[iOS源碼筆記]·第三方網(wǎng)絡(luò)圖片處理框架:SDWebImage網(wǎng)絡(luò)下載及緩存管理策略

目前，雖然AFNetworking也提供了圖片下載及緩存策略，但更專業(yè)的時(shí)候塞茅，圖片加載框架一般用SDWebImage嘴办，正如主流APP(如QQ和微信)會(huì)用到。這里介紹一個(gè)查詢主流APP主要框架的網(wǎng)站:AppSight乾巧。

官方Demo演示效果

1.框架簡(jiǎn)介

1.1 使用入門

Objective-C:

#import <SDWebImage/UIImageView+WebCache.h>
...
[imageView sd_setImageWithURL:[NSURL URLWithString:@"http://www.domain.com/path/to/image.jpg"]
             placeholderImage:[UIImage imageNamed:@"placeholder.png"]];

Swift:

import SDWebImage

imageView.sd_setImage(with: URL(string: "http://www.domain.com/path/to/image.jpg"), placeholderImage: UIImage(named: "placeholder.png"))

1.2 官方文檔

這里提供一個(gè)筆者翻譯的官方文檔翻譯版：
- [iOS學(xué)習(xí)筆記]·第三方網(wǎng)絡(luò)圖片處理框架:SDWebImage（官方文檔翻譯篇）

2.官方Demo調(diào)用實(shí)例解析

調(diào)用主體：

MasterViewController.m

導(dǎo)入文件：

#import <SDWebImage/UIImageView+WebCache.h>

調(diào)用方法：

...}


- 調(diào)用實(shí)例:

(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"Cell";

static UIImage *placeholderImage = nil;
if (!placeholderImage) {
placeholderImage = [UIImage imageNamed:@"placeholder"];
}

MyCustomTableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[MyCustomTableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
}

[cell.customImageView sd_setShowActivityIndicatorView:YES];
[cell.customImageView sd_setIndicatorStyle:UIActivityIndicatorViewStyleGray];

cell.customTextLabel.text = [NSString stringWithFormat:@"Image #%ld", (long)indexPath.row];
[cell.customImageView sd_setImageWithURL:[NSURL URLWithString:_objects[indexPath.row]]
placeholderImage:placeholderImage
options:indexPath.row == 0 ? SDWebImageRefreshCached : 0];
return cell;
}

## 3.SDWebImage框架調(diào)用棧

- 框架體系

![框架體系](http://upload-images.jianshu.io/upload_images/1283539-048d262fac5403a7.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

- 方法調(diào)用棧

![方法調(diào)用棧](http://upload-images.jianshu.io/upload_images/1283539-576675a2a13bf4c9.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)


#### 3.1 第一層
- 類 

> **FLAnimatedImageView+WebCache.m**

- 方法

> ```- (void)sd_setImageWithURL:(nullable NSURL *)url placeholderImage:(nullable UIImage *)placeholder options:(SDWebImageOptions)options {
...}

實(shí)現(xiàn)

- (void)sd_setImageWithURL:(nullable NSURL *)url placeholderImage:(nullable UIImage *)placeholder options:(SDWebImageOptions)options {
    [self sd_setImageWithURL:url placeholderImage:placeholder options:options progress:nil completed:nil];
}

3.2 第二層

FLAnimatedImageView+WebCache.m

方法：

      placeholderImage:(nullable UIImage *)placeholder
               options:(SDWebImageOptions)options
              progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
             completed:(nullable SDExternalCompletionBlock)completedBlock {

...}


- 作用：

> - 根據(jù)回調(diào)圖設(shè)置自己的image屬性
- 調(diào)用下一棧層句喜，獲取回調(diào)圖

- 實(shí)現(xiàn)：

(void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock {
__weak typeof(self)weakSelf = self;
[self sd_internalSetImageWithURL:url
placeholderImage:placeholder
options:options
operationKey:nil
setImageBlock:^(UIImage *image, NSData *imageData) {

                     SDImageFormat imageFormat = [NSData sd_imageFormatForImageData:imageData];
                     if (imageFormat == SDImageFormatGIF) {
                         weakSelf.animatedImage = [FLAnimatedImage animatedImageWithGIFData:imageData];
                         weakSelf.image = nil;
                     } else {
                         weakSelf.image = image;
                         weakSelf.animatedImage = nil;
                     }
                 }
                      progress:progressBlock
                     completed:completedBlock];

}

其中:

typedef void(^SDExternalCompletionBlock)(UIImage * _Nullable image, NSError * _Nullable error, SDImageCacheType cacheType, NSURL * _Nullable imageURL);


#### 3.3 第三層 

- 類：

> **UIView+WebCache.m**

- 方法：

> ```- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                      operationKey:(nullable NSString *)operationKey
                     setImageBlock:(nullable SDSetImageBlock)setImageBlock
                          progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                         completed:(nullable SDExternalCompletionBlock)completedBlock {
...}

作用：

判斷條件用占位圖返回給上一棧層

調(diào)用下一棧層，根據(jù)Url加載圖片

實(shí)現(xiàn)：

- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                      operationKey:(nullable NSString *)operationKey
                     setImageBlock:(nullable SDSetImageBlock)setImageBlock
                          progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                         completed:(nullable SDExternalCompletionBlock)completedBlock {
    
    NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);
    [self sd_cancelImageLoadOperationWithKey:validOperationKey];
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    if (!(options & SDWebImageDelayPlaceholder)) {
        dispatch_main_async_safe(^{
            [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
        });
    }
    
    if (url) {
        // check if activityView is enabled or not
        if ([self sd_showActivityIndicatorView]) {
            [self sd_addActivityIndicator];
        }
        
        __weak __typeof(self)wself = self;
        id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager loadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
            __strong __typeof (wself) sself = wself;
            [sself sd_removeActivityIndicator];
            if (!sself) {
                return;
            }
            dispatch_main_async_safe(^{
                if (!sself) {
                    return;
                }
                if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) {
                    completedBlock(image, error, cacheType, url);
                    return;
                } else if (image) {
                    [sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock];
                    [sself sd_setNeedsLayout];
                } else {
                    if ((options & SDWebImageDelayPlaceholder)) {
                        [sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
                        [sself sd_setNeedsLayout];
                    }
                }
                if (completedBlock && finished) {
                    completedBlock(image, error, cacheType, url);
                }
            });
        }];
        [self sd_setImageLoadOperation:operation forKey:validOperationKey];
    } else {
        dispatch_main_async_safe(^{
            [self sd_removeActivityIndicator];
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
                completedBlock(nil, error, SDImageCacheTypeNone, url);
            }
        });
    }
}

3.4 第四層

類名

SDWebImageManager.m

方法

                                 options:(SDWebImageOptions)options
                                progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                               completed:(nullable SDInternalCompletionBlock)completedBlock {

...}


- 作用

 - 緩存策略利用

 > 執(zhí)行完緩存策略后沟于，回調(diào)獲取緩存圖片數(shù)據(jù):
如滿足條件(`cachedImage && options & SDWebImageRefreshCached`)咳胃，則向上一棧層返回緩存圖片。
```[self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];

下載策略利用

執(zhí)行完下載策略后旷太，回調(diào)下載圖片:
如有需要(SDWebImageTransformAnimatedImage)展懈，轉(zhuǎn)換格式后，保存到緩存

[self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];

如無必要供璧，則直接保存到緩存

[self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];

實(shí)現(xiàn)

- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
                                     options:(SDWebImageOptions)options
                                    progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                   completed:(nullable SDInternalCompletionBlock)completedBlock {
    // Invoking this method without a completedBlock is pointless
    NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");

    // Very common mistake is to send the URL using NSString object instead of NSURL. For some strange reason, Xcode won't
    // throw any warning for this type mismatch. Here we failsafe this error by allowing URLs to be passed as NSString.
    if ([url isKindOfClass:NSString.class]) {
        url = [NSURL URLWithString:(NSString *)url];
    }

    // Prevents app crashing on argument type error like sending NSNull instead of NSURL
    if (![url isKindOfClass:NSURL.class]) {
        url = nil;
    }

    __block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
    __weak SDWebImageCombinedOperation *weakOperation = operation;

    BOOL isFailedUrl = NO;
    if (url) {
        @synchronized (self.failedURLs) {
            isFailedUrl = [self.failedURLs containsObject:url];
        }
    }

    if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
        [self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil] url:url];
        return operation;
    }

    @synchronized (self.runningOperations) {
        [self.runningOperations addObject:operation];
    }
    NSString *key = [self cacheKeyForURL:url];
    
    //緩存策略
    operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
        if (operation.isCancelled) {
            [self safelyRemoveOperationFromRunning:operation];
            return;
        }

        if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
            if (cachedImage && options & SDWebImageRefreshCached) {
                // If image was found in the cache but SDWebImageRefreshCached is provided, notify about the cached image
                // AND try to re-download it in order to let a chance to NSURLCache to refresh it from server.
                [self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            }

            // download if no image or requested to refresh anyway, and download allowed by delegate
            SDWebImageDownloaderOptions downloaderOptions = 0;
            if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
            if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
            if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;
            if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
            if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies;
            if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates;
            if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority;
            if (options & SDWebImageScaleDownLargeImages) downloaderOptions |= SDWebImageDownloaderScaleDownLargeImages;
            
            if (cachedImage && options & SDWebImageRefreshCached) {
                // force progressive off if image already cached but forced refreshing
                downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
                // ignore image read from NSURLCache if image if cached but force refreshing
                downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
            }
            
            //下載策略
            SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                if (!strongOperation || strongOperation.isCancelled) {
                    // Do nothing if the operation was cancelled
                    // See #699 for more details
                    // if we would call the completedBlock, there could be a race condition between this block and another completedBlock for the same object, so if this one is called second, we will overwrite the new data
                } else if (error) {
                    [self callCompletionBlockForOperation:strongOperation completion:completedBlock error:error url:url];

                    if (   error.code != NSURLErrorNotConnectedToInternet
                        && error.code != NSURLErrorCancelled
                        && error.code != NSURLErrorTimedOut
                        && error.code != NSURLErrorInternationalRoamingOff
                        && error.code != NSURLErrorDataNotAllowed
                        && error.code != NSURLErrorCannotFindHost
                        && error.code != NSURLErrorCannotConnectToHost
                        && error.code != NSURLErrorNetworkConnectionLost) {
                        @synchronized (self.failedURLs) {
                            [self.failedURLs addObject:url];
                        }
                    }
                }
                else {
                    if ((options & SDWebImageRetryFailed)) {
                        @synchronized (self.failedURLs) {
                            [self.failedURLs removeObject:url];
                        }
                    }
                    
                    BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);

                    if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
                        // Image refresh hit the NSURLCache cache, do not call the completion block
                    } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
                        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
                            UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];

                            if (transformedImage && finished) {
                                BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
                                // pass nil if the image was transformed, so we can recalculate the data from the image
                                [self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];
                            }
                            
                            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                        });
                    } else {
                        if (downloadedImage && finished) {
                            [self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
                        }
                        [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                    }
                }

                if (finished) {
                    [self safelyRemoveOperationFromRunning:strongOperation];
                }
            }];
            operation.cancelBlock = ^{
                [self.imageDownloader cancel:subOperationToken];
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                [self safelyRemoveOperationFromRunning:strongOperation];
            };
        } else if (cachedImage) {
            __strong __typeof(weakOperation) strongOperation = weakOperation;
            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            [self safelyRemoveOperationFromRunning:operation];
        } else {
            // Image not in cache and download disallowed by delegate
            __strong __typeof(weakOperation) strongOperation = weakOperation;
            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
            [self safelyRemoveOperationFromRunning:operation];
        }
    }];

    return operation;
}

其中存崖，緩存策略調(diào)用為

//緩存策略
operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
...}

下載策略調(diào)用為

//下載策略
SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
...}

4.緩存查詢策略

類名

SDImageCache.m

方法

...}


- 作用

1) 先查詢 Memory Cache
> `UIImage *image = [self imageFromMemoryCacheForKey:key];`

2) 如果 Memory Cache 查找不到， 就會(huì)查詢 Disk Cache

> `NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
UIImage *diskImage = [self diskImageForKey:key];
`

3) 如果 Disk Cache 查詢成功睡毒，還會(huì)把得到的圖片再次設(shè)置到 Memory Cache 中来惧。

> `[self.memCache setObject:diskImage forKey:key cost:cost];`

4) 向block返回圖片數(shù)據(jù)，如`doneBlock(image, diskData, SDImageCacheTypeMemory);`吕嘀。

> 其中违寞，關(guān)于image贞瞒，所有圖均需返回
`UIImage *image = [self imageFromMemoryCacheForKey:key];`。

> 關(guān)于diskData趁曼，如果為GIF圖军浆，需返回
`diskData = [self diskImageDataBySearchingAllPathsForKey:key];`，如果為非GIF圖挡闰，則返回nil乒融。

- 實(shí)現(xiàn)

(nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock {
if (!key) {
if (doneBlock) {
doneBlock(nil, nil, SDImageCacheTypeNone);
}
return nil;
}

// First check the in-memory cache...
UIImage *image = [self imageFromMemoryCacheForKey:key];
if (image) {
NSData *diskData = nil;
if ([image isGIF]) {
diskData = [self diskImageDataBySearchingAllPathsForKey:key];
}
if (doneBlock) {
doneBlock(image, diskData, SDImageCacheTypeMemory);
}
return nil;
}

NSOperation *operation = [NSOperation new];
dispatch_async(self.ioQueue, ^{
if (operation.isCancelled) {
// do not call the completion if cancelled
return;
}
```
  @autoreleasepool {
      NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
      UIImage *diskImage = [self diskImageForKey:key];
      if (diskImage && self.config.shouldCacheImagesInMemory) {
          NSUInteger cost = SDCacheCostForImage(diskImage);
          [self.memCache setObject:diskImage forKey:key cost:cost];
      }

      if (doneBlock) {
          dispatch_async(dispatch_get_main_queue(), ^{
              doneBlock(diskImage, diskData, SDImageCacheTypeDisk);
          });
      }
  }
```
});

return operation;
}

## 5.下載管理策略

- 類名

> **SDWebImageDownloader.h**

- 方法

>```- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
...}

作用
利用Url建立Request

NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:cachePolicy timeoutInterval:timeoutInterval];

利用Request初始化SDWebImageDownloaderOperation下載對(duì)象

SDWebImageDownloaderOperation *operation = [[sself.operationClass alloc] initWithRequest:request inSession:sself.session options:options];

實(shí)現(xiàn)

- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
    __weak SDWebImageDownloader *wself = self;

    return [self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^SDWebImageDownloaderOperation *{
        __strong __typeof (wself) sself = wself;
        NSTimeInterval timeoutInterval = sself.downloadTimeout;
        if (timeoutInterval == 0.0) {
            timeoutInterval = 15.0;
        }

        // In order to prevent from potential duplicate caching (NSURLCache + SDImageCache) we disable the cache for image requests if told otherwise
        NSURLRequestCachePolicy cachePolicy = NSURLRequestReloadIgnoringLocalCacheData;
        if (options & SDWebImageDownloaderUseNSURLCache) {
            if (options & SDWebImageDownloaderIgnoreCachedResponse) {
                cachePolicy = NSURLRequestReturnCacheDataDontLoad;
            } else {
                cachePolicy = NSURLRequestUseProtocolCachePolicy;
            }
        }
        
        NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:cachePolicy timeoutInterval:timeoutInterval];
        
        request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
        request.HTTPShouldUsePipelining = YES;
        if (sself.headersFilter) {
            request.allHTTPHeaderFields = sself.headersFilter(url, [sself.HTTPHeaders copy]);
        }
        else {
            request.allHTTPHeaderFields = sself.HTTPHeaders;
        }
        SDWebImageDownloaderOperation *operation = [[sself.operationClass alloc] initWithRequest:request inSession:sself.session options:options];
        operation.shouldDecompressImages = sself.shouldDecompressImages;
        
        if (sself.urlCredential) {
            operation.credential = sself.urlCredential;
        } else if (sself.username && sself.password) {
            operation.credential = [NSURLCredential credentialWithUser:sself.username password:sself.password persistence:NSURLCredentialPersistenceForSession];
        }
        
        if (options & SDWebImageDownloaderHighPriority) {
            operation.queuePriority = NSOperationQueuePriorityHigh;
        } else if (options & SDWebImageDownloaderLowPriority) {
            operation.queuePriority = NSOperationQueuePriorityLow;
        }

        [sself.downloadQueue addOperation:operation];
        if (sself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
            // Emulate LIFO execution order by systematically adding new operations as last operation's dependency
            [sself.lastAddedOperation addDependency:operation];
            sself.lastAddedOperation = operation;
        }

        return operation;
    }];
}

6.緩存管理策略

6.1 自動(dòng)化緩存管理

類名

SDImageCacheConfig.h

屬性

/**
 * The maximum length of time to keep an image in the cache, in seconds
 */
@property (assign, nonatomic) NSInteger maxCacheAge;

/**
 * The maximum size of the cache, in bytes.
 */
@property (assign, nonatomic) NSUInteger maxCacheSize;

類名

SDImageCacheConfig.m

實(shí)現(xiàn)

static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week

@implementation SDImageCacheConfig

- (instancetype)init {
    if (self = [super init]) {
        _shouldDecompressImages = YES;
        _shouldDisableiCloud = YES;
        _shouldCacheImagesInMemory = YES;
        _maxCacheAge = kDefaultCacheMaxCacheAge;
        _maxCacheSize = 0;
    }
    return self;
}

@end

類名

SDImageCache.m

添加通知

#if SD_UIKIT
        // Subscribe to app events
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(clearMemory)
                                                     name:UIApplicationDidReceiveMemoryWarningNotification
                                                   object:nil];

        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(deleteOldFiles)
                                                     name:UIApplicationWillTerminateNotification
                                                   object:nil];

        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(backgroundDeleteOldFiles)
                                                     name:UIApplicationDidEnterBackgroundNotification
                                                   object:nil];
#endif

內(nèi)存警告：清除所有內(nèi)存

- (void)clearMemory {
    [self.memCache removeAllObjects];
}

程序結(jié)束：刪除老文件（縮減一半）

if (self.config.maxCacheSize > 0 && currentCacheSize > self.config.maxCacheSize) {
            // Target half of our maximum cache size for this cleanup pass.
            const NSUInteger desiredCacheSize = self.config.maxCacheSize / 2;

            // Sort the remaining cache files by their last modification time (oldest first).
            NSArray<NSURL *> *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<NSString *, id> *resourceValues = cacheFiles[fileURL];
                    NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
                    currentCacheSize -= totalAllocatedSize.unsignedIntegerValue;

                    if (currentCacheSize < desiredCacheSize) {
                        break;
                    }
                }
            }
        }

進(jìn)入后臺(tái):清除一半緩存并結(jié)束后臺(tái)任務(wù)

#if SD_UIKIT
- (void)backgroundDeleteOldFiles {
    Class UIApplicationClass = NSClassFromString(@"UIApplication");
    if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) {
        return;
    }
    UIApplication *application = [UIApplication performSelector:@selector(sharedApplication)];
    __block UIBackgroundTaskIdentifier bgTask = [application beginBackgroundTaskWithExpirationHandler:^{
        // Clean up any unfinished task business by marking where you
        // stopped or ending the task outright.
        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];

    // Start the long-running task and return immediately.
    [self deleteOldFilesWithCompletionBlock:^{
        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];
}
#endif

6.2 手動(dòng)化內(nèi)存管理

手動(dòng)設(shè)置最大緩存空間

//調(diào)用處獲取單例進(jìn)行設(shè)置

[SDImageCache sharedImageCache].config.maxCacheSize = 1024 * 1024 * 50;    // 50M

手動(dòng)設(shè)置緩存清理周期

//SDImageCacheConfig.m

static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week

手動(dòng)清除緩存

- (void)clearDiskOnCompletion:(nullable SDWebImageNoParamsBlock)completion {
    dispatch_async(self.ioQueue, ^{
        [_fileManager removeItemAtPath:self.diskCachePath error:nil];
        [_fileManager createDirectoryAtPath:self.diskCachePath
                withIntermediateDirectories:YES
                                 attributes:nil
                                      error:NULL];

        if (completion) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completion();
            });
        }
    });
}

小結(jié)

學(xué)習(xí)最好的方式，是自己研讀里面的源碼摄悯，當(dāng)然限于篇幅赞季，本篇文章介紹的重點(diǎn)是緩存策略，而對(duì)SDWebImage的網(wǎng)絡(luò)管理部分一提而過奢驯。如果你愿意深入研究申钩，你會(huì)發(fā)現(xiàn)更多層的調(diào)用棧和線程管理的操作。下載官方源碼SDWebImage瘪阁，你可以研究更多撒遣。

最后編輯于：2017.12.08 06:53:18

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[iOS源碼筆記]·第三方網(wǎng)絡(luò)圖片處理框架:SDWebImage網(wǎng)絡(luò)下載及緩存管理策略

1.框架簡(jiǎn)介

1.1 使用入門

1.2 官方文檔

2.官方Demo調(diào)用實(shí)例解析

3.2 第二層

3.4 第四層

4.緩存查詢策略

6.緩存管理策略

6.1 自動(dòng)化緩存管理

6.2 手動(dòng)化內(nèi)存管理

小結(jié)

官方下載與說明

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