合填充模式实现效果
方法：按需求样式重绘，设置偏移量比例，偏移阈值，对图片进行裁剪。

重绘时机问题

选择：UIImageView自带的setImage方法，这是获取图片和进行重绘渲染的关口，从这里处理最恰当
方法：利用iOS底层方法，在运行时(runtime)进行方法交换，属性绑定等。

补充

因为运行时编程代码复杂冗余多，可以考虑运用第三方库方法解决这个问题，此处选择德国软件工程师Philipp Rentzsch开发的一个开源库[JRSwizzle](https://github.com/rentzsch/jrswizzle)辅助完成。
使用cocoapod集成JRSwizzle库

核心代码：

宏定义再次简化运行时代码

#import <JRSwizzle/JRSwizzle.h>
/* 方法交换宏*/
#define MYSwizzle(prefix, SEL)
do {
NSError *error = nil;
[[self class] jr_swizzleMethod: @selector(SEL)
withMethod: @selector(prefix####SEL)error:&error];
if (error) {NSLogError(@ "%@ swizzle %@ with %@ failed.",
NSStringFromClass(self),
NSStringFromSelector(@selector(SEL)),
NSStringFromSelector(@selector(prefix####SEL)));}
} while (0)

/添加属性宏/
#define MY_ASSOCIATE_OBJECT(getter, setter, association, type)
- (void)setter: (type)object {
[self willChangeValueForKey: @#getter];
objc_setAssociatedObject(self, cmd, object, OBJC_ASSOCIATION##association);
[self didChangeValueForKey: @#getter];
}
-(type)getter {
return objc_getAssociatedObject(self, @selector(setter:));
}
定义好红代码之后，写一个UIImageView的分类方法，记录加载原始图片imgOriginal,以及便宜设置类MYImaeViewOffset;
首先定义偏移重绘配置类MYImaeViewOffset：

/**
- 用于判断和控制UIImageView设置图片时候的偏移量的数据类。
- ratioOffsetX x轴方向的偏移量比例.
- ratioOffsetY y轴方向的偏移量比例.
- ratioThreshold 进行偏移重绘的阈值. 实值为 imgv_w / imgv_h.
- 如果设定ratioThreshold = 1.8，而当前UIImageView.size = {100, 50}
- 则实值为 2。此时超过偏移阈值，需对图片进行偏移裁剪重绘。
- @note ignoreThreshold 注意如果此值为YES，则忽略阈值，需要重绘。
  */
typedef NS_OPTIONS(NSUInteger, MYImageViewAlignmentMask) {
MYImageViewAlignmentMaskCenter = 1 << 0,
```
  MYImageViewAlignmentMaskLeft       = 1 << 1,
  MYImageViewAlignmentMaskRight      = 1 << 2,
  MYImageViewAlignmentMaskTop        = 1 << 3,
  MYImageViewAlignmentMaskBottom     = 1 << 4,
```
};

@interface MYImageViewOffset : NSObject
@property (nonatomic,assign) CGFloat ratioOffsetX;
@property (nonatomic,assign) CGFloat ratioOffsetY;
@property (nonatomic,assign) CGFloat ratioThreshold;//阈值
@property (nonatomic,assign) BOOL ignoreThreshold;//忽略阈值(默认阈值1.3)长图w/h >= 1.3
@property (nonatomic,assign) CGFloat cornerRadius;//圆角半径
@property (nonatomic,assign) MYImageViewAlignmentMask alignment;//对齐方式。
- (instancetype)offsetRatio:(CGFloat)ratioX ratioY:(CGFloat)ratioY;
- (instancetype)offsetRatio:(CGFloat)ratioX ratioY:(CGFloat)ratioY cornerRadius:(CGFloat)cornerRadius;
- (instancetype)offsetRatio:(CGFloat)ratioX ratioY:(CGFloat)ratioY cornerRadius:(CGFloat)cornerRadius align:(MYImageViewAlignmentMask)aligment;
  @end
@implementation MYImageViewOffset
- (instancetype)offsetRatio:(CGFloat)ratioX ratioY:(CGFloat)ratioY {
  return [MYImageViewOffset offsetRatio:ratioX ratioY:ratioY cornerRadius:0];
  }
- (instancetype)offsetRatio:(CGFloat)ratioX ratioY:(CGFloat)ratioY cornerRadius:(CGFloat)cornerRadius {
  MYImageViewOffset *offset = [MYImageViewOffset offsetRatio:ratioX ratioY:ratioY cornerRadius:cornerRadius align:MYImageViewAlignmentMaskCenter];
  return offset;
  }
- (instancetype)offsetRatio:(CGFloat)ratioX ratioY:(CGFloat)ratioY cornerRadius:(CGFloat)cornerRadius align:(MYImageViewAlignmentMask)aligment {
  MYImageViewOffset *offset = [MYImageViewOffset new];
  offset.ratioOffsetX = ratioX;
  offset.ratioOffsetY = ratioY;
  offset.ratioThreshold = 1.3;//默认阈值。
  offset.cornerRadius = cornerRadius;
  offset.alignment = aligment;
  return offset;
  }
  @end
然后实现UIImageVIew分类方法:

@interface UIImageView (ImageOffset)
@property (nonatomic, strong) UIImage *imgOriginal;
@property (nonatomic, strong) MYImageViewOffset *offset;
- (void)reloadOffsetJudge;
  @end
@implementation UIImageView (ImageOffset)
MY_ASSOCIATE_OBJECT(offset, setOffset, RETAIN, MYImageViewOffset *);
MY_ASSOCIATE_OBJECT(imgOriginal, setImgOriginal, RETAIN, UIImage *);
- (void)load {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
  MYSwizzle(MY, setImage:);
  MYSwizzle(MY, layoutSubviews);
  });
  }
- (void)MY_layoutSubviews {
  if ([self needImageRedrawJudge]) {
  UIImage *imgOffset = [self offsetImage:self.imgOriginal];
  if (imgOffset) {
  [self MY_setImage:imgOffset];
  }
  }
  [self MY_layoutSubviews];
  }
- (void)MY_setImage:(UIImage *)image {
  [self setImgOriginal:image];
  UIImage *destImg = nil;
  if ([self needImageRedrawJudge] && image) {
  destImg = [self offsetImage:image];
  }else{
  destImg = image;
  }
  [self MY_setImage:destImg];
  }
- (void)reloadOffsetJudge {
  if ([self needImageRedrawJudge]) {
  NSLogInfo(@"-- layout re");
  UIImage *imgOffset = [self offsetImage:self.imgOriginal];
  if (imgOffset) {
  [self MY_setImage:imgOffset];
  }
  }
  }
- (UIImage *)offsetImage:(UIImage *)imgOri {
  if (!imgOri) return nil;
  CGFloat offX = self.offset.ratioOffsetX * imgOri.size.width;
  CGFloat offY = self.offset.ratioOffsetY * imgOri.size.height;
  CGRect cutRect = CGRectMake(offX, offY, imgOri.size.width, imgOri.size.height);
  UIImage *newImage = [self image:imgOri cropToRect:cutRect cornerRadius:self.offset.cornerRadius];
  return newImage;
  }
- (BOOL)needImageRedrawJudge {
  if(self.offset.cornerRadius){
  return YES;
  }
  CGSize imgSize = self.imgOriginal.size;
  if (self.width <= 0 || self.height <= 0) return NO;
  if (imgSize.width / imgSize.height >= 1){
  //如果图片本身是一张横图(w>=h)则不需进行重绘
  return NO;
  }
  if (self.offset) {
  CGFloat ratioThreshold = self.width / self.height;
  if (ratioThreshold > self.offset.ratioThreshold) {
  //图片视图控件的阈值判断(视图控件UIImageView是一个通过了阈值的横框，方可进行重绘。)
  return YES;
  }
  }
  return NO;
  }
  /*
  图片剪裁接口
- 1.rect超过image.size则返回原图
- 2.rect可以是size的任何一个子集，比如取中间半区域 CG
  */
- (UIImage *)image:(UIImage *)imgOri cropToRect:(CGRect)rect cornerRadius:(CGFloat)cornerRadius{
  rect.origin.x *= imgOri.scale;
  rect.origin.y *= imgOri.scale;
  rect.size.width *= imgOri.scale;
  rect.size.height *= imgOri.scale;
  UIImage *outputImage = imgOri;
  if(rect.size.width > 0 && rect.size.height > 0){
  CGImageRef imageRef = CGImageCreateWithImageInRect(imgOri.CGImage, rect);
  UIImage *image = [UIImage imageWithCGImage:imageRef scale:imgOri.scale orientation:imgOri.imageOrientation];
  outputImage = image;
  CGImageRelease(imageRef);
  }
  if(self.width && self.height){
  if(cornerRadius > 0){
  CGFloat ratioCR = cornerRadius * (outputImage.size.width / self.width);
  UIGraphicsBeginImageContextWithOptions(outputImage.size, NO, outputImage.scale);
  CGContextRef context = UIGraphicsGetCurrentContext();
  CGRect imgRect = CGRectMake(0, 0, outputImage.size.width, outputImage.size.height);
```
      CGContextSaveGState(context);
      [[UIBezierPath bezierPathWithRoundedRect:imgRect cornerRadius:ratioCR] addClip];
      [outputImage drawInRect:imgRect];
      CGContextRestoreGState(context);
      UIImage *roundedImage = UIGraphicsGetImageFromCurrentImageContext();
      UIGraphicsEndImageContext();
      outputImage =  roundedImage;
  }
  
  if(self.offset.alignment != MYImageViewAlignmentMaskCenter){
      CGFloat logicH = (outputImage.size.height * self.width)/outputImage.size.width;
      CGFloat logicW = (outputImage.size.width * self.height)/outputImage.size.height;
      CGFloat offsetH = (self.height - logicH);
      CGFloat offsetV = (self.width - logicW);
      BOOL needRedraw = NO;
      if(offsetH > 0){ //横图（只有上下偏移）
          offsetV = 0;
          logicW = self.width;
          if(self.offset.alignment == MYImageViewAlignmentMaskTop){
              offsetH = 0;
              needRedraw = YES;
          }else if (self.offset.alignment == MYImageViewAlignmentMaskBottom){
              needRedraw = YES;
          }
      }
      if (offsetV > 0){ //竖图（只有左右偏移）
          logicH = self.height;
          offsetH = 0;
          if(self.offset.alignment == MYImageViewAlignmentMaskLeft){
              offsetV = 0;
              needRedraw = YES;
          }else if (self.offset.alignment == MYImageViewAlignmentMaskRight){
              needRedraw = YES;
          }
      }
      if(needRedraw){
          UIGraphicsBeginImageContextWithOptions(self.size, NO, 0.0);
          [outputImage drawInRect:CGRectMake(offsetV, offsetH, logicW, logicH)];
          UIImage *resizedImage = UIGraphicsGetImageFromCurrentImageContext();
          UIGraphicsEndImageContext();
          outputImage =  resizedImage;
      }
  }
```
  }
  return outputImage;
  }
  @end

补充说明：

说明

利用swizzle方法交换setImage 和layoutSubviews方法实现，它们将风别走向MY_setImage:和方法My_layoutSubviews从中获取到imgOriginal之后，再判断是否要进行重绘(needImageRedrawJudge),如果要进行重绘，重绘之后得到图片对象 destImg|imgOffset，这时候调用MY_setImage:方法，根据方法交换的逻辑，实际上就是调用了UIImageView的setImage:方法，从而实现拦截图片，并判断和重回图片在设置最终图片的效果。
至于图片重绘的具体逻辑，可根据具体需求进行灵活变换，此处重在说明如何判断需求和在拦截，以及拦截使用的方法，核心设计知识领域有：宏代码的编程简化、运行时方法、图片重绘