SDWebImage源码解析
SDWebImage源码解析
小春0618 发表于3年前
SDWebImage源码解析
  • 发表于 3年前
  • 阅读 5406
  • 收藏 7
  • 点赞 0
  • 评论 0

标题:腾讯云 新注册用户域名抢购1元起>>>   

摘要: 从网上看到的,讲的不错 ,留存

在开发项目的过程中会用到很多第三方库,比如AFNetWorking,SDWebImage,FMDB等,但一直都没去好好的研究一下,最近刚好项目不是太紧,闲下来可以给自己充充电,先研究一下SDWebImage的底层实现,源码地址:SDWebImage 

先介绍一下SDWebImage,我们使用较多的是它提供的UIImageView分类,支持从远程服务器下载并缓存图片。自从iOS5.0开始,NSURLCache也可以处理磁盘缓存,那么SDWebImage的优势在哪?首先NSURLCache是缓存原始数据(raw data)到磁盘或内存,因此每次使用的时候需要将原始数据转换成具体的对象,如UIImage等,这会导致额外的数据解析以及内存占用等,而SDWebImage则是缓存UIImage对象在内存,缓存在NSCache中,同时直接保存压缩过的图片到磁盘中;还有一个问题是当你第一次在UIImageView中使用image对象的时候,图片的解码是在主线程中运行的!而SDWebImage会强制将解码操作放到子线程中。下图是SDWebImage简单的类图关系:

下面从UIImageView的图片加载开始看起,Let's go! 首先我们在给UIImageView设置图片的时候会调用方法: -

(void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder;

其中url为远程图片的地址,而placeholder为预显示的图片。 其实还可以添加一些额外的参数,比如图片选项

SDWebImageOptions

typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
    SDWebImageRetryFailed = 1

 一般使用的是SDWebImageRetryFailed | SDWebImageLowPriority,

下面看看具体的函数调用:

- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock 
{
    [self sd_cancelCurrentImageLoad];//取消正在下载的操作
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);//关联该view对应的图片URL  
   /*...*/ 
    if (url) {
        __weak UIImageView *wself = self;//防止retain cricle
        //由SDWebImageManager负责图片的获取
        id  operation = [SDWebImageManager.sharedManager downloadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
              /*获取图片到主线层显示*/ 
        }];
        [self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
    } 
}

 

可以看出图片是从服务端、内存或者硬盘获取是由SDWebImageManager管理的,这个类有几个重要的属性: 

@property (strong, nonatomic, readwrite) SDImageCache *imageCache;//负责管理cache,涉及内存缓存和硬盘保存
@property (strong, nonatomic, readwrite) SDWebImageDownloader *imageDownloader;//负责从网络下载图片
@property (strong, nonatomic) NSMutableArray *runningOperations;//包含所有当前正在下载的操作对象

manager会根据URL先去imageCache中查找对应的图片,如果没有在使用downloader去下载,并在下载完成缓存图片到imageCache,接着看实现:

 - (id )downloadImageWithURL:(NSURL *)url options:(SDWebImageOptions)options
                                        progress:(SDWebImageDownloaderProgressBlock)progressBlock
                                       completed:(SDWebImageCompletionWithFinishedBlock)completedBlock
 {
     /*...*/
    //根据URL生成对应的key,没有特殊处理为[url absoluteString];
    NSString *key = [self cacheKeyForURL:url];
    //去imageCache中寻找图片
    operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType) 
    {
       /*...*/
       //如果图片没有找到,或者采用的SDWebImageRefreshCached选项,则从网络下载
        if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
                dispatch_main_sync_safe(^{
                  //如果图片找到了,但是采用的SDWebImageRefreshCached选项,通知获取到了图片,并再次从网络下载,使NSURLCache重新刷新
                     completedBlock(image, nil, cacheType, YES, url);
                });
            }
            /*下载选项设置*/ 
            //使用imageDownloader开启网络下载
            id  subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished) {
                /*...*/
               if (downloadedImage && finished) {
                     //下载完成后,先将图片保存到imageCache中,然后主线程返回
                     [self.imageCache storeImage:downloadedImage recalculateFromImage:NO imageData:data forKey:key toDisk:cacheOnDisk];
                        }
                     dispatch_main_sync_safe(^{
                            if (!weakOperation.isCancelled) {
                                completedBlock(downloadedImage, nil, SDImageCacheTypeNone, finished, url);
                            }
                        });
                    }
                }
          /*...*/
       }
        else if (image) {
          //在cache中找到图片了,直接返回
            dispatch_main_sync_safe(^{
                if (!weakOperation.isCancelled) {
                    completedBlock(image, nil, cacheType, YES, url);
                }
            });
        }
    }];
    return operation;
}


下面先看downloader从网络下载的过程,下载是放在NSOperationQueue中进行的,默认maxConcurrentOperationCount为6,timeout时间为15s: 

- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageDownloaderCompletedBlock)completedBlock {
    __block SDWebImageDownloaderOperation *operation;
    __weak SDWebImageDownloader *wself = self;
    /*...*/
    //防止NSURLCache和SDImageCache重复缓存
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy :NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];
    request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
    request.HTTPShouldUsePipelining = YES;
    request.allHTTPHeaderFields = wself.HTTPHeaders;//设置http头部
    //SDWebImageDownloaderOperation派生自NSOperation,负责图片下载工作
    operation = [[SDWebImageDownloaderOperation alloc] initWithRequest:request
                                                          options:options
                                                         progress:^(NSInteger receivedSize, NSInteger expectedSize) {}
                                                        completed:^(UIImage *image, NSData *data, NSError *error, BOOL finished) {}
                                                        cancelled:^{}];
    operation.shouldDecompressImages = wself.shouldDecompressImages;//是否需要解码
    if (wself.username && wself.password) {
            operation.credential = [NSURLCredential credentialWithUser:wself.username password:wself.password persistence:NSURLCredentialPersistenceForSession];
    }
    if (options & SDWebImageDownloaderHighPriority) {
            operation.queuePriority = NSOperationQueuePriorityHigh;
        } else if (options & SDWebImageDownloaderLowPriority) {
            operation.queuePriority = NSOperationQueuePriorityLow;
    }
        [wself.downloadQueue addOperation:operation];
        if (wself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
            // 如果下载顺序是后面添加的先运行
            [wself.lastAddedOperation addDependency:operation];
            wself.lastAddedOperation = operation;
        }
    }];
    return operation;
}

SDWebImageDownloaderOperation派生自NSOperation,通过NSURLConnection进行图片的下载,为了确保能够处理下载的数据,需要在后台运行runloop:

- (void)start {
  /*...*/
#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
        //开启后台下载
        if ([self shouldContinueWhenAppEntersBackground]) {
            __weak __typeof__ (self) wself = self;
            self.backgroundTaskId = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
                __strong __typeof (wself) sself = wself;
                if (sself) {
                    [sself cancel];
                    [[UIApplication sharedApplication] endBackgroundTask:sself.backgroundTaskId];
                    sself.backgroundTaskId = UIBackgroundTaskInvalid;
                }
            }];
        }
#endif
        self.executing = YES;
        self.connection = [[NSURLConnection alloc] initWithRequest:self.request delegate:self startImmediately:NO];
    }
    [self.connection start];

    if (self.connection) {
        if (self.progressBlock) {
            self.progressBlock(0, NSURLResponseUnknownLength);
        }
       //在主线程发通知,这样也保证在主线程收到通知
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self];
        });
       CFRunLoopRun();//在默认模式下运行当前runlooprun,直到调用CFRunLoopStop停止运行
        if (!self.isFinished) {
            [self.connection cancel];
            [self connection:self.connection didFailWithError:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorTimedOut userInfo:@{NSURLErrorFailingURLErrorKey : self.request.URL}]];
        }
    }
#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
    if (self.backgroundTaskId != UIBackgroundTaskInvalid) {
        [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTaskId];
        self.backgroundTaskId = UIBackgroundTaskInvalid;
    }
#endif
}

下载过程中,在代理 - (void)connection:(NSURLConnection )connection didReceiveData:(NSData )data中将接收到的数据保存到NSMutableData中,[self.imageData appendData:data],下载完成后在该线程完成图片的解码,并在完成的completionBlock中进行imageCache的缓存:

- (void)connectionDidFinishLoading:(NSURLConnection *)aConnection {
    SDWebImageDownloaderCompletedBlock completionBlock = self.completedBlock;
    @synchronized(self) {
        CFRunLoopStop(CFRunLoopGetCurrent());//停止当前对runloop
        /*...*/
        if (completionBlock) {
            /*...*/
            UIImage *image = [UIImage sd_imageWithData:self.imageData];
            NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
            image = [self scaledImageForKey:key image:image];

              // Do not force decoding animated GIFs
             if (!image.images) {
                 if (self.shouldDecompressImages) {
                    image = [UIImage decodedImageWithImage:image];//图片解码
                }
            }
            if (CGSizeEqualToSize(image.size, CGSizeZero)) {
                completionBlock(nil, nil, [NSError errorWithDomain:@"SDWebImageErrorDomain" code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}], YES);
            }
            else {
                completionBlock(image, self.imageData, nil, YES);
            }
        }
    }
    self.completionBlock = nil;
    [self done];
}

SDWebImageCache管理着SDWebImage的缓存,其中内存缓存采用NSCache,同时会创建一个ioQueue负责对硬盘的读写,并且会添加观察者,在收到内存警告、关闭或进入后台时完成对应的处理:

- (id)init {
     _memCache = [[NSCache alloc] init];
     _ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL);
     //收到内存警告时,清除NSCache:[self.memCache removeAllObjects];
     [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(clearMemory)
                                              name:UIApplicationDidReceiveMemoryWarningNotification
                                                   object:nil];
      //程序关闭时,会对硬盘文件做一些处理
      [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(cleanDisk)
                                                     name:UIApplicationWillTerminateNotification
                                                   object:nil];
      //程序进入后台时,也会进行硬盘文件处理
      [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(backgroundCleanDisk)
                                                     name:UIApplicationDidEnterBackgroundNotification
                                                   object:nil];
}

查询图片

每次向SDWebImageCache索取图片的时候,会先根据图片URL对应的key值先检查内存中是否有对应的图片,如果有则直接返回;如果没有则在ioQueue中去硬盘中查找,其中文件名是是根据URL生成的MD5值,找到之后先将图片缓存在内存中,然后在把图片返回:

- (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock {
    /*...*/
    // 首先查找内存缓存
    UIImage *image = [self imageFromMemoryCacheForKey:key];
    if (image) {
        doneBlock(image, SDImageCacheTypeMemory);
        return nil;
    }
    //硬盘查找
    NSOperation *operation = [NSOperation new];
    dispatch_async(self.ioQueue, ^{
        //创建自动释放池,内存及时释放
        @autoreleasepool {
            UIImage *diskImage = [self diskImageForKey:key];
            if (diskImage) {
                CGFloat cost = diskImage.size.height * diskImage.size.width * diskImage.scale * diskImage.scale;
                //缓存到NSCache中
                [self.memCache setObject:diskImage forKey:key cost:cost];
            }
            dispatch_async(dispatch_get_main_queue(), ^{
                doneBlock(diskImage, SDImageCacheTypeDisk);
            });
        }
    });
    return operation;
}

在硬盘查询的时候,会在后台将NSData转成UIImage,并完成相关的解码工作:

- (UIImage *)diskImageForKey:(NSString *)key {
    NSData *data = [self diskImageDataBySearchingAllPathsForKey:key];
    if (data) {
        UIImage *image = [UIImage sd_imageWithData:data];
        image = [self scaledImageForKey:key image:image];
        if (self.shouldDecompressImages) {
            image = [UIImage decodedImageWithImage:image];
        }
        return image;
    }
    else {
        return nil;
    }
}

保存图片

当下载完图片后,会先将图片保存到NSCache中,并把图片像素大小作为该对象的cost值,同时如果需要保存到硬盘,会先判断图片的格式,PNG或者JPEG,并保存对应的NSData到缓存路径中,文件名为URL的MD5值:

- (NSString *)cachedFileNameForKey:(NSString *)key {
    //根据key生成对应的MD5值作为文件名
    const char *str = [key UTF8String];
    if (str == NULL) {
        str = "";
    }
    unsigned char r[CC_MD5_DIGEST_LENGTH];
    CC_MD5(str, (CC_LONG)strlen(str), r);
    NSString *filename = [NSString stringWithFormat:@"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",
                                                    r[0], r[1], r[2], r[3], r[4], r[5], r[6], r[7], r[8], r[9], r[10], r[11], r[12], r[13], r[14], r[15]];

    return filename;
}
- (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk 
{
   //保存到NSCache,cost为像素值
   [self.memCache setObject:image forKey:key cost:image.size.height * image.size.width * image.scale * image.scale];
   if (toDisk) {
       dispatch_async(self.ioQueue, ^{
           NSData *data = imageData;
           if (image && (recalculate || !data)) {
              //判断图片格式
               BOOL imageIsPng = YES;
               // 查看imagedata的前缀是否是PNG的前缀格式
               if ([imageData length] >= [kPNGSignatureData length]) {
                   imageIsPng = ImageDataHasPNGPreffix(imageData);
               }
               if (imageIsPng) {
                   data = UIImagePNGRepresentation(image);
               }
               else {
                   data = UIImageJPEGRepresentation(image, (CGFloat)1.0);
               }
           }
           if (data) {
               if (![_fileManager fileExistsAtPath:_diskCachePath]) {
                   [_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
               }
               //保存data到指定的路径中
               [_fileManager createFileAtPath:[self defaultCachePathForKey:key] contents:data attributes:nil];
           }
       });
   }
}

硬盘文件的管理

在程序退出或者进入后台时,会出图片文件进行管理,具体的策略:

  • 清除过期的文件,默认一星期 

  • 如果设置了最大缓存,并且当前缓存的文件超过了这个限制,则删除最旧的文件,直到当前缓存文件的大小为最大缓存大小的一半

- (void)cleanDiskWithCompletionBlock:(SDWebImageNoParamsBlock)completionBlock {
   dispatch_async(self.ioQueue, ^{
       NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
       NSArray *resourceKeys = @[NSURLIsDirectoryKey, NSURLContentModificationDateKey, NSURLTotalFileAllocatedSizeKey];

       // This enumerator prefetches useful properties for our cache files.
       NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
                                                  includingPropertiesForKeys:resourceKeys
                                                                     options:NSDirectoryEnumerationSkipsHiddenFiles
                                                                errorHandler:NULL];

       NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.maxCacheAge];
       NSMutableDictionary *cacheFiles = [NSMutableDictionary dictionary];
       NSUInteger currentCacheSize = 0;

       // Enumerate all of the files in the cache directory.  This loop has two purposes:
       //
       //  1. Removing files that are older than the expiration date.
       //  2. Storing file attributes for the size-based cleanup pass.
       NSMutableArray *urlsToDelete = [[NSMutableArray alloc] init];
       for (NSURL *fileURL in fileEnumerator) {
           NSDictionary *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:NULL];

           // Skip directories.
           if ([resourceValues[NSURLIsDirectoryKey] boolValue]) {
               continue;
           }

           // Remove files that are older than the expiration date;
           NSDate *modificationDate = resourceValues[NSURLContentModificationDateKey];
           if ([[modificationDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
               [urlsToDelete addObject:fileURL];
               continue;
           }

           // Store a reference to this file and account for its total size.
           NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
           currentCacheSize += [totalAllocatedSize unsignedIntegerValue];
           [cacheFiles setObject:resourceValues forKey:fileURL];
       }

       for (NSURL *fileURL in urlsToDelete) {
           [_fileManager removeItemAtURL:fileURL error:nil];
       }

       // If our remaining disk cache exceeds a configured maximum size, perform a second
       // size-based cleanup pass.  We delete the oldest files first.
       if (self.maxCacheSize > 0 && currentCacheSize > self.maxCacheSize) {
           // Target half of our maximum cache size for this cleanup pass.
           const NSUInteger desiredCacheSize = self.maxCacheSize / 2;

           // Sort the remaining cache files by their last modification time (oldest first).
           NSArray *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
                                                           usingComparator:^NSComparisonResult(id obj1, id obj2) {
                                                               return [obj1[NSURLContentModificationDateKey] compare:obj2[NSURLContentModificationDateKey]];
                                                           }];

           // Delete files until we fall below our desired cache size.
           for (NSURL *fileURL in sortedFiles) {
               if ([_fileManager removeItemAtURL:fileURL error:nil]) {
                   NSDictionary *resourceValues = cacheFiles[fileURL];
                   NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
                   currentCacheSize -= [totalAllocatedSize unsignedIntegerValue];

                   if (currentCacheSize < desiredCacheSize) {
                       break;
                   }
               }
           }
       }
       if (completionBlock) {
           dispatch_async(dispatch_get_main_queue(), ^{
               completionBlock();
           });
       }
   });
}

总结

  • 接口设计简单
    通常我们使用较多的UIImageView分类:

[self.imageView sd_setImageWithURL:[NSURL URLWithString:@"url"]
               placeholderImage:[UIImage imageNamed:@"placeholder"]];
  • 一个简单的接口将其中复杂的实现细节全部隐藏:简单就是美。

  • 采用NSCache作为内存缓

  • 耗时较长的请求,都采用异步形式,在回调函数块中处理请求结果

  • NSOperation和NSOperationQueue:可以取消任务处理队列中的任务,设置最大并发数,设置operation之间的依赖关系。

  • 图片缓存清理的策略

  • dispatch_barrier_sync:前面的任务执行结束后它才执行,而且它后面的任务要等它执行完成之后才会执行。

  • 使用weak self strong self 防止retain circle

  • 如果子线程进需要不断处理一些事件,那么设置一个Run Loop是最好的处理方式


共有 人打赏支持
粉丝 8
博文 40
码字总数 10552
×
小春0618
如果觉得我的文章对您有用,请随意打赏。您的支持将鼓励我继续创作!
* 金额(元)
¥1 ¥5 ¥10 ¥20 其他金额
打赏人
留言
* 支付类型
微信扫码支付
打赏金额:
已支付成功
打赏金额: