第七篇
前言
本篇文章主要講解下載操作的相關(guān)知識(shí)，SDWebImageDownloaderOperation的主要任務(wù)是把一張圖片從服務(wù)器下載到內(nèi)存中。下載數(shù)據(jù)并不難，如何對(duì)下載這一系列的任務(wù)進(jìn)行設(shè)計(jì)，就很難了。接下來我們一步一步的分析作者在開發(fā)中的思路和使用到的技術(shù)細(xì)節(jié)。
NSOperation
NSOperation想必大家都知道，為了讓程序執(zhí)行的更快，我們用多線程異步的方式解決這個(gè)問題，GCD與NSOperation都能實(shí)現(xiàn)多線程，我們這里只介紹NSOperation。如果大家想了解更多NSOperation的知識(shí)，我覺得這篇文章寫得挺好：多線程之NSOperation簡介。
我們把NSOperation最核心的使用方法總結(jié)一下：
-
NSOperation有兩個(gè)方法：main()和start()。如果想使用同步，那么最簡單方法的就是把邏輯寫在main()中，使用異步，需要把邏輯寫到start()中，然后加入到隊(duì)列之中。 - 大家有沒有想過
NSOperation什么時(shí)候執(zhí)行呢？按照正常想法，難道要我們自己手動(dòng)調(diào)用main()和start()嗎？這樣肯定也是行的。當(dāng)手動(dòng)調(diào)用start()或者main()方法的時(shí)候，和調(diào)用普通的方法沒什么區(qū)別，當(dāng)被加入operationQueue中后，情況不同，operationQueue中所有的NSOperation都是異步執(zhí)行的，也就是說start()會(huì)在子線程執(zhí)行，至于是串行還是并發(fā)，都由maxConcurrentOperationCount控制，當(dāng)maxConcurrentOperationCount == 1時(shí)，相當(dāng)于串行了。另外一種方法就是加入到operationQueue中，operationQueue會(huì)盡快執(zhí)行NSOperation，如果operationQueue是同步的，那么它會(huì)等到NSOperation的isFinished等于YES后，在執(zhí)行下一個(gè)任務(wù)，如果是異步的，通過設(shè)置maxConcurrentOperationCount來控制同事執(zhí)行的最大操作，某個(gè)操作完成后，繼續(xù)其他的操作。 - 并不是調(diào)用了canche就一定取消了，如果
NSOperation沒有執(zhí)行，那么就會(huì)取消，如果執(zhí)行了，只會(huì)將isCancelled設(shè)置為YES。所以，在我們的操作中，我們應(yīng)該在每個(gè)操作開始前，或者在每個(gè)有意義的實(shí)際操作完成后，先檢查下這個(gè)屬性是不是已經(jīng)設(shè)置為YES。如果是YES，則后面操作都可以不用在執(zhí)行了。
能夠引起思考的地方就是，比如說我有一系列的任務(wù)要執(zhí)行，我有兩種選擇，一種是通過數(shù)組控制數(shù)據(jù)的取出順序，另外一種就是使用隊(duì)列
通知
extern NSString * _Nonnull const SDWebImageDownloadStartNotification;
extern NSString * _Nonnull const SDWebImageDownloadReceiveResponseNotification;
extern NSString * _Nonnull const SDWebImageDownloadStopNotification;
extern NSString * _Nonnull const SDWebImageDownloadFinishNotification;
SDWebImageDownloaderOperation有四種情況會(huì)發(fā)送通知：
- 任務(wù)開始
- 接收到數(shù)據(jù)
- 暫停
- 完成
不知道大家發(fā)現(xiàn)沒有，在設(shè)計(jì)一個(gè)功能的時(shí)候，作者都會(huì)用通知的形式暴露出關(guān)鍵的節(jié)點(diǎn)。不管使用者需不需要使用這些通知。這是一個(gè)很好地方法，可以在自定義控件的時(shí)候參考這個(gè)設(shè)計(jì)。
SDWebImageDownloaderOperationInterface
/**
Describes a downloader operation. If one wants to use a custom downloader op, it needs to inherit from `NSOperation` and conform to this protocol
*/
@protocol SDWebImageDownloaderOperationInterface<NSObject>
- (nonnull instancetype)initWithRequest:(nullable NSURLRequest *)request
inSession:(nullable NSURLSession *)session
options:(SDWebImageDownloaderOptions)options;
- (nullable id)addHandlersForProgress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock;
- (BOOL)shouldDecompressImages;
- (void)setShouldDecompressImages:(BOOL)value;
- (nullable NSURLCredential *)credential;
- (void)setCredential:(nullable NSURLCredential *)value;
@end
按照作者的注釋，如果我們想要實(shí)現(xiàn)一個(gè)自定義的下載操作，就必須繼承自NSOperation,同時(shí)實(shí)現(xiàn)SDWebImageDownloaderOperationInterface這個(gè)協(xié)議，我們不去看其他的代碼，只做一個(gè)簡單的猜測:很可能在別的類中，只使用SDWebImageDownloaderOperationInterface和NSOperation中的方法和屬性。
- 使用
NSURLRequest,NSURLSession和SDWebImageDownloaderOptions初始化 - 可以為每一個(gè)
NSOperation自由的添加相應(yīng)對(duì)象 - 設(shè)置是否需要解壓圖片
- 設(shè)置是否需要設(shè)置憑證
@interface SDWebImageDownloaderOperation : NSOperation
關(guān)于SDWebImageDownloaderOperation.h的設(shè)計(jì)，有幾點(diǎn)值得我們注意，首先它是遵守SDWebImageDownloaderOperationInterface協(xié)議的，所以上一節(jié)的那些方法，都必須實(shí)現(xiàn)，我們?cè)谠O(shè)計(jì)這個(gè).h的時(shí)候呢，可以把協(xié)議中的方法再次寫到這個(gè).h中，這樣別人在使用的時(shí)候，就會(huì)更加直觀
。
/**
* The credential used for authentication challenges in `-connection:didReceiveAuthenticationChallenge:`.
*
* This will be overridden by any shared credentials that exist for the username or password of the request URL, if present.
*/
@property (nonatomic, strong, nullable) NSURLCredential *credential;
通過聲明一個(gè)屬性，就實(shí)現(xiàn)了SDWebImageDownloaderOperationInterface協(xié)議中的
- (nullable NSURLCredential *)credential;
- (void)setCredential:(nullable NSURLCredential *)value;
一般情況下，我們?cè)谥鲃?dòng)指明初始化方法的時(shí)候，肯定會(huì)為初始化方法設(shè)定幾個(gè)參數(shù)。那么這些參數(shù)就應(yīng)該以只讀的方式暴露給他人。比如：
初始化方法：
/**
* Initializes a `SDWebImageDownloaderOperation` object
*
* @see SDWebImageDownloaderOperation
*
* @param request the URL request
* @param session the URL session in which this operation will run
* @param options downloader options
*
* @return the initialized instance
*/
- (nonnull instancetype)initWithRequest:(nullable NSURLRequest *)request
inSession:(nullable NSURLSession *)session
options:(SDWebImageDownloaderOptions)options NS_DESIGNATED_INITIALIZER;
只讀的屬性：
/**
* The request used by the operation's task.
*/
@property (strong, nonatomic, readonly, nullable) NSURLRequest *request;
/**
* The operation's task
*/
@property (strong, nonatomic, readonly, nullable) NSURLSessionTask *dataTask;
/**
* The SDWebImageDownloaderOptions for the receiver.
*/
@property (assign, nonatomic, readonly) SDWebImageDownloaderOptions options;
其他的屬性：
/**
* The expected size of data.
*/
@property (assign, nonatomic) NSInteger expectedSize;
/**
* The response returned by the operation's connection.
*/
@property (strong, nonatomic, nullable) NSURLResponse *response;
取消方法：
/**
* Cancels a set of callbacks. Once all callbacks are canceled, the operation is cancelled.
*
* @param token the token representing a set of callbacks to cancel
*
* @return YES if the operation was stopped because this was the last token to be canceled. NO otherwise.
*/
- (BOOL)cancel:(nullable id)token;
這個(gè)方法不是取消任務(wù)的，而是取消任務(wù)中的響應(yīng)，當(dāng)時(shí)當(dāng)任務(wù)中沒有響應(yīng)者的時(shí)候，任務(wù)也會(huì)被取消。
SDWebImageDownloaderOperation.m
我們的目的是下載一張圖片，那么我們最核心的邏輯是什么呢？
- 初始化一個(gè)task
- 添加響應(yīng)者
- 開啟下載任務(wù)
- 處理下載過程和結(jié)束后的事情
也就是說.m中所有的代碼，都是圍繞著上邊4點(diǎn)來設(shè)計(jì)的。 那么我們就詳細(xì)的對(duì)每一步進(jìn)行分析：
1.初始化一個(gè)task
- (nonnull instancetype)init {
return [self initWithRequest:nil inSession:nil options:0];
}
- (nonnull instancetype)initWithRequest:(nullable NSURLRequest *)request
inSession:(nullable NSURLSession *)session
options:(SDWebImageDownloaderOptions)options {
if ((self = [super init])) {
_request = [request copy];
_shouldDecompressImages = YES;
_options = options;
_callbackBlocks = [NSMutableArray new];
_executing = NO;
_finished = NO;
_expectedSize = 0;
_unownedSession = session;
responseFromCached = YES; // Initially wrong until `- URLSession:dataTask:willCacheResponse:completionHandler: is called or not called
_barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderOperationBarrierQueue", DISPATCH_QUEUE_CONCURRENT);
}
return self;
}
- (void)dealloc {
SDDispatchQueueRelease(_barrierQueue);
}
這個(gè)初始化方法里邊有很多我們?cè)?h沒有見過的屬性。我們有必要在此做一些解釋，在接下來的解讀中，就不會(huì)再做出解釋了。
- _callbackBlocks
@property (strong, nonatomic, nonnull) NSMutableArray<SDCallbacksDictionary *> *callbackBlocks;是一個(gè)數(shù)組，數(shù)組中存放的是SDCallbacksDictionary類型的數(shù)據(jù)，那么這個(gè)SDCallbacksDictionary其實(shí)就是一個(gè)字典，typedef NSMutableDictionary<NSString *, id> SDCallbacksDictionary;,key是一個(gè)字符串，這個(gè)字符串有兩種情況:kProgressCallbackKey和kCompletedCallbackKey,也就是說進(jìn)度和完成的回調(diào)都是放到一個(gè)數(shù)組中的。那么字典的值就是回調(diào)的block了。 - _unownedSession
@property (weak, nonatomic, nullable) NSURLSession *unownedSession;,這個(gè)屬性是我們初始化時(shí)候傳進(jìn)來的參數(shù)，作者提到。這個(gè)參數(shù)不一定是可用的。也就是說是不安全的。當(dāng)出現(xiàn)不可用的情況的時(shí)候，就需要使用@property (strong, nonatomic, nullable) NSURLSession *ownedSession;. - responseFromCached 用于設(shè)置是否需要緩存響應(yīng)，默認(rèn)為YES
- _barrierQueue 隊(duì)列，這個(gè)會(huì)在后邊的使用中講解到
2.添加響應(yīng)者
- (nullable id)addHandlersForProgress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
SDCallbacksDictionary *callbacks = [NSMutableDictionary new];
if (progressBlock) callbacks[kProgressCallbackKey] = [progressBlock copy];
if (completedBlock) callbacks[kCompletedCallbackKey] = [completedBlock copy];
dispatch_barrier_async(self.barrierQueue, ^{
[self.callbackBlocks addObject:callbacks];
});
return callbacks;
}
看這段代碼，也很好理解。就是把字典添加到數(shù)組中去，但是這里邊有一個(gè)很重要的知識(shí)點(diǎn)：dispatch_barrier_async,我們做一個(gè)簡單的介紹。
我們可以創(chuàng)建兩種類型的隊(duì)列，串行和并行，也就是DISPATCH_QUEUE_SERIAL,DISPATCH_QUEUE_CONCURRENT。那么dispatch_barrier_async和dispatch_barrier_sync究竟有什么不同之處呢？
barrier這個(gè)詞是柵欄的意思，也就是說是用來做攔截功能的，上邊的這另種都能夠攔截任務(wù)，換句話說，就是只有我的任務(wù)完成后，隊(duì)列后邊的任務(wù)才能完成。
不同之處就是，dispatch_barrier_sync控制了任務(wù)往隊(duì)列添加這一過程，只有當(dāng)我的任務(wù)完成之后，才能往隊(duì)列中添加任務(wù)。dispatch_barrier_async不會(huì)控制隊(duì)列添加任務(wù)。但是只有當(dāng)我的任務(wù)完成后，隊(duì)列中后邊的任務(wù)才會(huì)執(zhí)行。
那么在這里的任務(wù)是往數(shù)組中添加數(shù)據(jù)，對(duì)順序沒什么要求，我們采取dispatch_barrier_async就可以了，已經(jīng)能保證數(shù)據(jù)添加的安全性了。
- (nullable NSArray<id> *)callbacksForKey:(NSString *)key {
__block NSMutableArray<id> *callbacks = nil;
dispatch_sync(self.barrierQueue, ^{
// We need to remove [NSNull null] because there might not always be a progress block for each callback
callbacks = [[self.callbackBlocks valueForKey:key] mutableCopy];
[callbacks removeObjectIdenticalTo:[NSNull null]];
});
return [callbacks copy]; // strip mutability here
}
這個(gè)方法是根據(jù)key取出所有符合key的block，這里采用了同步的方式，相當(dāng)于加鎖。比較有意思的是[self.callbackBlocks valueForKey:key]這段代碼，self.callbackBlocks是一個(gè)數(shù)組，我們假定他的結(jié)構(gòu)是這樣的：
@[@{@"completed" : Block1},
@{@"progress" : Block2},
@{@"completed" : Block3},
@{@"progress" : Block4},
@{@"completed" : Block5},
@{@"progress" : Block6}]
調(diào)用[self.callbackBlocks valueForKey:@"progress"]后會(huì)得到[Block2, Block4, Block6].
removeObjectIdenticalTo:這個(gè)方法會(huì)移除數(shù)組中指定相同地址的元素。
- (BOOL)cancel:(nullable id)token {
__block BOOL shouldCancel = NO;
dispatch_barrier_sync(self.barrierQueue, ^{
[self.callbackBlocks removeObjectIdenticalTo:token];
if (self.callbackBlocks.count == 0) {
shouldCancel = YES;
}
});
if (shouldCancel) {
[self cancel];
}
return shouldCancel;
}
這個(gè)函數(shù)，就是取消某一回調(diào)。使用了dispatch_barrier_sync，保證，必須該隊(duì)列之前的任務(wù)都完成，且該取消任務(wù)結(jié)束后，在將其他的任務(wù)加入隊(duì)列。
3.開啟下載任務(wù)
- (void)start {
@synchronized (self) {
if (self.isCancelled) {
self.finished = YES;
[self reset];
return;
}
#if SD_UIKIT
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;
}
}];
}
#endif
NSURLSession *session = self.unownedSession;
if (!self.unownedSession) {
NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
sessionConfig.timeoutIntervalForRequest = 15;
/**
* Create the session for this task
* We send nil as delegate queue so that the session creates a serial operation queue for performing all delegate
* method calls and completion handler calls.
*/
self.ownedSession = [NSURLSession sessionWithConfiguration:sessionConfig
delegate:self
delegateQueue:nil];
session = self.ownedSession;
}
self.dataTask = [session dataTaskWithRequest:self.request];
self.executing = YES;
}
[self.dataTask resume];
if (self.dataTask) {
for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
progressBlock(0, NSURLResponseUnknownLength, self.request.URL);
}
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self];
});
} else {
[self callCompletionBlocksWithError:[NSError errorWithDomain:NSURLErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Connection can't be initialized"}]];
}
#if SD_UIKIT
Class UIApplicationClass = NSClassFromString(@"UIApplication");
if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) {
return;
}
if (self.backgroundTaskId != UIBackgroundTaskInvalid) {
UIApplication * app = [UIApplication performSelector:@selector(sharedApplication)];
[app endBackgroundTask:self.backgroundTaskId];
self.backgroundTaskId = UIBackgroundTaskInvalid;
}
#endif
}
這一塊也分為幾個(gè)子任務(wù)
-
如果該任務(wù)已經(jīng)被設(shè)置為取消了，那么就無需開啟下載任務(wù)了。并重置。別忘了設(shè)置finished為YES
- (void)reset { dispatch_barrier_async(self.barrierQueue, ^{ [self.callbackBlocks removeAllObjects]; }); self.dataTask = nil; self.imageData = nil; if (self.ownedSession) { [self.ownedSession invalidateAndCancel]; self.ownedSession = nil; } } -
確保能夠開啟下載任務(wù)，我之前在網(wǎng)上跟別的哥們討論過，一開始不太明白下邊的方法的用途，后來想通了，也不知道對(duì)不對(duì)，start方法的目的只是開啟下載任務(wù)，它所要保證的就是調(diào)用start時(shí)，任務(wù)能夠開啟，至于是否下載成功，那不是start 應(yīng)該關(guān)心的事情。
#if SD_UIKIT 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; } }]; } #endif 開啟后，確保關(guān)閉后臺(tái)任務(wù) #if SD_UIKIT Class UIApplicationClass = NSClassFromString(@"UIApplication"); if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) { return; } if (self.backgroundTaskId != UIBackgroundTaskInvalid) { UIApplication * app = [UIApplication performSelector:@selector(sharedApplication)]; [app endBackgroundTask:self.backgroundTaskId]; self.backgroundTaskId = UIBackgroundTaskInvalid; } #endif -
task開啟前的準(zhǔn)備工作
NSURLSession *session = self.unownedSession; if (!self.unownedSession) { NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration]; sessionConfig.timeoutIntervalForRequest = 15; /** * Create the session for this task * We send nil as delegate queue so that the session creates a serial operation queue for performing all delegate * method calls and completion handler calls. */ self.ownedSession = [NSURLSession sessionWithConfiguration:sessionConfig delegate:self delegateQueue:nil]; session = self.ownedSession; } self.dataTask = [session dataTaskWithRequest:self.request]; self.executing = YES; -
開啟task 并處理回調(diào)
[self.dataTask resume]; if (self.dataTask) { for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) { progressBlock(0, NSURLResponseUnknownLength, self.request.URL); } dispatch_async(dispatch_get_main_queue(), ^{ [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self]; }); } else { [self callCompletionBlocksWithError:[NSError errorWithDomain:NSURLErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Connection can't be initialized"}]]; }
4.處理下載過程和結(jié)束后的事情
- (void)URLSession:(NSURLSession *)session
dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {
//'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;
for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
progressBlock(0, expected, self.request.URL);
}
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.dataTask cancel];
}
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
});
[self callCompletionBlocksWithError:[NSError errorWithDomain:NSURLErrorDomain code:((NSHTTPURLResponse *)response).statusCode userInfo:nil]];
[self done];
}
if (completionHandler) {
completionHandler(NSURLSessionResponseAllow);
}
}
上邊的代碼，處理了當(dāng)收到響應(yīng)后要做的事情。我們規(guī)定，把沒有收到響應(yīng)碼或者響應(yīng)碼小于400認(rèn)定為正常的情況，其中304比較特殊，因?yàn)楫?dāng)stateCode為304的時(shí)候，便是這個(gè)響應(yīng)沒有變化，可以再緩存中讀取。那么其他的情況，就可以認(rèn)定為錯(cuò)誤的請(qǐng)求。
當(dāng)一切順利的時(shí)候，基本上就是給早已定義的屬性賦值，上邊的代碼邏輯比較簡單，在這里就不做介紹了。
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
[self.imageData appendData:data];
if ((self.options & SDWebImageDownloaderProgressiveDownload) && self.expectedSize > 0) {
// The following code is from http://www.cocoaintheshell.com/2011/05/progressive-images-download-imageio/
// Thanks to the author @Nyx0uf
// 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 didCompleteWithError.) So save it here and pass it on later.
#if SD_UIKIT || SD_WATCH
orientation = [[self class] orientationFromPropertyValue:(orientationValue == -1 ? 1 : orientationValue)];
#endif
}
}
if (width + height > 0 && totalSize < self.expectedSize) {
// Create the image
CGImageRef partialImageRef = CGImageSourceCreateImageAtIndex(imageSource, 0, NULL);
#if SD_UIKIT || SD_WATCH
// 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) {
#if SD_UIKIT || SD_WATCH
UIImage *image = [UIImage imageWithCGImage:partialImageRef scale:1 orientation:orientation];
#elif SD_MAC
UIImage *image = [[UIImage alloc] initWithCGImage:partialImageRef size:NSZeroSize];
#endif
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);
[self callCompletionBlocksWithImage:image imageData:nil error:nil finished:NO];
}
}
CFRelease(imageSource);
}
for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
progressBlock(self.imageData.length, self.expectedSize, self.request.URL);
}
}
上邊的方法處理了接收到數(shù)據(jù)的邏輯。按照正常想法，當(dāng)我們接受到數(shù)據(jù)的時(shí)候，只要把收據(jù)拼接起來，根據(jù)設(shè)置選項(xiàng)，調(diào)用process回調(diào)就行了。那么為什么這個(gè)方法中用了如此大的篇幅來處理圖片數(shù)據(jù)呢？
答案就是，即使圖片沒有下載完，我們也能根據(jù)已經(jīng)獲取的圖片數(shù)據(jù)，來顯示一張數(shù)據(jù)不完整的圖片。 通過這樣一個(gè)細(xì)節(jié)，我想到了很多應(yīng)用場景，比如說，之前看到過一個(gè)場景，通過滑動(dòng)slider 自上而下的顯示一張圖片的部分內(nèi)容，我們完全可以通過上邊的代碼來實(shí)現(xiàn)。根據(jù)slider的value來控制整個(gè)NSData的大小，來合成圖片。當(dāng)然這也跟圖片的組成有關(guān)。
- (void)URLSession:(NSURLSession *)session
dataTask:(NSURLSessionDataTask *)dataTask
willCacheResponse:(NSCachedURLResponse *)proposedResponse
completionHandler:(void (^)(NSCachedURLResponse *cachedResponse))completionHandler {
responseFromCached = NO; // If this method is called, it means the response wasn't read from cache
NSCachedURLResponse *cachedResponse = proposedResponse;
if (self.request.cachePolicy == NSURLRequestReloadIgnoringLocalCacheData) {
// Prevents caching of responses
cachedResponse = nil;
}
if (completionHandler) {
completionHandler(cachedResponse);
}
}
該方法用于響應(yīng)的緩存設(shè)置，如果把回調(diào)的參數(shù)設(shè)置為nil，那么就不會(huì)緩存響應(yīng)，總之，真正緩存的數(shù)據(jù)就是回調(diào)中的參數(shù)。
- (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];
}
該方法是處理了圖片下載完成之后的邏輯，也沒有很特別的東西，比較復(fù)雜的是對(duì)完成后的數(shù)據(jù)的處理更加完善。要做到這一點(diǎn)，確實(shí)需要N多知識(shí)的積累啊。
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler {
NSURLSessionAuthChallengeDisposition disposition = NSURLSessionAuthChallengePerformDefaultHandling;
__block NSURLCredential *credential = nil;
if ([challenge.protectionSpace.authenticationMethod isEqualToString:NSURLAuthenticationMethodServerTrust]) {
if (!(self.options & SDWebImageDownloaderAllowInvalidSSLCertificates)) {
disposition = NSURLSessionAuthChallengePerformDefaultHandling;
} else {
credential = [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust];
disposition = NSURLSessionAuthChallengeUseCredential;
}
} else {
if (challenge.previousFailureCount == 0) {
if (self.credential) {
credential = self.credential;
disposition = NSURLSessionAuthChallengeUseCredential;
} else {
disposition = NSURLSessionAuthChallengeCancelAuthenticationChallenge;
}
} else {
disposition = NSURLSessionAuthChallengeCancelAuthenticationChallenge;
}
}
if (completionHandler) {
completionHandler(disposition, credential);
}
}
這個(gè)方法跟HTTPS有點(diǎn)關(guān)系，要想說明白這個(gè)方法究竟干了什么事？ 需要對(duì)驗(yàn)證有點(diǎn)了解才行。
當(dāng)我們發(fā)出了一個(gè)請(qǐng)求，這個(gè)請(qǐng)求到達(dá)服務(wù)器后，假定服務(wù)器設(shè)置了需要驗(yàn)證。那么這個(gè)方法就會(huì)被調(diào)用。服務(wù)器會(huì)返回去一個(gè)NSURLAuthenticationChallenge。通過NSURLAuthenticationChallenge的protectionSpace,獲取授權(quán)method。如果這個(gè)metho是服務(wù)器信任的， 那么我們就可以直接使用服務(wù)器返回的證書，當(dāng)然，我們也可以使用自己的證書，其他情況都會(huì)被認(rèn)為驗(yàn)證失敗，當(dāng)前請(qǐng)求將會(huì)被取消。當(dāng)有了證書后，客戶端就可以使用證書中的公鑰對(duì)數(shù)據(jù)進(jìn)行加密了。
其他的方法：
#if SD_UIKIT || SD_WATCH
+ (UIImageOrientation)orientationFromPropertyValue:(NSInteger)value {
switch (value) {
case 1:
return UIImageOrientationUp;
case 3:
return UIImageOrientationDown;
case 8:
return UIImageOrientationLeft;
case 6:
return UIImageOrientationRight;
case 2:
return UIImageOrientationUpMirrored;
case 4:
return UIImageOrientationDownMirrored;
case 5:
return UIImageOrientationLeftMirrored;
case 7:
return UIImageOrientationRightMirrored;
default:
return UIImageOrientationUp;
}
}
#endif
- (nullable UIImage *)scaledImageForKey:(nullable NSString *)key image:(nullable UIImage *)image {
return SDScaledImageForKey(key, image);
}
- (BOOL)shouldContinueWhenAppEntersBackground {
return self.options & SDWebImageDownloaderContinueInBackground;
}
- (void)callCompletionBlocksWithError:(nullable NSError *)error {
[self callCompletionBlocksWithImage:nil imageData:nil error:error finished:YES];
}
- (void)callCompletionBlocksWithImage:(nullable UIImage *)image
imageData:(nullable NSData *)imageData
error:(nullable NSError *)error
finished:(BOOL)finished {
NSArray<id> *completionBlocks = [self callbacksForKey:kCompletedCallbackKey];
dispatch_main_async_safe(^{
for (SDWebImageDownloaderCompletedBlock completedBlock in completionBlocks) {
completedBlock(image, imageData, error, finished);
}
});
}
總結(jié)
SDWebImageDownloaderOperation提供了下載單張圖片的能力，在真實(shí)開發(fā)中。圖片往往都是一組一組的出現(xiàn)的。那么該如何管理這些組圖片呢？下一篇文章將會(huì)揭曉答案。
由于個(gè)人知識(shí)有限，如有錯(cuò)誤之處，還望各路大俠給予指出啊
