SDWebImage

SDWebImage具有缓存支持的异步映像下载程序。并添加了像UI元素分类类UIImageView、UIButton、MKAnnotationView，可以直接为这些UI元素添加图片。

日常使用

在日常的使用中，通常是加载网络图片到UIImageView上展示，所以一般在需要使用SDWebImage的文件中只引用#import "UIImageView+WebCache.h"头文件。

最简单的加载方式是只加载图片地址：

UIImageView *imageView = [[UIImageView alloc] initWithFrame:self.view.bounds];
[self.view addSubview:imageView];
    
[imageView sd_setImageWithURL:[NSURL URLWithString:@"http://www.domain.com/path/to/image.jpg"]];

当然，SDWebImage也提供了其他的加载方法，不过点击方法进入查看后，发现最终都是调用其全能方法：

- (void)sd_setImageWithURL:(nullable NSURL *)url {
    [self sd_setImageWithURL:url placeholderImage:nil options:0 progress:nil completed:nil];
}

全能方法除了必需的的图片地址，还提供了占位图、可选项、加载进度和完成回调。

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

紧接着我们点击进入全能方法中：

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                 completed:(nullable SDExternalCompletionBlock)completedBlock {
    [self sd_internalSetImageWithURL:url
                    placeholderImage:placeholder
                             options:options
                        operationKey:nil
                       setImageBlock:nil
                            progress:progressBlock
                           completed:completedBlock];
}

可以发现，全能方法并没有什么实际的实现，只是对另一个方法的封装。

一些主要功能

对UIImageView、UIButton、MKAnnotationView添加Web图像和告诉缓存管理
异步图像下载器
具有自动缓存到期处理的异步内存+磁盘映像缓存
背景图像解压缩
对动画图像的支持
可以自定义和组合的转换，可在下载后立即应用于图像
可以自定义加载器（如照片库）来扩展图像加载功能
加载中的indicator显示
保证不会下载相同的URL
下载过程或者资源保存过程用到了GCD和ARC
提前将获取到的图片放到主线程，保证不会阻塞主线程

图片加载全过程

在这里插入图片描述

我们在使用SDWebImage时调用了[imageView sd_setImageWithURL:[NSURL URLWithString:@"http://www.domain.com/path/to/image.jpg"]];这个简单的分类方法，然后就静静的等着图片被设置到UIImageView类对象上。
经过一系列调用，我们首先来到UIView+WebCache分类中，在这个分类中，首先保障了图片加载的唯一性，然后就开始了核心的加载操作。
接着就进入了SDWebImageManager类中，在这个类中，首先去查找是否有缓存，没有缓存的话才去服务器下载。
想要查找缓存我们要进入SDImageCache这个类中，在这个类中，首先去内存中查看是否有对应的缓存，如果没有再去硬盘中查找是否有对应的缓存，但是从硬盘中获取的是图片的数据，要想获得图片还要经历解码、缩放和解压。当然如果都没有缓存的话就去下载。
负责下载的是SDWebImageDownloader这个类，在这个类中，将图片的下载操作封装成了自定义的一个类SDWebImageDownloaderOperation，然后添加到了操作队列中。
当操作队列调用这个操作时，会调用操作对象的- (void)start方法，在重写的这个方法中，生成了任务对象dataTask，并调用resume开始执行任务。
因为SDWebImageDownloaderOperation类遵守了dataTask对象的协议，所以dataTask执行的结果会通过代理方法进行回调。在代理方法中，获取并保存了服务器返回的数据，并在任务执行结束后，对数据进行解码、缩放和解压。处理完成后就进行回调。
通过重重回调，要回调的数据沿着SDWebImageDownloaderOperation->SDWebImageDownloader->SDWebImageManager->UIView+WebCache一路流动，其中流动到SDWebImageManager中时对图片进行了缓存，最后在UIView+WebCache中为UIImageView设置了处理好的图片。

源码分析

架构图

在这里插入图片描述
从SDWebImage中提供的架构图中我们可以大概的看出，整个库分为两层，Top Level、Base Module。

Top Level：当UIImageView调用加载image方法，会进入SDWebImage中的UIImageView分类，在分类中调用负责加载UIImage的核心代码块ImageManager，其主要负责调度Image Cache/Image Loader，这两者分别从缓存或者网络端加载图片，并且又进行了细分。Cache中获取图片业务，拆分到了memory/disk两个分类中；Image Loader中又分为从网络端获取或者从系统的Photos中获取。
Base Module：获取到图片的二进制数据处理，二进制解压缩，二进制中格式字节判断出具体的图片类型。

结构

(设计思路借鉴：提供多种接口，到底层调用到同一个方法，减少调用方对可选参数的传递)

UIImageView+WebCache和UIButton+WebCache直接为表层的 UIKit框架提供接口,
SDWebImageManger（SDWebImageManager是SDWebImage的核心类，也是我们经常接触到的类）负责处理和协调SDWebImageDownloader和SDWebImageCache, 并与UIKit层进行交互。SDWebImageDownloaderOperation真正执行下载请求；
SDWebImageCompat 是最基础的配置文件，为了兼容苹果各个平台
SDWebImageDecoder sd解码图片这个类其实是UIImage的一个分类UIImage+ForceDecode，主要用来解码UIImage

SDWebImageManager

SDWebImageManager是SDWebImage的核心类，也是我们经常接触到的类，我们将一起看下是如何实现的

1. SDWebImageOptions

typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
    /**
     * By default, when a URL fail to be downloaded, the URL is blacklisted so the library won't keep trying.
     * This flag disable this blacklisting.
     不会重新下载已经失败过的图片
     */
    SDWebImageRetryFailed = 1 << 0,

    /**
     * By default, image downloads are started during UI interactions, this flags disable this feature,
     * leading to delayed download on UIScrollView deceleration for instance.
     *默认情况下，图像下载是在UI交互期间启动的，此标志禁用此功能，
     *导致延迟下载UIScrollView减速为例。
     */
    SDWebImageLowPriority = 1 << 1,

    /**
     * This flag disables on-disk caching
     禁止在磁盘缓存，只在缓存中存在
     
     */
    SDWebImageCacheMemoryOnly = 1 << 2,

    /**
     * This flag enables progressive download, the image is displayed progressively during download as a browser would do.
     * By default, the image is only displayed once completely downloaded.
     此标志支持逐行下载，图像在下载过程中逐步显示，就像浏览器所做的那样。
     *默认情况下，图像只显示一次，完全下载。
     
     */
    SDWebImageProgressiveDownload = 1 << 3,

    /**
     * Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.
     * The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.
     * This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.
     * If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.
     *
     * Use this flag only if you can't make your URLs static with embedded cache busting parameter.
     任何图片都从新下载，不使用http cache，比如一个图片发送变化，但是url没有变化，用改options刷新数据
     */
    SDWebImageRefreshCached = 1 << 4,

    /**
     * In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
     * extra time in background to let the request finish. If the background task expires the operation will be cancelled.
     后台下载
     */
    SDWebImageContinueInBackground = 1 << 5,

    /**
     * Handles cookies stored in NSHTTPCookieStore by setting
     * NSMutableURLRequest.HTTPShouldHandleCookies = YES;
     存储在cookie nshttpcookiestore
     */
    SDWebImageHandleCookies = 1 << 6,

    /**
     * Enable to allow untrusted SSL certificates.
     * Useful for testing purposes. Use with caution in production.
     允许不信任的ssl证书
     */
    SDWebImageAllowInvalidSSLCertificates = 1 << 7,

    /**
     * By default, images are loaded in the order in which they were queued. This flag moves them to
     * the front of the queue.
     默认情况下，图像按它们排队的顺序加载。这个标志是将图片放在队列的最前面
     */
    SDWebImageHighPriority = 1 << 8,
    
    /**
     * By default, placeholder images are loaded while the image is loading. This flag will delay the loading
     * of the placeholder image until after the image has finished loading.
     占位符图像是在图像加载时加载的，完全加载完才暂时
     */
    SDWebImageDelayPlaceholder = 1 << 9,

    /**
     * We usually don't call transformDownloadedImage delegate method on animated images,
     * as most transformation code would mangle it.
     * Use this flag to transform them anyway.
     转换图像
     */
    SDWebImageTransformAnimatedImage = 1 << 10,
    
    /**
     * By default, image is added to the imageView after download. But in some cases, we want to
     * have the hand before setting the image (apply a filter or add it with cross-fade animation for instance)
     * Use this flag if you want to manually set the image in the completion when success
     默认情况下，image是在下载完成后加载，但是在一些情况下，我们想要在设置图像之前使用（例如应用过滤器或添加交叉淡入淡出动画），如果您想在成功完成时手动设置图像，请使用此标志
     */
    SDWebImageAvoidAutoSetImage = 1 << 11,
    
    /**
     * By default, images are decoded respecting their original size. On iOS, this flag will scale down the
     * images to a size compatible with the constrained memory of devices.
     * If `SDWebImageProgressiveDownload` flag is set the scale down is deactivated.
     默认情况下，图像是进行解码的，这个标志是按比例缩小images的尺寸，来缩小占用的手机内存，如果` sdwebimageprogressivedownload `标志设置的情况下被停用。，压缩大的图片
     */
    SDWebImageScaleDownLargeImages = 1 << 12
};

2. SDWebImageManagerDelegate

@protocol SDWebImageManagerDelegate <NSObject>

@optional

/**
 * Controls which image should be downloaded when the image is not found in the cache.
 *
 * @param imageManager The current `SDWebImageManager`
 * @param imageURL     The url of the image to be downloaded
 *
 * @return Return NO to prevent the downloading of the image on cache misses. If not implemented, YES is implied.
 //当缓存没有发现当前图片，那么会查看调用者是否实现改方法，如果return一个no，则不会继续下载这张图片

 */
- (BOOL)imageManager:(nonnull SDWebImageManager *)imageManager shouldDownloadImageForURL:(nullable NSURL *)imageURL;

/**
 * Allows to transform the image immediately after it has been downloaded and just before to cache it on disk and memory.
 * NOTE: This method is called from a global queue in order to not to block the main thread.
 *
 * @param imageManager The current `SDWebImageManager`
 * @param image        The image to transform
 * @param imageURL     The url of the image to transform
 *
 * @return The transformed image object.
 //当图片下载完成但是未添加到缓存里面，这时候调用该方法可以给图片旋转方向，注意是异步执行， 防止组织主线程
 
 */
- (nullable UIImage *)imageManager:(nonnull SDWebImageManager *)imageManager transformDownloadedImage:(nullable UIImage *)image withURL:(nullable NSURL *)imageURL;

@end

SDWebImageCompat

SDWebImage库中SDWebImageCompat是最基础的配置文件，为了兼容苹果各个平台。

SDWebImageCompat.h

#import <TargetConditionals.h>

//不支持OC的GC内存管理
#ifdef __OBJC_GC__
    #error SDWebImage does not support Objective-C Garbage Collection
#endif

//苹果的`TargetConditionals.h`文件有点怪，在所有平台上TARGET_OS_MAC都被定义了，所以只能通过TARGET_OS_IPHONE=0这种方式进行判断
#if !TARGET_OS_IPHONE && !TARGET_OS_IOS && !TARGET_OS_TV && !TARGET_OS_WATCH
    #define SD_MAC 1
#else
    #define SD_MAC 0
#endif

//判断是否支持UIKIT, iOS 与 tvOS都支持。
#if TARGET_OS_IOS || TARGET_OS_TV
    #define SD_UIKIT 1
#else
    #define SD_UIKIT 0
#endif

//是否是iOS系统
#if TARGET_OS_IOS
    #define SD_IOS 1
#else
    #define SD_IOS 0
#endif

//是否是TV系统
#if TARGET_OS_TV
    #define SD_TV 1
#else
    #define SD_TV 0
#endif

//是否是watchOS系统
#if TARGET_OS_WATCH
    #define SD_WATCH 1
#else
    #define SD_WATCH 0
#endif


#if SD_MAC
    #import <AppKit/AppKit.h>
    #ifndef UIImage
        #define UIImage NSImage
    #endif
    #ifndef UIImageView
        #define UIImageView NSImageView
    #endif
    #ifndef UIView
        #define UIView NSView
    #endif
#else
    #if __IPHONE_OS_VERSION_MIN_REQUIRED != 20000 && __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_5_0
        #error SDWebImage doesn't support Deployment Target version < 5.0
    #endif

    #if SD_UIKIT
        #import <UIKit/UIKit.h>
    #endif
    #if SD_WATCH
        #import <WatchKit/WatchKit.h>
        #ifndef UIView
            #define UIView WKInterfaceObject
        #endif
        #ifndef UIImageView
            #define UIImageView WKInterfaceImage
        #endif
    #endif
#endif

#ifndef NS_ENUM
#define NS_ENUM(_type, _name) enum _name : _type _name; enum _name : _type
#endif

#ifndef NS_OPTIONS
#define NS_OPTIONS(_type, _name) enum _name : _type _name; enum _name : _type
#endif

FOUNDATION_EXPORT UIImage *SDScaledImageForKey(NSString *key, UIImage *image);

typedef void(^SDWebImageNoParamsBlock)(void);

FOUNDATION_EXPORT NSString *const SDWebImageErrorDomain;

#ifndef dispatch_queue_async_safe
#define dispatch_queue_async_safe(queue, block)\
    if (dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL) == dispatch_queue_get_label(queue)) {\
        block();\
    } else {\
        dispatch_async(queue, block);\
    }
#endif

#ifndef dispatch_main_async_safe
#define dispatch_main_async_safe(block) dispatch_queue_async_safe(dispatch_get_main_queue(), block)
#endif

SDWebImageCompat.m

#import "SDWebImageCompat.h"
#import "UIImage+MultiFormat.h"

//只能支持ARC
#if !__has_feature(objc_arc)
    #error SDWebImage is ARC only. Either turn on ARC for the project or use -fobjc-arc flag
#endif

#if !OS_OBJECT_USE_OBJC
    #error SDWebImage need ARC for dispatch object
#endif

//获取合适的scale的UIImage
inline UIImage *SDScaledImageForKey(NSString * _Nullable key, UIImage * _Nullable image) {
    if (!image) {
        return nil;
    }
    
#if SD_MAC
    return image;
#elif SD_UIKIT || SD_WATCH
    if ((image.images).count > 0) {
        NSMutableArray<UIImage *> *scaledImages = [NSMutableArray array];

        for (UIImage *tempImage in image.images) {
            [scaledImages addObject:SDScaledImageForKey(key, tempImage)];
        }
        
        UIImage *animatedImage = [UIImage animatedImageWithImages:scaledImages duration:image.duration];
        if (animatedImage) {
            animatedImage.sd_imageLoopCount = image.sd_imageLoopCount;
        }
        return animatedImage;
    } else {
#if SD_WATCH
        if ([[WKInterfaceDevice currentDevice] respondsToSelector:@selector(screenScale)]) {
#elif SD_UIKIT
        if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)]) {
#endif
            CGFloat scale = 1;
            if (key.length >= 8) {
                NSRange range = [key rangeOfString:@"@2x."];
                if (range.location != NSNotFound) {
                    scale = 2.0;
                }
                
                range = [key rangeOfString:@"@3x."];
                if (range.location != NSNotFound) {
                    scale = 3.0;
                }
            }

            UIImage *scaledImage = [[UIImage alloc] initWithCGImage:image.CGImage scale:scale orientation:image.imageOrientation];
            image = scaledImage;
        }
        return image;
    }
#endif
}

NSString *const SDWebImageErrorDomain = @"SDWebImageErrorDomain";