SDWebImage庫中SDWebImageCompat是最基礎(chǔ)的配置文件，為了兼容蘋果各個平臺。

SDWebImageCompat.h

#import <TargetConditionals.h>

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

//蘋果的`TargetConditionals.h`文件有點怪，在所有平臺上TARGET_OS_MAC都被定義了，所以只能通過TARGET_OS_IPHONE=0這種方式進(jìn)行判斷
#if !TARGET_OS_IPHONE && !TARGET_OS_IOS && !TARGET_OS_TV && !TARGET_OS_WATCH
    #define SD_MAC 1
#else
    #define SD_MAC 0
#endif

//判斷是否支持UIKIT, iOS 與 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系統(tǒng)
#if TARGET_OS_IOS
    #define SD_IOS 1
#else
    #define SD_IOS 0
#endif

//是否是TV系統(tǒng)
#if TARGET_OS_TV
    #define SD_TV 1
#else
    #define SD_TV 0
#endif

//是否是watchOS系統(tǒng)
#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

//獲取合適的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";