SD源码中下载只涉及到两个类，一个是SDWebImageDownloader，另外一个是SDWebImageDownloaderOperation。由文件命名不难看出，SD采用的是NSOperation异步下载的方式。下面就说说它的几个细节处理。
- 如何实现一个下载manager？
- 如何处理一个异步下载？
- 一个下载任务具体实现？
- 其他细节处理
1. 如何实现一个下载manager？
通常设计一个Manager就是对外提供一个统一的接口，Manager应该具备的功能主要有就是对被操作对象进行管理（增加，删除，修改）和处理操作（具体就是被操作对象的功能方法）并且维护任务状态。要实现一个Manager，常规做法就是维护一个对象容器，并对该容器进行相关操作。在此处，SDWebImageDownloader也是这样来实现的。其具体做法就是内部维护一个NSOperationqueue，来处理外部下载任务。同时，它也维护一个下载任务的容器，用来维护对应任务的一个状态。它提供了实以下两个方法，如下：
添加并执行一个下载任务
- (id
)downloadImageWithURL:(NSURL *)url
options:(SDWebImageDownloaderOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageDownloaderCompletedBlock)completedBlock;
设置是否挂起一个任务
- (void)setSuspended:(BOOL)suspended;
除了以上方法，SDWebImageDownloader提供了一些配置类操作，比如设置最大并发数，设置Http相关配置等。
2. 如何处理一个异步下载？
SDWebImageDownloader处理一个下载任务很简单，就只是把下载任务添加到并发队列，同时维护该下载任务的一个状态即可。
1. 维护任务状态
一个下载任务通常具有以下几个状态：1.下载开始，2.下载中，3.下载结束，4.下载取消。SDWebImageDownloader对外通过提供block回调来处理下载结束，下载中的一个状态。具体有以下几个block：
typedef void(^SDWebImageDownloaderProgressBlock)(NSInteger receivedSize, NSInteger expectedSize);
typedef void(^SDWebImageDownloaderCompletedBlock)(UIImage *image, NSData *data, NSError *error, BOOL finished);
typedef void(^SDWebImageNoParamsBlock)();
SDWebImageDownloader在外部调用downloadImageWithURL时，通过被请求URL来创建一个字典用来维护该任务的一个状态处理block，在这里，SDWebImageDownloader对相同URL请求进行了过滤，即多次请求相同URL任务只会被处理一次。
dispatch_barrier_sync(self.barrierQueue, ^{
BOOL first = NO;
if (!self.URLCallbacks[url]) {
self.URLCallbacks[url] = [NSMutableArray new];
first = YES;
}
// Handle single download of simultaneous download request for the same URL
NSMutableArray *callbacksForURL = self.URLCallbacks[url];
NSMutableDictionary *callbacks = [NSMutableDictionary new];
if (progressBlock) callbacks[kProgressCallbackKey] = [progressBlock copy];
if (completedBlock) callbacks[kCompletedCallbackKey] = [completedBlock copy];
[callbacksForURL addObject:callbacks];
self.URLCallbacks[url] = callbacksForURL;
if (first) {
createCallback();
}
});
block处理下载中状态
SDWebImageDownloader *sself = wself;
if (!sself) return;
__block NSArray *callbacksForURL;
dispatch_sync(sself.barrierQueue, ^{
//获取当前状态维护队列
callbacksForURL = [sself.URLCallbacks[url] copy];
});
for (NSDictionary *callbacks in callbacksForURL) { dispatch_async(dispatch_get_main_queue(), ^{
//获取Url对应的下载处理block
SDWebImageDownloaderProgressBlock callback = callbacks[kProgressCallbackKey];
if (callback) callback(receivedSize, expectedSize);
});
}
block处理下载完成状态
SDWebImageDownloader *sself = wself;
if (!sself) return;
__block NSArray *callbacksForURL;
dispatch_barrier_sync(sself.barrierQueue, ^{
callbacksForURL = [sself.URLCallbacks[url] copy];
if (finished) {
[sself.URLCallbacks removeObjectForKey:url];
}
});
for (NSDictionary *callbacks in callbacksForURL) {
//获取下载完成处理block
SDWebImageDownloaderCompletedBlock callback = callbacks[kCompletedCallbackKey];
if (callback) callback(image, data, error, finished);
}
block处理下载取消状态
SDWebImageDownloader *sself = wself;
if (!sself) return;
dispatch_barrier_async(sself.barrierQueue, ^{
[sself.URLCallbacks removeObjectForKey:url];
});
2. 创建任务并执行
由于SDWebImageDownloader是通过NSOperation来处理并发任务，所以实现起来比较简单。创建一个NSOperation，并把它添加到Queue队列即可。在此处，SDWebImageDownloader创建了一个并发队列，默认的最大并发数6。
SDWebImageDownloaderOperation是下载任务的具体实现类，它继承NSOperation，且该类通过Url请求来进行初始化。在处理队列中任务时，提供了两种处理方式，一种是FIFO，一种是LIFO。如果采用的是LIFO方式处理，则在把新任务加到队列的同时，需要更新一下最后入队列的任务，并且该任务依赖当前新加入的任务。具体实现如下：
[wself.downloadQueue addOperation:operation];
if (wself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
[wself.lastAddedOperation addDependency:operation];
wself.lastAddedOperation = operation;
}
3. 一个下载任务具体实现
SDWebImageDownloaderOperation是下载任务的具体实现类。该类维护了自生的一个状态，一个NSURLConnection，一个下载线程，还有具体的下载后数据。在NSOperation的start方法中。创建一个NSURLConnection并且开始这个connection，同时，开启一个后台线程用于下载数据。
- (void)start {
//初始化当前状态
@synchronized (self) {
if (self.isCancelled) {
self.finished = YES;
[self reset];
return;
}
...
...
self.executing = YES;
self.connection = [[NSURLConnection alloc] initWithRequest:self.request delegate:self startImmediately:NO];
self.thread = [NSThread currentThread];
}
//执行connection
[self.connection start];
if (self.connection) {
if (self.progressBlock) {
self.progressBlock(0, NSURLResponseUnknownLength);
}
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self];
});
//开启线程
if (floor(NSFoundationVersionNumber) <= NSFoundationVersionNumber_iOS_5_1) {
CFRunLoopRunInMode(kCFRunLoopDefaultMode, 10, false);
}
else {
CFRunLoopRun();
}
if (!self.isFinished) {
[self.connection cancel];
[self connection:self.connection didFailWithError:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorTimedOut userInfo:@{NSURLErrorFailingURLErrorKey : self.request.URL}]];
}
}
else {
if (self.completedBlock) {
self.completedBlock(nil, nil, [NSError errorWithDomain:NSURLErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Connection can't be initialized"}], YES);
}
}
由于SDWebImageDownloaderOperation是通过NSURLConnection来下载图片。所以，具体的下载动作都由NSURLConnection的delegate来处理。
收到http的response时的回调，在该回调里面，首先根据response返回码来判断请求是否成功。请求成功，则通过respone中文件大小来创建一个下载数据缓存区。请求失败，则主动取消改请求，并且停止后台下载线程，复位当前任务状态。
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
//'304 Not Modified' is an exceptional one
if (![response respondsToSelector:@selector(statusCode)] || ([((NSHTTPURLResponse *)response) statusCode] < 400 && [((NSHTTPURLResponse *)response) statusCode] != 304)) {
NSInteger expected = response.expectedContentLength > 0 ? (NSInteger)response.expectedContentLength : 0;
self.expectedSize = expected;
if (self.progressBlock) {
self.progressBlock(0, expected);
}
self.imageData = [[NSMutableData alloc] initWithCapacity:expected];
self.response = response;
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadReceiveResponseNotification object:self];
});
}
else {
NSUInteger code = [((NSHTTPURLResponse *)response) statusCode];
//This is the case when server returns '304 Not Modified'. It means that remote image is not changed.
//In case of 304 we need just cancel the operation and return cached image from the cache.
if (code == 304) {
[self cancelInternal];
} else {
[self.connection cancel];
}
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
});
if (self.completedBlock) {
self.completedBlock(nil, nil, [NSError errorWithDomain:NSURLErrorDomain code:[((NSHTTPURLResponse *)response) statusCode] userInfo:nil], YES);
}
CFRunLoopStop(CFRunLoopGetCurrent());
[self done];
}
}
收到具体下载数据时候的回调，在该回调里面，首先对下载到的数据进行缓存，然后根据当前缓存到的数据，来获取到图片的长和宽，同时，根据请求下载的数据大小来做判断，如果符合，则生成下载的图片并回调给上层。
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
[self.imageData appendData:data];
if ((self.options & SDWebImageDownloaderProgressiveDownload) && self.expectedSize > 0 && self.completedBlock) {
// Get the total bytes downloaded
const NSInteger totalSize = self.imageData.length;
// Update the data source, we must pass ALL the data, not just the new bytes
CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)self.imageData, NULL);
if (width + height == 0) {
CFDictionaryRef properties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);
if (properties) {
NSInteger orientationValue = -1;
CFTypeRef val = CFDictionaryGetValue(properties, kCGImagePropertyPixelHeight);
if (val) CFNumberGetValue(val, kCFNumberLongType, &height);
val = CFDictionaryGetValue(properties, kCGImagePropertyPixelWidth);
if (val) CFNumberGetValue(val, kCFNumberLongType, &width);
val = CFDictionaryGetValue(properties, kCGImagePropertyOrientation);
if (val) CFNumberGetValue(val, kCFNumberNSIntegerType, &orientationValue);
CFRelease(properties);
// When we draw to Core Graphics, we lose orientation information,
// which means the image below born of initWithCGIImage will be
// oriented incorrectly sometimes. (Unlike the image born of initWithData
// in connectionDidFinishLoading.) So save it here and pass it on later.
orientation = [[self class] orientationFromPropertyValue:(orientationValue == -1 ? 1 : orientationValue)];
}
}
if (width + height > 0 && totalSize < self.expectedSize) {
// Create the image
CGImageRef partialImageRef = CGImageSourceCreateImageAtIndex(imageSource, 0, NULL);
#ifdef TARGET_OS_IPHONE
// Workaround for iOS anamorphic image
if (partialImageRef) {
const size_t partialHeight = CGImageGetHeight(partialImageRef);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef bmContext = CGBitmapContextCreate(NULL, width, height, 8, width * 4, colorSpace, kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst);
CGColorSpaceRelease(colorSpace);
if (bmContext) {
CGContextDrawImage(bmContext, (CGRect){.origin.x = 0.0f, .origin.y = 0.0f, .size.width = width, .size.height = partialHeight}, partialImageRef);
CGImageRelease(partialImageRef);
partialImageRef = CGBitmapContextCreateImage(bmContext);
CGContextRelease(bmContext);
}
else {
CGImageRelease(partialImageRef);
partialImageRef = nil;
}
}
#endif
if (partialImageRef) {
UIImage *image = [UIImage imageWithCGImage:partialImageRef scale:1 orientation:orientation];
NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
UIImage *scaledImage = [self scaledImageForKey:key image:image];
if (self.shouldDecompressImages) {
image = [UIImage decodedImageWithImage:scaledImage];
}else {
image = scaledImage;
}
CGImageRelease(partialImageRef);
dispatch_main_sync_safe(^{
if (self.completedBlock) {
self.completedBlock(image, nil, nil, NO);
}
});
}
}
CFRelease(imageSource);
}
if (self.progressBlock) {
self.progressBlock(self.imageData.length, self.expectedSize);
}
}
请求失败时的回调处理，终止后台下载线程，并复位当前请求。
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
@synchronized(self) {
CFRunLoopStop(CFRunLoopGetCurrent());
self.thread = nil;
self.connection = nil;
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
});
}
if (self.completedBlock) {
self.completedBlock(nil, nil, error, YES);
}
self.completionBlock = nil;
[self done];
}
请求加载完成时的回调处理，在该回调处理中，首先就是停止了后台下载线程，复位当前请求。然后判断该请求是否已经被缓存了，如果已经缓存，则取出缓存数据进行解码操作，生成相应图片并回调给上层
- (void)connectionDidFinishLoading:(NSURLConnection *)aConnection {
SDWebImageDownloaderCompletedBlock completionBlock = self.completedBlock;
@synchronized(self) {
CFRunLoopStop(CFRunLoopGetCurrent());
self.thread = nil;
self.connection = nil;
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadFinishNotification object:self];
});
}
if (![[NSURLCache sharedURLCache] cachedResponseForRequest:_request]) {
responseFromCached = NO;
}
if (completionBlock) {
if (self.options & SDWebImageDownloaderIgnoreCachedResponse && responseFromCached) {
completionBlock(nil, nil, nil, 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) {
image = [UIImage decodedImageWithImage:image];
}
}
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);
}
} else {
completionBlock(nil, nil, [NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}], YES);
}
}
self.completionBlock = nil;
[self done];
}
除了以上几个比较重要的回调处理。SDWebImageDownloaderOperation还处理了http鉴权，加载等相关回调。
4. 其他细节处理
1. 通知上层应用
针对具体的下载事件，需要及时通知上层，SDWebImageDownloaderOperation提供以下几个通知来做到上层同步。
NSString *const SDWebImageDownloadStartNotification = @"SDWebImageDownloadStartNotification";
NSString *const SDWebImageDownloadReceiveResponseNotification = @"SDWebImageDownloadReceiveResponseNotification";
NSString *const SDWebImageDownloadStopNotification = @"SDWebImageDownloadStopNotification";
NSString *const SDWebImageDownloadFinishNotification = @"SDWebImageDownloadFinishNotification";
2. 后台线程的处理
针对系统后台运行的处理
Class UIApplicationClass = NSClassFromString(@"UIApplication");
BOOL hasApplication = UIApplicationClass && [UIApplicationClass respondsToSelector:@selector(sharedApplication)];
if (hasApplication && [self shouldContinueWhenAppEntersBackground]) {
__weak __typeof__ (self) wself = self;
UIApplication * app = [UIApplicationClass performSelector:@selector(sharedApplication)];
self.backgroundTaskId = [app beginBackgroundTaskWithExpirationHandler:^{
__strong __typeof (wself) sself = wself;
if (sself) {
[sself cancel];
[app endBackgroundTask:sself.backgroundTaskId];
sself.backgroundTaskId = UIBackgroundTaskInvalid;
}
}];
}
针对后台下载线程的处理
if (floor(NSFoundationVersionNumber) <= NSFoundationVersionNumber_iOS_5_1) {
CFRunLoopRunInMode(kCFRunLoopDefaultMode, 10, false);
}
else {
CFRunLoopRun();
}
总结
- 掌握Manager与operatior的基本实现方式
- 加深NSOperation，NSURLConnection的应用