ios UITableView 异步加载图片并防止错位

叁歲伎倆 2022-08-09 09:44 154阅读 0赞

原文连接：[http://www.cnblogs.com/lesliefang/p/3906708.html][http_www.cnblogs.com_lesliefang_p_3906708.html]

本人在使用过程，觉得对于界面刷新部分需要修改。如果列表下载大量的图片，频繁调用主线程就会出现问题，建议在最后block回调中比对成功后再调用主线程刷新界面。

**修改前代码如下：**

- (void)downloadImageWithURL:(NSURL *)url complete:(ImageDownloadedBlock)completeBlock{
        LeslieImageCache *imageCache = [LeslieImageCache sharedCache];
        NSString *imageUrl = [url absoluteString];
        UIImage *image = [imageCache getImageFromMemoryForkey:imageUrl];
        // 先从内存中取
        if (image) {
            if (completeBlock) {
                NSLog(@"image exists in memory");
                completeBlock(image,nil,url);
            }
            
            return;
        }
        
        // 再从文件中取
        image = [imageCache getImageFromFileForKey:imageUrl];
        if (image) {
            if (completeBlock) {
                NSLog(@"image exists in file");
                completeBlock(image,nil,url);
            }
            
            // 重新加入到 NSCache 中
            [imageCache cacheImageToMemory:image forKey:imageUrl];
            
            return;
        }
        
        // 内存和文件中都没有再从网络下载
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSError * error;
            NSData *imgData = [NSData dataWithContentsOfURL:url options:NSDataReadingMappedIfSafe error:&error];
            
            dispatch_async(dispatch_get_main_queue(), ^{
         
                UIImage *image = [UIImage imageWithData:imgData];
                
                if (image) {
                    // 先缓存图片到内存
                    [imageCache cacheImageToMemory:image forKey:imageUrl];
                    
                    // 再缓存图片到文件系统
                    NSString *extension = [[imageUrl substringFromIndex:imageUrl.length-3] lowercaseString];
                    NSString *imageType = @"jpg";
                    
                    if ([extension isEqualToString:@"jpg"]) {
                        imageType = @"jpg";
                    }else{
                        imageType = @"png";
                    }
                    
                    [imageCache cacheImageToFile:image forKey:imageUrl ofType:imageType];
                }
                
                if (completeBlock) {
                    completeBlock(image,error,url);
                }
            });
        });
    }

- (void)setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder{
        // 给  ImageView 设置 tag, 指向当前 url
        self.tag = [url absoluteString];
        
        // 预设一个图片，可以为 nil
        // 主要是为了清除由于复用以前可能存在的图片
        self.image = placeholder;
        
        if (url) {
            // 异步下载图片
            LeslieAsyncImageDownloader *imageLoader = [LeslieAsyncImageDownloader sharedImageLoader];
            [imageLoader downloadImageWithURL:url
                                     complete:^(UIImage *image, NSError *error, NSURL *imageURL) {
                                         // 通过 tag 保证图片被正确的设置
                                         if (image && [self.tag isEqualToString:[imageURL absoluteString]]) {
                                             self.image = image;
                                         }else{
                                             NSLog(@"error when download:%@", error);
                                         }
                                     }];
        }
    }

**修改后代码如下：**

- (void)downloadImageWithURL:(NSURL *)url complete:(ImageDownloadedBlock)completeBlock{
        LeslieImageCache *imageCache = [LeslieImageCache sharedCache];
        NSString *imageUrl = [url absoluteString];
        UIImage *image = [imageCache getImageFromMemoryForkey:imageUrl];
        // 先从内存中取
        if (image) {
            if (completeBlock) {
                NSLog(@"image exists in memory");
                completeBlock(image,nil,url);
            }
            
            return;
        }
        
        // 再从文件中取
        image = [imageCache getImageFromFileForKey:imageUrl];
        if (image) {
            if (completeBlock) {
                NSLog(@"image exists in file");
                completeBlock(image,nil,url);
            }
            
            // 重新加入到 NSCache 中
            [imageCache cacheImageToMemory:image forKey:imageUrl];
            
            return;
        }
        
        // 内存和文件中都没有再从网络下载
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSError * error;
            NSData *imgData = [NSData dataWithContentsOfURL:url options:NSDataReadingMappedIfSafe error:&error];
            
            //dispatch_async(dispatch_get_main_queue(), ^{
         
                UIImage *image = [UIImage imageWithData:imgData];
                
                if (image) {
                    // 先缓存图片到内存
                    [imageCache cacheImageToMemory:image forKey:imageUrl];
                    
                    // 再缓存图片到文件系统
                    NSString *extension = [[imageUrl substringFromIndex:imageUrl.length-3] lowercaseString];
                    NSString *imageType = @"jpg";
                    
                    if ([extension isEqualToString:@"jpg"]) {
                        imageType = @"jpg";
                    }else{
                        imageType = @"png";
                    }
                    
                    [imageCache cacheImageToFile:image forKey:imageUrl ofType:imageType];
                }
                
                if (completeBlock) {
                    completeBlock(image,error,url);
                }
            //});
        });
    }

dispatch_async(dispatch_get_main_queue(), ^{
                                                   	self.image = image;

});                                      }else{                                         NSLog(@"error when download:%@", error);                                     }                                 }];    }}

[http_www.cnblogs.com_lesliefang_p_3906708.html]: http://www.cnblogs.com/lesliefang/p/3906708.html