一. SDWebImage的使用
1. 基本使用
// ViewController.m
#import "ViewController.h"
#import "UIImageView+WebCache.h"
#import "SDWebImageManager.h"
#import "SDWebImageDownloader.h"
#import "UIImage+GIF.h"
#import "NSData+ImageContentType.h"
@interface ViewController ()
@property (weak, nonatomic) IBOutlet UIImageView *imageView;
@end
@implementation ViewController
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
[self download];
}
//1.下载图片且需要获取下载进度
//内存缓存&磁盘缓存
-(void)download
{
[self.imageView sd_setImageWithURL:[NSURL URLWithString:@"http://img4.duitang.com/uploads/blog/201310/18/20131018213446_smUw4.thumb.600_0.jpeg"] placeholderImage:[UIImage imageNamed:@"Snip20160221_306"] options:SDWebImageCacheMemoryOnly | SDWebImageProgressiveDownload progress:^(NSInteger receivedSize, NSInteger expectedSize) {
} completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
switch (cacheType) {
case SDImageCacheTypeNone:
NSLog(@"直接下载");
break;
case SDImageCacheTypeDisk:
NSLog(@"磁盘缓存");
break;
case SDImageCacheTypeMemory:
NSLog(@"内存缓存");
break;
default:
break;
}
}];
NSLog(@"%@",[NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject]);
}
//2.只需要简单获得一张图片,不设置
//内存缓存&磁盘缓存
-(void)download2
{
[[SDWebImageManager sharedManager] downloadImageWithURL:[NSURL URLWithString:@"http://img4.duitang.com/uploads/blog/201310/18/20131018213446_smUw4.thumb.600_0.jpeg"] options:0 progress:^(NSInteger receivedSize, NSInteger expectedSize) {
NSLog(@"%f",1.0 * receivedSize / expectedSize);
} completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
//得到图片
self.imageView.image = image;
}];
}
//3.不需要任何的缓存处理
//没有做任何缓存处理
-(void)download3
{
//data:图片的二进制数据
[[SDWebImageDownloader sharedDownloader] downloadImageWithURL:[NSURL URLWithString:@"http://img4.duitang.com/uploads/blog/201310/18/20131018213446_smUw4.thumb.600_0.jpeg"] options:0 progress:^(NSInteger receivedSize, NSInteger expectedSize) {
} completed:^(UIImage *image, NSData *data, NSError *error, BOOL finished) {
[[NSOperationQueue mainQueue]addOperationWithBlock:^{
self.imageView.image = image;
}];
}];
}
//4.播放Gif图片
-(void)gif
{
NSLog(@"%s",__func__);
//self.imageView.image = [UIImage imageNamed:@"39e805d5ad6eddc4f80259d23bdbb6fd536633ca"];
UIImage *image = [UIImage sd_animatedGIFNamed:@"39e805d5ad6eddc4f80259d23bdbb6fd536633ca"];
self.imageView.image = image;
}
// 获取图片类型
-(void)type
{
NSData *imageData = [NSData dataWithContentsOfFile:@"/Users/xiaomage/Desktop/Snip20160221_306.png"];
NSString *typeStr = [NSData sd_contentTypeForImageData:imageData];
NSLog(@"%@",typeStr);
}
@end
2. 内存紧张的处理
如果内存紧张，在AppDelegate.m里面会做相应的处理，如下：
// AppDelegate.m
// 04-掌握-SDWebImage的基本使用
#import "AppDelegate.h"
#import "SDWebImageManager.h"
@interface AppDelegate ()
@end
@implementation AppDelegate
-(void)applicationDidReceiveMemoryWarning:(UIApplication *)application
{
//1.清空缓存
//clear:直接删除缓存目录下面的文件,然后重新创建空的缓存文件
//clean:清除过期缓存,计算当前缓存的大小,和设置的最大缓存数量比较,如果超出那么会继续删除(按照文件了创建的先后顺序)
//过期时间:7天
[[SDWebImageManager sharedManager].imageCache clearMemory];
//2.取消当前所有的操作
[[SDWebImageManager sharedManager] cancelAll];
//3.最大并发数量 == 6
//4.缓存文件的保存名称如何处理? 拿到图片的URL路径,对该路径进行MD5加密
//5.该框架内部对内存警告的处理方式? 内部通过监听通知的方式清理缓存
//6.该框架进行缓存处理的方式:可变字典--->NSCache
//7.如何判断图片的类型: 在判断图片类型的时候，只匹配第一个字节
//8.队列中任务的处理方式:FIFO 先进先出
//9.如何下载图片的? 发送网络请求下载图片，NSURLConnection
//10.请求超时的时间 15秒
//[NSData dataWithContentsOfURL:<#(nonnull NSURL *)#>]
}
@end
3. 面试相关
1> 面试题
1] 如何防止一个url对应的图片重复下载
* "cell下载图片思路 – 有沙盒缓存"
2] SDWebImage的默认缓存时长是多少？
* 7天
3] SDWebImage底层是怎么实现的？
* 上课PPT的"cell下载图片思路 – 有沙盒缓存"
2> 常用方法
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder;
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options;
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder completed:(SDWebImageCompletionBlock)completedBlock;
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock;
3> 内存处理：当app接收到内存警告时，SDWebImage做了什么？
- SDWebImage会监听系统的UIApplicationDidReceiveMemoryWarningNotification通知，一旦收到通知，就会清理内存
- 应用程序将要终止的通知UIApplicationWillTerminateNotification, 清理磁盘
- 应用程序进入后台的通知UIApplicationDidEnterBackgroundNotification，也会清理磁盘
4> SDWebImageOptions
* SDWebImageRetryFailed : 下载失败后，会自动重新下载
* SDWebImageLowPriority : 当正在进行UI交互时，自动暂停内部的一些下载操作
* SDWebImageRetryFailed | SDWebImageLowPriority : 拥有上面2个功能
// 图片的格式
PNG: 无损压缩！压缩比较低。 PNG图片一般会JPG大。
- GPU解压缩的消耗非常小。解压缩的速度比较快，比较清晰，苹果推荐使用
JPG: 有损压缩！压缩比非常高！照相机使用。
- GPU解压缩的消耗比较大
GIF: 可动画的图片
BMP: (位图)，没有任何压缩。几乎不用
二. 源码分析
相信对于广大的iOS开发者，对SDWebImage并不会陌生，这个框架通过给UIImageView和UIButton添加分类，实现一个异步下载图片并且支持缓存的功能。整个框架的接口非常简洁，每个类的分工都很明确，是很值得大家学习的。
在使用这个框架的时候，只需要提供一个下载的url和占位图就可以在回调里拿到下载后的图片：
[imageview sd_setImageWithURL:[NSURL URLWithString:@“pic.jpg”] placeholderImage:[UIImage imageNamed:@“placeholder”] completed:^(UIImage * _Nullable image, NSError * _Nullable error, SDImageCacheType cacheType, NSURL * _Nullable imageURL) {
imageview.image = image;
NSLog(@"图片加载完成");
}];
而且我们还可以不设置占位图片，也可以不使用回调的block，非常灵活：
//图片下载完成后直接显示下载后的图片
[imageview sd_setImageWithURL:[NSURL URLWithString:@“pic.jpg”]];
SDWebImage框架结构如下：
SDWebImage.png
- 这个框架的核心类是SDWebImageManger，在外部有UIImageView+WebCache 和 UIButton+WebCache 为下载图片的操作提供接口。
- 内部有SDWebImageManger负责处理和协调 SDWebImageDownloader 和 SDImageCache。
- SDWebImageDownloader负责具体的下载任务，SDImageCache负责关于缓存的工作：添加，删除，查询缓存。
该框架分为两个层：UIKit层（负责接收下载参数）和工具层（负责下载操作和缓存），基本流程大概清楚了，我们看一下每个层具体实现吧！
1. UIKit层
UIKit层主要包括：UIImageView+WebCache、UIButton+WebCache、UIView+WebCache
该框架最外层的类是UIImageView +WebCache，我们将图片的URL，占位图片直接给这个类。这个类提供的接口非常灵活，可以根据我们自己的需求来调用其中某一个方法，而这些方法到最后都会走到：
//所有的方法最终都会调用这个方法
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
context:(nullable SDWebImageContext *)context
progress:(nullable SDImageLoaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock {
//实际调用了UIView+WebCache分类的这个方法
[self sd_internalSetImageWithURL:url
placeholderImage:placeholder
options:options
context:context
setImageBlock:nil
progress:progressBlock
completed:^(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, SDImageCacheType cacheType, BOOL finished, NSURL * _Nullable imageURL) {
if (completedBlock) {
completedBlock(image, error, cacheType, imageURL);
}
}];
}
可以看出，这个方法实际上会调用UIView+WebCache分类的sd_internalSetImageWithURL:url 方法。
为什么不是UIImageView+WebCache而要上一层到UIView的分类里呢？
因为SDWebImage框架也支持UIButton的下载图片等方法，所以需要在它们的父类：UIView里面统一一个下载方法。
下面看一下这个方法的实现：
//实际调用的方法
- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
context:(nullable SDWebImageContext *)context
setImageBlock:(nullable SDSetImageBlock)setImageBlock
progress:(nullable SDImageLoaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
context = [context copy]; // copy to avoid mutable object
//先看看当前是否已存在任务下载
NSString *validOperationKey = context[SDWebImageContextSetImageOperationKey];
if (!validOperationKey) {
validOperationKey = NSStringFromClass([self class]);
}
self.sd_latestOperationKey = validOperationKey;
//下面这行代码是保证没有当前正在进行的异步下载操作, 使它不会与即将进行的操作发生冲突
[self sd_cancelImageLoadOperationWithKey:validOperationKey];
self.sd_imageURL = url;
//先将占位图设置上去（在不延迟添加占位图的option下）
if (!(options & SDWebImageDelayPlaceholder)) {
dispatch_main_async_safe(^{
[self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock cacheType:SDImageCacheTypeNone imageURL:url];
});
}
//如果url存在,开始下载
if (url) {
// 重置进度条
NSProgress *imageProgress = objc_getAssociatedObject(self, @selector(sd_imageProgress));
if (imageProgress) {
imageProgress.totalUnitCount = 0;
imageProgress.completedUnitCount = 0;
}
#if SD_UIKIT || SD_MAC
// 检查并开始下载进度指示器
[self sd_startImageIndicator];
id<SDWebImageIndicator> imageIndicator = self.sd_imageIndicator;
#endif
// 创建一个SDWebImageManager
SDWebImageManager *manager = context[SDWebImageContextCustomManager];
if (!manager) {
manager = [SDWebImageManager sharedManager];
}
//定义combinedProgressBlock进度block
SDImageLoaderProgressBlock combinedProgressBlock = ^(NSInteger receivedSize, NSInteger expectedSize, NSURL * _Nullable targetURL) {
if (imageProgress) {
imageProgress.totalUnitCount = expectedSize;
imageProgress.completedUnitCount = receivedSize;
}
#if SD_UIKIT || SD_MAC
if ([imageIndicator respondsToSelector:@selector(updateIndicatorProgress:)]) {
double progress = 0;
if (expectedSize != 0) {
progress = (double)receivedSize / expectedSize;
}
progress = MAX(MIN(progress, 1), 0); // 0.0 - 1.0
dispatch_async(dispatch_get_main_queue(), ^{
[imageIndicator updateIndicatorProgress:progress];
});
}
#endif
if (progressBlock) {
progressBlock(receivedSize, expectedSize, targetURL);
}
};
@weakify(self);
//调用SDWebImageManager的loadImageWithURL方法
id <SDWebImageOperation> operation = [manager loadImageWithURL:url options:options context:context progress:combinedProgressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
//图片下载完成的回调
@strongify(self);
if (!self) { return; }
// if the progress not been updated, mark it to complete state
if (imageProgress && finished && !error && imageProgress.totalUnitCount == 0 && imageProgress.completedUnitCount == 0) {
imageProgress.totalUnitCount = SDWebImageProgressUnitCountUnknown;
imageProgress.completedUnitCount = SDWebImageProgressUnitCountUnknown;
}
#if SD_UIKIT || SD_MAC
// 检查并结束下载进度指示器
if (finished) {
[self sd_stopImageIndicator];
}
#endif
BOOL shouldCallCompletedBlock = finished || (options & SDWebImageAvoidAutoSetImage);
BOOL shouldNotSetImage = ((image && (options & SDWebImageAvoidAutoSetImage)) ||
(!image && !(options & SDWebImageDelayPlaceholder)));
//定义下载完成之后的回调block
SDWebImageNoParamsBlock callCompletedBlockClojure = ^{
if (!self) { return; }
if (!shouldNotSetImage) {
[self sd_setNeedsLayout];
}
if (completedBlock && shouldCallCompletedBlock) {
completedBlock(image, data, error, cacheType, finished, url);
}
};
// case 1a: we got an image, but the SDWebImageAvoidAutoSetImage flag is set
// OR
// case 1b: we got no image and the SDWebImageDelayPlaceholder is not set
if (shouldNotSetImage) {
dispatch_main_async_safe(callCompletedBlockClojure);
return;
}
UIImage *targetImage = nil;
NSData *targetData = nil;
if (image) {
// case 2a: we got an image and the SDWebImageAvoidAutoSetImage is not set
targetImage = image;
targetData = data;
} else if (options & SDWebImageDelayPlaceholder) {
// case 2b: we got no image and the SDWebImageDelayPlaceholder flag is set
targetImage = placeholder;
targetData = nil;
}
#if SD_UIKIT || SD_MAC
// check whether we should use the image transition
SDWebImageTransition *transition = nil;
if (finished && (options & SDWebImageForceTransition || cacheType == SDImageCacheTypeNone)) {
transition = self.sd_imageTransition;
}
#endif
// 保证在主线程执行,然后设置图片
dispatch_main_async_safe(^{
#if SD_UIKIT || SD_MAC
[self sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock transition:transition cacheType:cacheType imageURL:imageURL];
#else
[self sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock cacheType:cacheType imageURL:imageURL];
#endif
callCompletedBlockClojure();
});
}];
// 在操作缓存字典（operationDictionary）里添加operation，表示当前的操作正在进行
[self sd_setImageLoadOperation:operation forKey:validOperationKey];
} else {
//如果url不存在
#if SD_UIKIT || SD_MAC
//取消下载进度指示器
[self sd_stopImageIndicator];
#endif
//在completedBlock里传入error（url为空）
dispatch_main_async_safe(^{
if (completedBlock) {
NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidURL userInfo:@{NSLocalizedDescriptionKey : @"Image url is nil"}];
completedBlock(nil, nil, error, SDImageCacheTypeNone, YES, url);
}
});
}
}
可以发现，上面方法主要做了：
- 创建一个SDWebImageManager
- 调用SDWebImageManager类的loadImageWithURL:方法
- 下载完成后，在主线程设置图片
- 在操作缓存字典（operationDictionary）里添加operation，表示当前的操作正在进行
上面代码的核心方法是SDWebImageManager类的loadImageWithURL:方法。
值得一提的是，在这一层，使用一个字典operationDictionary专门用作存储操作的缓存，随时添加，删除操作任务，而这个字典是UIView+WebCacheOperation分类的关联对象，它的存取方法使用运行时来操作。
static char loadOperationKey;
// key is strong, value is weak because operation instance is retained by SDWebImageManager's runningOperations property
// we should use lock to keep thread-safe because these method may not be acessed from main queue
typedef NSMapTable<NSString *, id<SDWebImageOperation>> SDOperationsDictionary;
@implementation UIView (WebCacheOperation)
//使用一个字典operationDictionary专门用作存储操作的缓存，随时添加，删除操作任务。而这个字典是UIView+WebCacheOperation分类的关联对象，它的存取方法使用运行时来操作
- (SDOperationsDictionary *)sd_operationDictionary {
@synchronized(self) {
SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
if (operations) {
return operations;
}
operations = [[NSMapTable alloc] initWithKeyOptions:NSPointerFunctionsStrongMemory valueOptions:NSPointerFunctionsWeakMemory capacity:0];
objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
return operations;
}
}
为什么不直接在UIImageView+WebCache里直接关联这个对象呢？我觉得这里作者应该是遵从面向对象的单一职责原则（SRP：Single responsibility principle），就连类都要履行这个职责，何况分类呢？这里作者专门创造一个分类UIView+WebCacheOperation来管理操作缓存（字典）。
到这里，UIKit层上面的东西都讲完了，现在开始正式讲解工具层。
2. 工具层
工具层主要包括：SDWebImageManger、SDWebImageDownloader、SDImageCache
我们先看一下一次图片下载的流程图：
流程图.png
SDWebImageManager同时管理SDImageCache和SDWebImageDownloader两个类，它是这一层的老大哥。
在下载任务开始的时候，SDWebImageManager首先访问SDImageCache来查询是否存在缓存，如果有缓存，直接返回缓存的图片。如果没有缓存，就命令SDWebImageDownloader来下载图片，下载成功后，存入缓存，显示图片。
我们先看一下SDWebImageManager的几个重要属性：
@interface SDWebImageManager ()
@property (strong, nonatomic, readwrite, nonnull) SDImageCache *imageCache; //管理缓存
@property (strong, nonatomic, readwrite, nonnull) id<SDImageLoader> imageLoader; //下载器
@property (strong, nonatomic, nonnull) NSMutableSet<NSURL *> *failedURLs; //记录失效url的名单
@property (strong, nonatomic, nonnull) dispatch_semaphore_t failedURLsLock; // a lock to keep the access to `failedURLs` thread-safe
@property (strong, nonatomic, nonnull) NSMutableSet<SDWebImageCombinedOperation *> *runningOperations; //记录当前正在执行的操作
@property (strong, nonatomic, nonnull) dispatch_semaphore_t runningOperationsLock; // a lock to keep the access to `runningOperations` thread-safe
@end
SDWebImageManager下载图片的方法只有一个，也就是上面我们说的loadImageWithURL:方法，如下：
//SDWebImageManager类的loadImageWithURL:方法
- (SDWebImageCombinedOperation *)loadImageWithURL:(nullable NSURL *)url
options:(SDWebImageOptions)options
context:(nullable SDWebImageContext *)context
progress:(nullable SDImageLoaderProgressBlock)progressBlock
completed:(nonnull SDInternalCompletionBlock)completedBlock {
// Invoking this method without a completedBlock is pointless
NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");
//如果传进来的是一个NSString，则把NSString转化为NSURL
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;
}
SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
operation.manager = self;
BOOL isFailedUrl = NO;
if (url) {
//SDWebImage使用信号量锁
SD_LOCK(self.failedURLsLock);
//self.failedURLs是NSURL的黑名单，一般情况下，如果URL在这个黑名单里，那么就不会尝试加载这个图片，直接返回
isFailedUrl = [self.failedURLs containsObject:url];
SD_UNLOCK(self.failedURLsLock);
}
//SDWebImageRetryFailed即使URL被加入了黑名单，也要尝试加载这个URL对应的图片
//如果URL长度为0，或者URL被加入了黑名单并且没有设置SDWebImageRetryFailed，那么就直接回调完成的block
if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
//那么就直接回调完成的block(SDWebImageErrorDomain使用了const)
[self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidURL userInfo:@{NSLocalizedDescriptionKey : @"Image url is nil"}] url:url];
return operation;
}
SD_LOCK(self.runningOperationsLock);
[self.runningOperations addObject:operation];
SD_UNLOCK(self.runningOperationsLock);
//获取SDWebImageOptionsResult
SDWebImageOptionsResult *result = [self processedResultForURL:url options:options context:context];
// 开始从缓存中加载图片
[self callCacheProcessForOperation:operation url:url options:result.options context:result.context progress:progressBlock completed:completedBlock];
return operation;
}
上面方法的核心方法是callCacheProcessForOperation:方法，也就是开始从缓存中加载图片，进入此方法：
// 查询缓存进程
// Query cache process
- (void)callCacheProcessForOperation:(nonnull SDWebImageCombinedOperation *)operation
url:(nonnull NSURL *)url
options:(SDWebImageOptions)options
context:(nullable SDWebImageContext *)context
progress:(nullable SDImageLoaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
// 判断是否需要查缓存
BOOL shouldQueryCache = !SD_OPTIONS_CONTAINS(options, SDWebImageFromLoaderOnly);
if (shouldQueryCache) { //需要查缓存
id<SDWebImageCacheKeyFilter> cacheKeyFilter = context[SDWebImageContextCacheKeyFilter];
NSString *key = [self cacheKeyForURL:url cacheKeyFilter:cacheKeyFilter];
@weakify(operation);
//queryImageForKey从缓存中加载图片
operation.cacheOperation = [self.imageCache queryImageForKey:key options:options context:context completion:^(UIImage * _Nullable cachedImage, NSData * _Nullable cachedData, SDImageCacheType cacheType) {
@strongify(operation);
if (!operation || operation.isCancelled) {
// Image combined operation cancelled by user
[self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorCancelled userInfo:nil] url:url];
[self safelyRemoveOperationFromRunning:operation];
return;
}
// Continue download process
// 如果缓存中没有拿到image,就从网络上下载图片
[self callDownloadProcessForOperation:operation url:url options:options context:context cachedImage:cachedImage cachedData:cachedData cacheType:cacheType progress:progressBlock completed:completedBlock];
}];
} else { //不需要查缓存,直接下载
// Continue download process
[self callDownloadProcessForOperation:operation url:url options:options context:context cachedImage:nil cachedData:nil cacheType:SDImageCacheTypeNone progress:progressBlock completed:completedBlock];
}
}
可以发现，上面代码：
- 判断是否需要查缓存
- 如果不需要查缓存就直接调用callDownloadProcessForOperation:下载，如果需要查缓存，往下走
- 调用queryImageForKey:从缓存中加载图片，如果缓存中没有拿到image，就从网络上下载图片
上面步骤有两个核心方法：
① queryImageForKey:从缓存中加载图片
② callDownloadProcessForOperation:下载图片
① queryImageForKey:从缓存中加载图片
- (id<SDWebImageOperation>)queryImageForKey:(NSString *)key options:(SDWebImageOptions)options context:(nullable SDWebImageContext *)context completion:(nullable SDImageCacheQueryCompletionBlock)completionBlock {
//由于我们在使用API的时候只设置SDWebImageOptions，所以这里就是根据用户设置的SDWebImageOptions去设置SDImageCacheOptions
SDImageCacheOptions cacheOptions = 0;
if (options & SDWebImageQueryMemoryData) cacheOptions |= SDImageCacheQueryMemoryData;
if (options & SDWebImageQueryMemoryDataSync) cacheOptions |= SDImageCacheQueryMemoryDataSync;
if (options & SDWebImageQueryDiskDataSync) cacheOptions |= SDImageCacheQueryDiskDataSync;
if (options & SDWebImageScaleDownLargeImages) cacheOptions |= SDImageCacheScaleDownLargeImages;
if (options & SDWebImageAvoidDecodeImage) cacheOptions |= SDImageCacheAvoidDecodeImage;
if (options & SDWebImageDecodeFirstFrameOnly) cacheOptions |= SDImageCacheDecodeFirstFrameOnly;
if (options & SDWebImagePreloadAllFrames) cacheOptions |= SDImageCachePreloadAllFrames;
if (options & SDWebImageMatchAnimatedImageClass) cacheOptions |= SDImageCacheMatchAnimatedImageClass;
//这里开始调用SDImageCache对象的queryCacheOperationForKey:方法去缓存中查找有没有这个URL对应的图片
return [self queryCacheOperationForKey:key options:cacheOptions context:context done:completionBlock];
}
代码解释如上注释，再进入queryCacheOperationForKey:
//核心方法：查询缓存
//去缓存中查找有没有这个URL对应的图片
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key options:(SDImageCacheOptions)options context:(nullable SDWebImageContext *)context done:(nullable SDImageCacheQueryCompletionBlock)doneBlock {
if (!key) {
if (doneBlock) {
doneBlock(nil, nil, SDImageCacheTypeNone);
}
return nil;
}
id<SDImageTransformer> transformer = context[SDWebImageContextImageTransformer];
if (transformer) {
// grab the transformed disk image if transformer provided
NSString *transformerKey = [transformer transformerKey];
key = SDTransformedKeyForKey(key, transformerKey);
}
// 首先检查内存中有没有
UIImage *image = [self imageFromMemoryCacheForKey:key];
if (image) {
if (options & SDImageCacheDecodeFirstFrameOnly) {
// 保证静态图片
// Ensure static image
Class animatedImageClass = image.class;
if (image.sd_isAnimated || ([animatedImageClass isSubclassOfClass:[UIImage class]] && [animatedImageClass conformsToProtocol:@protocol(SDAnimatedImage)])) {
#if SD_MAC
image = [[NSImage alloc] initWithCGImage:image.CGImage scale:image.scale orientation:kCGImagePropertyOrientationUp];
#else
image = [[UIImage alloc] initWithCGImage:image.CGImage scale:image.scale orientation:image.imageOrientation];
#endif
}
} else if (options & SDImageCacheMatchAnimatedImageClass) {
// Check image class matching
Class animatedImageClass = image.class;
Class desiredImageClass = context[SDWebImageContextAnimatedImageClass];
if (desiredImageClass && ![animatedImageClass isSubclassOfClass:desiredImageClass]) {
image = nil;
}
}
}
//如果内存中image有值,并且可以只查内存,那就回调block,传入image
BOOL shouldQueryMemoryOnly = (image && !(options & SDImageCacheQueryMemoryData));
if (shouldQueryMemoryOnly) {
if (doneBlock) {
doneBlock(image, nil, SDImageCacheTypeMemory);
}
return nil;
}
// 如果内存中没有或者需要查磁盘缓存,再检查磁盘中有没有
NSOperation *operation = [NSOperation new];
// Check whether we need to synchronously query disk
// 1. in-memory cache hit & memoryDataSync
// 2. in-memory cache miss & diskDataSync
BOOL shouldQueryDiskSync = ((image && options & SDImageCacheQueryMemoryDataSync) ||
(!image && options & SDImageCacheQueryDiskDataSync));
//定义查询磁盘缓存的回调block
void(^queryDiskBlock)(void) = ^{
// 在用之前就判断operation是否被取消了，作者考虑的非常严谨
if (operation.isCancelled) {
if (doneBlock) {
doneBlock(nil, nil, SDImageCacheTypeNone);
}
return;
}
@autoreleasepool {
//去磁盘读取图片data
NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
UIImage *diskImage;
SDImageCacheType cacheType = SDImageCacheTypeNone;
if (image) {
// the image is from in-memory cache, but need image data
diskImage = image;
cacheType = SDImageCacheTypeMemory;
} else if (diskData) {
cacheType = SDImageCacheTypeDisk;
// decode image data only if in-memory cache missed
//如果内存中没有缓存,就将diskData转成diskImage
diskImage = [self diskImageForKey:key data:diskData options:options context:context];
if (diskImage && self.config.shouldCacheImagesInMemory) {
//cost被用来计算缓存中所有对象的代价。当内存受限或者所有缓存对象的总代价超过了最大允许的值时，缓存会移除其中的一些对象
NSUInteger cost = diskImage.sd_memoryCost;
//再将图片缓存到memoryCache里面
[self.memoryCache setObject:diskImage forKey:key cost:cost];
}
}
//完成回调
if (doneBlock) {
if (shouldQueryDiskSync) {
doneBlock(diskImage, diskData, cacheType);
} else {
dispatch_async(dispatch_get_main_queue(), ^{
doneBlock(diskImage, diskData, cacheType);
});
}
}
}
};
// 调用查询磁盘缓存的回调block
// Query in ioQueue to keep IO-safe
if (shouldQueryDiskSync) {
//唯一的子线程：self.ioQueue 保证IO操作安全
dispatch_sync(self.ioQueue, queryDiskBlock);
} else {
dispatch_async(self.ioQueue, queryDiskBlock);
}
return operation;
}
可以发现：
- 首先检查内存中有没有
- 如果内存中image有值，并且可以只查内存，那就回调block，传入image
- 如果内存中没有或者需要查磁盘缓存，再检查磁盘中有没有
- 磁盘中读取到图片后，再将图片缓存到memoryCache里面
上面方法比较简单，只看去磁盘读取图片data的方法：diskImageDataBySearchingAllPathsForKey:
- (nullable NSData *)diskImageDataBySearchingAllPathsForKey:(nullable NSString *)key {
if (!key) {
return nil;
}
NSData *data = [self.diskCache dataForKey:key];
if (data) {
return data;
}
// Addtional cache path for custom pre-load cache
if (self.additionalCachePathBlock) {
NSString *filePath = self.additionalCachePathBlock(key);
if (filePath) {
data = [NSData dataWithContentsOfFile:filePath options:self.config.diskCacheReadingOptions error:nil];
}
}
return data;
}
[self.diskCache dataForKey:key];
- (NSData *)dataForKey:(NSString *)key {
NSParameterAssert(key);
NSString *filePath = [self cachePathForKey:key];
NSData *data = [NSData dataWithContentsOfFile:filePath options:self.config.diskCacheReadingOptions error:nil];
if (data) {
return data;
}
// fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
// checking the key with and without the extension
data = [NSData dataWithContentsOfFile:filePath.stringByDeletingPathExtension options:self.config.diskCacheReadingOptions error:nil];
if (data) {
return data;
}
return nil;
}
这俩方法都很简单，就是从磁盘中读取data，没什么可说的。
② callDownloadProcessForOperation:下载图片
// Download process
// 网络下载图片
- (void)callDownloadProcessForOperation:(nonnull SDWebImageCombinedOperation *)operation
url:(nonnull NSURL *)url
options:(SDWebImageOptions)options
context:(SDWebImageContext *)context
cachedImage:(nullable UIImage *)cachedImage
cachedData:(nullable NSData *)cachedData
cacheType:(SDImageCacheType)cacheType
progress:(nullable SDImageLoaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
// 判断是否需要从网上下载图片
BOOL shouldDownload = !SD_OPTIONS_CONTAINS(options, SDWebImageFromCacheOnly);
shouldDownload &= (!cachedImage || options & SDWebImageRefreshCached);
shouldDownload &= (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url]);
shouldDownload &= [self.imageLoader canRequestImageForURL:url];
if (shouldDownload) { //需要从网上下载图片
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:operation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
// Pass the cached image to the image loader. The image loader should check whether the remote image is equal to the cached image.
SDWebImageMutableContext *mutableContext;
if (context) {
mutableContext = [context mutableCopy];
} else {
mutableContext = [NSMutableDictionary dictionary];
}
mutableContext[SDWebImageContextLoaderCachedImage] = cachedImage;
context = [mutableContext copy];
}
// `SDWebImageCombinedOperation` -> `SDWebImageDownloadToken` -> `downloadOperationCancelToken`, which is a `SDCallbacksDictionary` and retain the completed block below, so we need weak-strong again to avoid retain cycle
@weakify(operation);
//网络图片下载的逻辑
operation.loaderOperation = [self.imageLoader requestImageWithURL:url options:options context:context progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
@strongify(operation);
//如果任务被取消，直接调用完成block
if (!operation || operation.isCancelled) {
// Image combined operation cancelled by user
[self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorCancelled userInfo:nil] url:url];
} else if (cachedImage && options & SDWebImageRefreshCached && [error.domain isEqualToString:SDWebImageErrorDomain] && error.code == SDWebImageErrorCacheNotModified) {
// Image refresh hit the NSURLCache cache, do not call the completion block
} else if ([error.domain isEqualToString:SDWebImageErrorDomain] && error.code == SDWebImageErrorCancelled) {
// Download operation cancelled by user before sending the request, don't block failed URL
[self callCompletionBlockForOperation:operation completion:completedBlock error:error url:url];
} else if (error) { //如果有错误,在completedBlock里传入error
[self callCompletionBlockForOperation:operation completion:completedBlock error:error url:url];
//是否需要将失败url放入失败url名单里
BOOL shouldBlockFailedURL = [self shouldBlockFailedURLWithURL:url error:error];
if (shouldBlockFailedURL) { //将url放入失败url黑名单里
SD_LOCK(self.failedURLsLock);
[self.failedURLs addObject:url];
SD_UNLOCK(self.failedURLsLock);
}
} else {
//如果需要下载失败后重新下载，则将当前url从失败url名单里移除
if ((options & SDWebImageRetryFailed)) {
SD_LOCK(self.failedURLsLock);
[self.failedURLs removeObject:url];
SD_UNLOCK(self.failedURLsLock);
}
//图片下载成功将图片存入缓存
[self callStoreCacheProcessForOperation:operation url:url options:options context:context downloadedImage:downloadedImage downloadedData:downloadedData finished:finished progress:progressBlock completed:completedBlock];
}
if (finished) {
//移除当前的的下载操作（线程安全）
[self safelyRemoveOperationFromRunning:operation];
}
}];
} else if (cachedImage) { //有缓存图片直接设置缓存图片
//调用完成的block
[self callCompletionBlockForOperation:operation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
//移除当前的的下载操作（线程安全）
[self safelyRemoveOperationFromRunning:operation];
} else { //没有缓存的图片，而且下载被代理终止了
// Image not in cache and download disallowed by delegate
//调用完成的block
[self callCompletionBlockForOperation:operation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
//移除当前的的下载操作（线程安全）
[self safelyRemoveOperationFromRunning:operation];
}
}
上面代码的核心方法是调用requestImageWithURL:下载图片，下载成功之后的回调操作如下：
- 如果图片下载成功，就使用callStoreCacheProcessForOperation:将图片存入缓存
- 其他情况，调用完成的block（callCompletionBlockForOperation:），并且移除当前的下载操作（线程安全）（safelyRemoveOperationFromRunning）
进入requestImageWithURL:方法，代码如下：
//网络图片下载逻辑
- (id<SDWebImageOperation>)requestImageWithURL:(NSURL *)url options:(SDWebImageOptions)options context:(SDWebImageContext *)context progress:(SDImageLoaderProgressBlock)progressBlock completed:(SDImageLoaderCompletedBlock)completedBlock {
UIImage *cachedImage = context[SDWebImageContextLoaderCachedImage];
SDWebImageDownloaderOptions downloaderOptions = 0;
if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
if (options & SDWebImageProgressiveLoad) downloaderOptions |= SDWebImageDownloaderProgressiveLoad;
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 (options & SDWebImageAvoidDecodeImage) downloaderOptions |= SDWebImageDownloaderAvoidDecodeImage;
if (options & SDWebImageDecodeFirstFrameOnly) downloaderOptions |= SDWebImageDownloaderDecodeFirstFrameOnly;
if (options & SDWebImagePreloadAllFrames) downloaderOptions |= SDWebImageDownloaderPreloadAllFrames;
if (options & SDWebImageMatchAnimatedImageClass) downloaderOptions |= SDWebImageDownloaderMatchAnimatedImageClass;
if (cachedImage && options & SDWebImageRefreshCached) {
// force progressive off if image already cached but forced refreshing
downloaderOptions &= ~SDWebImageDownloaderProgressiveLoad;
// ignore image read from NSURLCache if image if cached but force refreshing
downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
}
return [self downloadImageWithURL:url options:downloaderOptions context:context progress:progressBlock completed:completedBlock];
}
进入downloadImageWithURL:方法，代码如下：
//核心方法：下载图片
//真正的网络图片下载代码
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
options:(SDWebImageDownloaderOptions)options
context:(nullable SDWebImageContext *)context
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
// The URL will be used as the key to the callbacks dictionary so it cannot be nil. If it is nil immediately call the completed block with no image or data.
//如果url为nil会会立刻回调完成block
if (url == nil) {
if (completedBlock) {
NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidURL userInfo:@{NSLocalizedDescriptionKey : @"Image url is nil"}];
completedBlock(nil, nil, error, YES);
}
return nil;
}
SD_LOCK(self.operationsLock);
id downloadOperationCancelToken;
NSOperation<SDWebImageDownloaderOperation> *operation = [self.URLOperations objectForKey:url];
// There is a case that the operation may be marked as finished or cancelled, but not been removed from `self.URLOperations`.
if (!operation || operation.isFinished || operation.isCancelled) {
//通过url来初始化operation
operation = [self createDownloaderOperationWithUrl:url options:options context:context];
if (!operation) {
SD_UNLOCK(self.operationsLock);
if (completedBlock) {
NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidDownloadOperation userInfo:@{NSLocalizedDescriptionKey : @"Downloader operation is nil"}];
completedBlock(nil, nil, error, YES);
}
return nil;
}
@weakify(self);
operation.completionBlock = ^{
@strongify(self);
if (!self) {
return;
}
SD_LOCK(self.operationsLock);
[self.URLOperations removeObjectForKey:url];
SD_UNLOCK(self.operationsLock);
};
self.URLOperations[url] = operation;
// Add operation to operation queue only after all configuration done according to Apple's doc.
// `addOperation:` does not synchronously execute the `operation.completionBlock` so this will not cause deadlock.
//在下载队列里添加下载操作，执行下载操作
[self.downloadQueue addOperation:operation];
// 给回调字典赋值
downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];
} else {
// When we reuse the download operation to attach more callbacks, there may be thread safe issue because the getter of callbacks may in another queue (decoding queue or delegate queue)
// So we lock the operation here, and in `SDWebImageDownloaderOperation`, we use `@synchonzied (self)`, to ensure the thread safe between these two classes.
//当我们重用下载操作来附加更多回调时，可能会出现线程安全问题，因为回调的getter可能在另一个队列中(解码队列或委托队列)
//所以我们在这里锁定操作，在' SDWebImageDownloaderOperation '中，我们使用' @synchonzied (self) '，以确保线程在这两个类之间的安全。
@synchronized (operation) {
//这里真正保存两个block的方法是addHandlersForProgress: completed:：
downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];
}
//如果operation不在执行中,就根据设置的优先级给operation设置操作优先级
if (!operation.isExecuting) {
if (options & SDWebImageDownloaderHighPriority) {
operation.queuePriority = NSOperationQueuePriorityHigh;
} else if (options & SDWebImageDownloaderLowPriority) {
operation.queuePriority = NSOperationQueuePriorityLow;
} else {
operation.queuePriority = NSOperationQueuePriorityNormal;
}
}
}
SD_UNLOCK(self.operationsLock);
//创建SDWebImageDownloadToken,并返回
SDWebImageDownloadToken *token = [[SDWebImageDownloadToken alloc] initWithDownloadOperation:operation];
token.url = url;
token.request = operation.request;
token.downloadOperationCancelToken = downloadOperationCancelToken;
token.downloader = self;
return token;
}
上面方法，会先判断operation是否可用，如果不可用，就通过url来初始化operation，最后再创建SDWebImageDownloadToken，并返回。
通过url来初始化operation，代码如下：
//通过url来初始化operation
- (nullable NSOperation<SDWebImageDownloaderOperation> *)createDownloaderOperationWithUrl:(nonnull NSURL *)url
options:(SDWebImageDownloaderOptions)options
context:(nullable SDWebImageContext *)context {
//设置超时时间
NSTimeInterval timeoutInterval = self.config.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 = options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData;
NSMutableURLRequest *mutableRequest = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:cachePolicy timeoutInterval:timeoutInterval];
mutableRequest.HTTPShouldHandleCookies = SD_OPTIONS_CONTAINS(options, SDWebImageDownloaderHandleCookies);
mutableRequest.HTTPShouldUsePipelining = YES;
SD_LOCK(self.HTTPHeadersLock);
mutableRequest.allHTTPHeaderFields = self.HTTPHeaders;
SD_UNLOCK(self.HTTPHeadersLock);
id<SDWebImageDownloaderRequestModifier> requestModifier;
if ([context valueForKey:SDWebImageContextDownloadRequestModifier]) {
requestModifier = [context valueForKey:SDWebImageContextDownloadRequestModifier];
} else {
requestModifier = self.requestModifier;
}
NSURLRequest *request;
if (requestModifier) {
NSURLRequest *modifiedRequest = [requestModifier modifiedRequestWithRequest:[mutableRequest copy]];
// If modified request is nil, early return
if (!modifiedRequest) {
return nil;
} else {
request = [modifiedRequest copy];
}
} else {
request = [mutableRequest copy];
}
Class operationClass = self.config.operationClass;
if (operationClass && [operationClass isSubclassOfClass:[NSOperation class]] && [operationClass conformsToProtocol:@protocol(SDWebImageDownloaderOperation)]) {
// Custom operation class
} else {
operationClass = [SDWebImageDownloaderOperation class];
}
//创建下载操作：SDWebImageDownloaderOperation用于请求网络资源的操作，它是一个 NSOperation 的子类
NSOperation<SDWebImageDownloaderOperation> *operation = [[operationClass alloc] initWithRequest:request inSession:self.session options:options context:context];
//url证书
if ([operation respondsToSelector:@selector(setCredential:)]) {
if (self.config.urlCredential) {
operation.credential = self.config.urlCredential;
} else if (self.config.username && self.config.password) {
operation.credential = [NSURLCredential credentialWithUser:self.config.username password:self.config.password persistence:NSURLCredentialPersistenceForSession];
}
}
if ([operation respondsToSelector:@selector(setMinimumProgressInterval:)]) {
operation.minimumProgressInterval = MIN(MAX(self.config.minimumProgressInterval, 0), 1);
}
//优先级
if (options & SDWebImageDownloaderHighPriority) {
operation.queuePriority = NSOperationQueuePriorityHigh;
} else if (options & SDWebImageDownloaderLowPriority) {
operation.queuePriority = NSOperationQueuePriorityLow;
}
//后进先出,通过设置依赖实现
if (self.config.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
// Emulate LIFO execution order by systematically adding new operations as last operation's dependency
[self.lastAddedOperation addDependency:operation];
self.lastAddedOperation = operation;
}
return operation;
}
Demo地址：SDWebImage笔记
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