1. SDWebImage怎么实现缓存的？
分为内存缓存（利用SDImageCache类的NSCache属性），磁盘缓存（利用NSFileManager），和操作缓存（利用runtime关联的字典属性）。下载之前先查询缓存，没有就下载并在下载后保存图片到缓存。
(1). 查询图片缓存 的内部API
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock {
- 调用位置 -- SDWebImageManager.m
- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
调用位置
(2). 保存图片到缓存 的内部API
- (void)storeImage:(nullable UIImage *)image
imageData:(nullable NSData *)imageData
forKey:(nullable NSString *)key
toDisk:(BOOL)toDisk
completion:(nullable SDWebImageNoParamsBlock)completionBlock {
- 调用位置 -- SDWebImageManager.m
- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
调用位置
- 实现原理 --- SDImageCache.m
缓存数据
- 其中，数据转换部分 原理为：
- (nullable NSData *)sd_imageDataAsFormat:(SDImageFormat)imageFormat {
NSData *imageData = nil;
if (self) {
#if SD_UIKIT || SD_WATCH
int alphaInfo = CGImageGetAlphaInfo(self.CGImage);
BOOL hasAlpha = !(alphaInfo == kCGImageAlphaNone ||
alphaInfo == kCGImageAlphaNoneSkipFirst ||
alphaInfo == kCGImageAlphaNoneSkipLast);
BOOL usePNG = hasAlpha;
// the imageFormat param has priority here. But if the format is undefined, we relly on the alpha channel
if (imageFormat != SDImageFormatUndefined) {
usePNG = (imageFormat == SDImageFormatPNG);
}
if (usePNG) {
imageData = UIImagePNGRepresentation(self);
} else {
imageData = UIImageJPEGRepresentation(self, (CGFloat)1.0);
}
#else
NSBitmapImageFileType imageFileType = NSJPEGFileType;
if (imageFormat == SDImageFormatGIF) {
imageFileType = NSGIFFileType;
} else if (imageFormat == SDImageFormatPNG) {
imageFileType = NSPNGFileType;
}
imageData = [NSBitmapImageRep representationOfImageRepsInArray:self.representations
usingType:imageFileType
properties:@{}];
#endif
}
return imageData;
}
其中，保存到沙盒部分 原理为：
- (void)storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key {
if (!imageData || !key) {
return;
}
[self checkIfQueueIsIOQueue];
if (![_fileManager fileExistsAtPath:_diskCachePath]) {
[_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
}
// get cache Path for image key
NSString *cachePathForKey = [self defaultCachePathForKey:key];
// transform to NSUrl
NSURL *fileURL = [NSURL fileURLWithPath:cachePathForKey];
[_fileManager createFileAtPath:cachePathForKey contents:imageData attributes:nil];
// disable iCloud backup
if (self.config.shouldDisableiCloud) {
[fileURL setResourceValue:@YES forKey:NSURLIsExcludedFromBackupKey error:nil];
}
}
2. SDWebImage下载后的图片在什么时候用到解码？
在NSURLSession下载完成后的代理方法中，具体文件是SDWebImageDownloaderOperation.m。
- 内部API
+ (nullable UIImage *)decodedImageWithImage:(nullable UIImage *)image {
- 调用位置1 -- SDWebImageDownloaderOperation.m代理1
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
@synchronized(self) {
self.dataTask = nil;
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
if (!error) {
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadFinishNotification object:self];
}
});
}
if (error) {
[self callCompletionBlocksWithError:error];
} else {
if ([self callbacksForKey:kCompletedCallbackKey].count > 0) {
/**
* See #1608 and #1623 - apparently, there is a race condition on `NSURLCache` that causes a crash
* Limited the calls to `cachedResponseForRequest:` only for cases where we should ignore the cached response
* and images for which responseFromCached is YES (only the ones that cannot be cached).
* Note: responseFromCached is set to NO inside `willCacheResponse:`. This method doesn't get called for large images or images behind authentication
*/
if (self.options & SDWebImageDownloaderIgnoreCachedResponse && responseFromCached && [[NSURLCache sharedURLCache] cachedResponseForRequest:self.request]) {
// hack
[self callCompletionBlocksWithImage:nil imageData:nil error:nil finished:YES];
} else if (self.imageData) {
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) {
if (self.options & SDWebImageDownloaderScaleDownLargeImages) {
#if SD_UIKIT || SD_WATCH
image = [UIImage decodedAndScaledDownImageWithImage:image];
[self.imageData setData:UIImagePNGRepresentation(image)];
#endif
} else {
image = [UIImage decodedImageWithImage:image];
}
}
}
if (CGSizeEqualToSize(image.size, CGSizeZero)) {
[self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}]];
} else {
[self callCompletionBlocksWithImage:image imageData:self.imageData error:nil finished:YES];
}
} else {
[self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}]];
}
}
}
[self done];
}
- 调用位置2 -- SDWebImageDownloaderOperation.m代理1
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
//省略...
- 实现原理 -- SDWebImageDecoder.m
+ (nullable UIImage *)decodedImageWithImage:(nullable UIImage *)image {
if (![UIImage shouldDecodeImage:image]) {
return image;
}
// autorelease the bitmap context and all vars to help system to free memory when there are memory warning.
// on iOS7, do not forget to call [[SDImageCache sharedImageCache] clearMemory];
@autoreleasepool{
CGImageRef imageRef = image.CGImage;
CGColorSpaceRef colorspaceRef = [UIImage colorSpaceForImageRef:imageRef];
size_t width = CGImageGetWidth(imageRef);
size_t height = CGImageGetHeight(imageRef);
size_t bytesPerRow = kBytesPerPixel * width;
// kCGImageAlphaNone is not supported in CGBitmapContextCreate.
// Since the original image here has no alpha info, use kCGImageAlphaNoneSkipLast
// to create bitmap graphics contexts without alpha info.
CGContextRef context = CGBitmapContextCreate(NULL,
width,
height,
kBitsPerComponent,
bytesPerRow,
colorspaceRef,
kCGBitmapByteOrderDefault|kCGImageAlphaNoneSkipLast);
if (context == NULL) {
return image;
}
// Draw the image into the context and retrieve the new bitmap image without alpha
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGImageRef imageRefWithoutAlpha = CGBitmapContextCreateImage(context);
UIImage *imageWithoutAlpha = [UIImage imageWithCGImage:imageRefWithoutAlpha
scale:image.scale
orientation:image.imageOrientation];
CGContextRelease(context);
CGImageRelease(imageRefWithoutAlpha);
return imageWithoutAlpha;
}
}
3. 怎样安全地在主线程执行一个Block？
有时候会把将要执行的内容放到主线程里面执行，但如果已经是主线程里面的代码调用dispatch_async的时候偶尔会出现crash，所以就需要判断是否已经在主线程里面了。
老版本的SDWebImage这样封装了一个宏：
//主线程同步队列
#define dispatch_main_sync_safe(block)\
if ([NSThread isMainThread]) {\
block();\
} else {\
dispatch_sync(dispatch_get_main_queue(), block);\
}
//主线程异步队列
#define dispatch_main_async_safe(block)\
if ([NSThread isMainThread]) {\
block();\
} else {\
dispatch_async(dispatch_get_main_queue(), block);\
}
//用法
dispatch_main_async_safe(^{
//需要执行的代码片段;
});
新版本的SDWebImage是这样封装的宏：
#ifndef dispatch_main_async_safe
#define dispatch_main_async_safe(block)\
if (strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(dispatch_get_main_queue())) == 0) {\
block();\
} else {\
dispatch_async(dispatch_get_main_queue(), block);\
}
#endif
其中的strcmp是字符串比较函数:
int strcmp(char *str1， char *str2)
当str1>str2时，返回一个正数；
当str1<str2时，返回一个负数；
当str1=str2时，返回0。
最后应该注意的是：两个字符串比较时，是按asiic码大小逐个比较的，当发现某一个大或者小时，就停止比较、返回一个值。否则比较到最后一个字母。
注意的问题是，宏里面的block是无法打断点调试的。你如果步进查看，可以发现会跳到汇编的文件里面步进。
4. 怎样区分SDWebImageDownloader和SDWebImageManager的工作？
-
SDWebImageManager提供的关键API是loadImageWithURL开头的，负责加载的，加载load这个词跟下载download不同，比它更广，加载负责管理下载之前的操作：
- 管理下载操作的开始和取消
- 下载之前查询图片的内存缓存和磁盘缓存
- 下载之后保存图片到内存缓存和磁盘缓存
- 返回一个操作对象给上级对象UIImageView+WebCache作为操作缓存数组属性中去
SDWebImageDownloader提供的关键API是downloadImageWithURL开头的，可见它仅仅管理下载的操作，没有缓存的管理功能。
