SDWebImage中，我们可以使用URL给ImageView赋值，实现异步赋值，保持正确并且不卡顿，现在有这样一个需求，图片是张二维码，我们需要耗时计算出二维码的图片，添加到tableView中，为了保证正确，因此二维码的生成是同步的，因此造成了tableView的卡顿，以及页面跳转的延迟。

• 为了优化该问题，这里参考并使用了SDWebImage的一些Category，做了一个中间件，用来实现异步给图片赋值，但不使用URL, 图片的生成方式就很多种多样了，可以充分发挥想想，比如cpu，gpu算出来，比如接口上获取的base64转出来，等等。

UIView+WebCacheOperation.h

这里借用了，SDWebImageView的UIView+WebCacheOperation.h, 这个分类主要是定义了异步设置图片的操作”Operation“的缓存、取消

UIImageView+AsyncImage.h

然后，新建异步设置图片的新类以代替UIImageView+WebCache.h，仅仅是异步赋值的壳子，没有涉及图片的缓存！！

代码很简单，我这就直接贴了

//
//  UIImageView+AsyncImage.h
//  mmspc
//
//  Created by admin on 2021/2/5.
//

#import <UIKit/UIKit.h>

NS_ASSUME_NONNULL_BEGIN

typedef UIImage *_Nullable(^InputBlock)(void);
typedef InputBlock _Nullable(^InputExecuteBlock)(void);
typedef void(^OutputBlock)(UIImage *);

/// 异步设置图片，使用了SDWebImage的分类，
@interface UIImageView (AsyncImage)
/// 异步设置图片，进负责设置图片，不负责图片的缓存
/// @param asyncBlock 设置图片的方法，通过传回一个返回imageView的block实现图片的设置
/// @param placeholder 默认图
/// @param complete 完成事件
- (void)ky_asyncSetImageWithBlock:(nullable InputExecuteBlock)asyncBlock
                  placeholderImage:(nullable UIImage *)placeholder
                     completeBlock:(OutputBlock)complete;
@end

NS_ASSUME_NONNULL_END

//
//  UIImageView+AsyncImage.m
//  mmspc
//
//  Created by admin on 2021/2/5.
//

#import "UIImageView+AsyncImage.h"
#import "UIView+WebCacheOperation.h"


@interface KYWebImageCombinedOperation : NSObject <SDWebImageOperation>

@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;
@property (copy, nonatomic, nullable) SDWebImageNoParamsBlock cancelBlock;
@property (strong, nonatomic, nullable) NSOperation *transformOperation;
@end

@implementation UIImageView (AsyncImage)

- (void)ky_asyncSetImageWithBlock:(nullable InputExecuteBlock)asyncBlock
                  placeholderImage:(nullable UIImage *)placeholder
                     completeBlock:(OutputBlock)complete {
    
    NSString *validOperationKey = NSStringFromClass([self class]);
    [self sd_cancelImageLoadOperationWithKey:validOperationKey];
    
    dispatch_main_async_safe(^{
        [self ky_setImage:placeholder];
    });
    
    InputBlock block = asyncBlock();
    
    __weak __typeof(self)wself = self;
    NSOperation *operation = [NSOperation new];
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        if (operation.isCancelled) {
            // do not call the completion if cancelled
            return;
        }
        
        @autoreleasepool {
            UIImage * qrImage = block();//[SGQRCodeGenerateManager generateWithLogoQRCodeData:@"扫描上方的二维码，立即申请 %@ 贷款！" logoImageName:@"wechat" logoScaleToSuperView:0.3];
            NSLog(@"%@", NSThread.currentThread);
            
            __strong __typeof (wself) sself = wself;
            //            [sself ky_removeActivityIndicator];
            if (!sself) {
                return;
            }
            if (complete) {
                complete(qrImage);
            }
            dispatch_main_async_safe(^{
                if (!sself) {
                    return;
                }
                if (qrImage) {
                    [sself ky_setImage:qrImage];
                    [sself ky_setNeedsLayout];
                } else {
                    [sself ky_setImage:placeholder];
                    [sself ky_setNeedsLayout];
                }
            });
        }
    });
    
    [self sd_setImageLoadOperation:operation forKey:validOperationKey];
    
}

- (void)ky_setImage:(UIImage *)image {

    if ([self isKindOfClass:[UIImageView class]]) {
        UIImageView *imageView = (UIImageView *)self;
        imageView.image = image;
    }
}

- (void)ky_setNeedsLayout {
    [self setNeedsLayout];
}


@end
// 模仿WebCache的Opreation，创建自己的KYWebImageCombinedOperation
@implementation KYWebImageCombinedOperation

- (void)setCancelBlock:(nullable SDWebImageNoParamsBlock)cancelBlock {
    // check if the operation is already cancelled, then we just call the cancelBlock
    if (self.isCancelled) {
        if (cancelBlock) {
            cancelBlock();
        }
        _cancelBlock = nil; // don't forget to nil the cancelBlock, otherwise we will get crashes
    } else {
        _cancelBlock = [cancelBlock copy];
    }
}

- (void)cancel {
    self.cancelled = YES;
//    if (self.cacheOperation) {
//        [self.cacheOperation cancel];
//        self.cacheOperation = nil;
//    }
    if (self.cancelBlock) {
        self.cancelBlock();
        
        // TODO: this is a temporary fix to #809.
        // Until we can figure the exact cause of the crash, going with the ivar instead of the setter
//        self.cancelBlock = nil;
        _cancelBlock = nil;
    }
}
@end

使用方式

    [self.codeImageView ky_asyncSetImageWithBlock:^InputBlock _Nullable {

        InputBlock block = ^UIImage * _Nullable {
            UIImage * image = model.qrCodeImage;
            if (!image) {
                NSString * logoName = model.codeScanBy == 0 ? @"wechat" : @"logo";
                 // 这里就是一个同步计算图片的方法，如果在主线程的话，就会引起卡顿了
                image = [SGQRCodeGenerateManager generateWithLogoQRCodeData:@"1" logoImageName:logoName logoScaleToSuperView:0.3];
            }
            return image;
        };
        return block;
    } placeholderImage:[UIImage imageNamed:@"logo"] completeBlock:^(UIImage * _Nonnull image) {
        model.qrCodeImage = image;
    }];