/**不同图片类型的枚举- SDImageFormatUndefined: 未知- SDImageFormatJPEG: JPG(FFD8FFE1)- SDImageFormatPNG: PNG(89504E47)- SDImageFormatGIF: GIF(47494638)- SDImageFormatTIFF: TIFF(49492A00或4D4D002A)- SDImageFormatWebP: WebP(524946462A73010057454250, 52494646对应ASCII字符为RIFF，57454250对应ASCII字符为WEBP。当第一个字节为52时，如果长度<12 我们就认定为不是图片。因此返回SDImageFormatUndefined。)*/
typedef NS_ENUM(NSInteger, SDImageFormat) {SDImageFormatUndefined = -1,SDImageFormatJPEG = 0,SDImageFormatPNG,SDImageFormatGIF,SDImageFormatTIFF,SDImageFormatWebP
};/**根据图片NSData获取图片的类型@param data NSData数据@return 图片数据类型*/
+ (SDImageFormat)sd_imageFormatForImageData:(nullable NSData *)data {if (!data) {return SDImageFormatUndefined;}uint8_t c;//获取图片数据的第一个字节数据[data getBytes:&c length:1];//根据字母的ASCII码比较switch (c) {case 0xFF:return SDImageFormatJPEG;case 0x89:return SDImageFormatPNG;case 0x47:return SDImageFormatGIF;case 0x49:case 0x4D:return SDImageFormatTIFF;case 0x52:// R as RIFF for WEBPif (data.length < 12) {return SDImageFormatUndefined;}NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(0, 12)] encoding:NSASCIIStringEncoding];if ([testString hasPrefix:@"RIFF"] && [testString hasSuffix:@"WEBP"]) {return SDImageFormatWebP;}}return SDImageFormatUndefined;
}

/**根据图片的data数据，生成对应的图片对象@param data 图片的data@return 图片对象*/
+ (nullable UIImage *)sd_imageWithData:(nullable NSData *)data {if (!data) {return nil;}UIImage *image;//获取data的图片类型，png，gif，jpgSDImageFormat imageFormat = [NSData sd_imageFormatForImageData:data];if (imageFormat == SDImageFormatGIF) {//gif处理：返回一张只包含数据第一张image 的gif图片image = [UIImage sd_animatedGIFWithData:data];}
#ifdef SD_WEBPelse if (imageFormat == SDImageFormatWebP){image = [UIImage sd_imageWithWebPData:data];}
#endifelse {image = [[UIImage alloc] initWithData:data];
#if SD_UIKIT || SD_WATCH//获取方向UIImageOrientation orientation = [self sd_imageOrientationFromImageData:data];//如果不是向上的，还需要再次生成图片if (orientation != UIImageOrientationUp) {image = [UIImage imageWithCGImage:image.CGImagescale:image.scaleorientation:orientation];}
#endif}return image;
}#if SD_UIKIT || SD_WATCH/**根据图片数据获取图片的方向@param imageData 图片数据@return 方向*/
+(UIImageOrientation)sd_imageOrientationFromImageData:(nonnull NSData *)imageData {//默认是向上的UIImageOrientation result = UIImageOrientationUp;CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)imageData, NULL);if (imageSource) {//获取图片的属性列表CFDictionaryRef properties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);if (properties) {CFTypeRef val;int exifOrientation;//获取图片方向val = CFDictionaryGetValue(properties, kCGImagePropertyOrientation);if (val) {CFNumberGetValue(val, kCFNumberIntType, &exifOrientation);result = [self sd_exifOrientationToiOSOrientation:exifOrientation];} // else - if it's not set it remains at up
            CFRelease((CFTypeRef) properties);} else {//NSLog(@"NO PROPERTIES, FAIL");
        }CFRelease(imageSource);}return result;
}#pragma mark EXIF orientation tag converter
// Convert an EXIF image orientation to an iOS one.
// reference see here: http://sylvana.net/jpegcrop/exif_orientation.html/**根据不同的值返回不同的图片方向@param exifOrientation 输入值@return 图片的方向*/
+ (UIImageOrientation) sd_exifOrientationToiOSOrientation:(int)exifOrientation {UIImageOrientation orientation = UIImageOrientationUp;switch (exifOrientation) {case 1:orientation = UIImageOrientationUp;break;case 3:orientation = UIImageOrientationDown;break;case 8:orientation = UIImageOrientationLeft;break;case 6:orientation = UIImageOrientationRight;break;case 2:orientation = UIImageOrientationUpMirrored;break;case 4:orientation = UIImageOrientationDownMirrored;break;case 5:orientation = UIImageOrientationLeftMirrored;break;case 7:orientation = UIImageOrientationRightMirrored;break;default:break;}return orientation;
}
#endif- (nullable NSData *)sd_imageData {return [self sd_imageDataAsFormat:SDImageFormatUndefined];
}/**根据指定的图片类型，把image对象转换为对应格式的data@param imageFormat 指定的image格式@return 返回data对象*/
- (nullable NSData *)sd_imageDataAsFormat:(SDImageFormat)imageFormat {NSData *imageData = nil;if (self) {
#if SD_UIKIT || SD_WATCHint alphaInfo = CGImageGetAlphaInfo(self.CGImage);//是否有透明度BOOL hasAlpha = !(alphaInfo == kCGImageAlphaNone ||alphaInfo == kCGImageAlphaNoneSkipFirst ||alphaInfo == kCGImageAlphaNoneSkipLast);//只有png图片有alpha属性BOOL usePNG = hasAlpha;// the imageFormat param has priority here. But if the format is undefined, we relly on the alpha channel//是否是PNG类型的图片if (imageFormat != SDImageFormatUndefined) {usePNG = (imageFormat == SDImageFormatPNG);}//根据不同的图片类型获取到对应的图片dataif (usePNG) {imageData = UIImagePNGRepresentation(self);} else {imageData = UIImageJPEGRepresentation(self, (CGFloat)1.0);}
#elseNSBitmapImageFileType imageFileType = NSJPEGFileType;if (imageFormat == SDImageFormatGIF) {imageFileType = NSGIFFileType;} else if (imageFormat == SDImageFormatPNG) {imageFileType = NSPNGFileType;}imageData = [NSBitmapImageRep representationOfImageRepsInArray:self.representationsusingType:imageFileTypeproperties:@{}];
#endif}return imageData;
}

/**根据Gif图片的data生成对应的UIImage对象(只会取GIF图片的第一张UIImage)@param data data@return 生成的image对象。这里只获取Gif图片的第一张图像，如果要实现Gif完整图像，使用FLAnimatedImageView*/
+ (UIImage *)sd_animatedGIFWithData:(NSData *)data {if (!data) {return nil;}CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);//获取NSData中的图片数量size_t count = CGImageSourceGetCount(source);UIImage *staticImage;//如果图片数量小于或者等于1，直接转换if (count <= 1) {staticImage = [[UIImage alloc] initWithData:data];} else {// we will only retrieve the 1st frame. the full GIF support is available via the FLAnimatedImageView category.// this here is only code to allow drawing animated images as static ones
#if SD_WATCHCGFloat scale = 1;scale = [WKInterfaceDevice currentDevice].screenScale;
#elif SD_UIKITCGFloat scale = 1;scale = [UIScreen mainScreen].scale;
#endif//获取第一张UIImage对象CGImageRef CGImage = CGImageSourceCreateImageAtIndex(source, 0, NULL);
#if SD_UIKIT || SD_WATCH//获取Gif图片的第一张图片UIImage *frameImage = [UIImage imageWithCGImage:CGImage scale:scale orientation:UIImageOrientationUp];//用第一张图片生成一个新的Gif图片staticImage = [UIImage animatedImageWithImages:@[frameImage] duration:0.0f];
#elif SD_MACstaticImage = [[UIImage alloc] initWithCGImage:CGImage size:NSZeroSize];
#endifCGImageRelease(CGImage);}CFRelease(source);return staticImage;
}/**判断一张图片是否GIF图片@return YES/NO*/
- (BOOL)isGIF {return (self.images != nil);
}

#if SD_UIKIT || SD_WATCH
static const size_t kBytesPerPixel = 4;  //!<每个像素占用的字节数(图像在iOS设备上是以像素为单位显示的)
static const size_t kBitsPerComponent = 8;  //!<每一个组件占多少位(比方说RGBA，其中R（红色）G（绿色）B（蓝色）A（透明度）是4个组件，每个像素由这4个组件组成，那么我们就用8位来表示着每一个组件，所以这个RGBA就是8*4 = 32位)/**解码图片@param image UIImage对象@return 返回解码以后的图片*/
+ (nullable UIImage *)decodedImageWithImage:(nullable UIImage *)image {//图片是否能够解码if (![UIImage shouldDecodeImage:image]) {return image;}// autorelease the bitmap context and all vars to help system to free memory when there are memory warning.// on iOS7, do not forget to call [[SDImageCache sharedImageCache] clearMemory];//解码操作放入一个自动释放池里面，以便自动释放所有的变量
    @autoreleasepool{//获取和图像相关的各种参数CGImageRef imageRef = image.CGImage;//获取图片的色彩空间CGColorSpaceRef colorspaceRef = [UIImage colorSpaceForImageRef:imageRef];//宽度和高度size_t width = CGImageGetWidth(imageRef);size_t height = CGImageGetHeight(imageRef);//计算出每行的像素数size_t bytesPerRow = kBytesPerPixel * width;// kCGImageAlphaNone is not supported in CGBitmapContextCreate.// Since the original image here has no alpha info, use kCGImageAlphaNoneSkipLast// to create bitmap graphics contexts without alpha info.//创建一个绘制图片的上下文//这里创建的contexts是没有透明因素的。在UI渲染的时候，实际上是把多个图层按像素叠加计算的过程，需要对每一个像素进行 RGBA 的叠加计算。当某个 layer 的是不透明的，也就是 opaque 为 YES 时，GPU 可以直接忽略掉其下方的图层，这就减少了很多工作量。CGContextRef context = CGBitmapContextCreate(NULL,width,height,kBitsPerComponent,bytesPerRow,colorspaceRef,kCGBitmapByteOrderDefault|kCGImageAlphaNoneSkipLast);if (context == NULL) {return image;}// Draw the image into the context and retrieve the new bitmap image without alpha//绘制一个和图片大小一样的图片CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);//创建一个没有alpha通道的图片CGImageRef imageRefWithoutAlpha = CGBitmapContextCreateImage(context);//得到解码以后的图片UIImage *imageWithoutAlpha = [UIImage imageWithCGImage:imageRefWithoutAlphascale:image.scaleorientation:image.imageOrientation];CGContextRelease(context);CGImageRelease(imageRefWithoutAlpha);return imageWithoutAlpha;}
}/** Defines the maximum size in MB of the decoded image when the flag `SDWebImageScaleDownLargeImages` is set* Suggested value for iPad1 and iPhone 3GS: 60.* Suggested value for iPad2 and iPhone 4: 120.* Suggested value for iPhone 3G and iPod 2 and earlier devices: 30.*/
static const CGFloat kDestImageSizeMB = 60.0f;  //!<最大支持压缩图像源的大小，默认为60MB。当我们要压缩一张图像的时候，首先就是要定义最大支持的源文件的大小，不能没有任何限制。/** Defines the maximum size in MB of a tile used to decode image when the flag `SDWebImageScaleDownLargeImages` is set* Suggested value for iPad1 and iPhone 3GS: 20.* Suggested value for iPad2 and iPhone 4: 40.* Suggested value for iPhone 3G and iPod 2 and earlier devices: 10.*/
static const CGFloat kSourceImageTileSizeMB = 20.0f;  //!<原图方块的大小，默认为20MB，这个方块将会被用来分割原图static const CGFloat kBytesPerMB = 1024.0f * 1024.0f;  //!<1MB有多少字节
static const CGFloat kPixelsPerMB = kBytesPerMB / kBytesPerPixel;  //!<1MB可以存储多少像素
static const CGFloat kDestTotalPixels = kDestImageSizeMB * kPixelsPerMB;  //!<目标总像素
static const CGFloat kTileTotalPixels = kSourceImageTileSizeMB * kPixelsPerMB;  //!<原图方块总像素static const CGFloat kDestSeemOverlap = 2.0f;   //重叠像素大小/**如果原始图片占用的空间太大，则按照一定的比例解码，从而不让解码以后的图片占用的空间太大@param image UIImage对象@return 返回处理结束的UIImage对象*/
+ (nullable UIImage *)decodedAndScaledDownImageWithImage:(nullable UIImage *)image {//图片是否支持解码if (![UIImage shouldDecodeImage:image]) {return image;}//图片不需要处理，直接解码if (![UIImage shouldScaleDownImage:image]) {return [UIImage decodedImageWithImage:image];}CGContextRef destContext;// autorelease the bitmap context and all vars to help system to free memory when there are memory warning.// on iOS7, do not forget to call [[SDImageCache sharedImageCache] clearMemory];
    @autoreleasepool {//获取和图像相关的各种参数CGImageRef sourceImageRef = image.CGImage;//获取原始图片的像素CGSize sourceResolution = CGSizeZero;sourceResolution.width = CGImageGetWidth(sourceImageRef);sourceResolution.height = CGImageGetHeight(sourceImageRef);//计算原始图片的总像素float sourceTotalPixels = sourceResolution.width * sourceResolution.height;// Determine the scale ratio to apply to the input image// that results in an output image of the defined size.// see kDestImageSizeMB, and how it relates to destTotalPixels.//根据一定的比例设置目标图片的宽度和高度float imageScale = kDestTotalPixels / sourceTotalPixels;//计算目标像素CGSize destResolution = CGSizeZero;destResolution.width = (int)(sourceResolution.width*imageScale);destResolution.height = (int)(sourceResolution.height*imageScale);// current color space//获取原始图片的像素空间，默认是RGBCGColorSpaceRef colorspaceRef = [UIImage colorSpaceForImageRef:sourceImageRef];//每一行像素占用的内存空间大小size_t bytesPerRow = kBytesPerPixel * destResolution.width;// Allocate enough pixel data to hold the output image.//目标图片占用的总内存空间大小，一行占用内存空间大小*高度void* destBitmapData = malloc( bytesPerRow * destResolution.height );if (destBitmapData == NULL) {return image;}// kCGImageAlphaNone is not supported in CGBitmapContextCreate.// Since the original image here has no alpha info, use kCGImageAlphaNoneSkipLast// to create bitmap graphics contexts without alpha info.//根据各种设置创建一个上下文环境destContext = CGBitmapContextCreate(destBitmapData,destResolution.width,destResolution.height,kBitsPerComponent,bytesPerRow,colorspaceRef,kCGBitmapByteOrderDefault|kCGImageAlphaNoneSkipLast);if (destContext == NULL) {free(destBitmapData);return image;}//设置目标图片的压缩质量
        CGContextSetInterpolationQuality(destContext, kCGInterpolationHigh);// Now define the size of the rectangle to be used for the// incremental blits from the input image to the output image.// we use a source tile width equal to the width of the source// image due to the way that iOS retrieves image data from disk.// iOS must decode an image from disk in full width 'bands', even// if current graphics context is clipped to a subrect within that// band. Therefore we fully utilize all of the pixel data that results// from a decoding opertion by achnoring our tile size to the full// width of the input image.//计算第一个原图方块，这个方块的宽度同原图一样，高度根据方块容量计算CGRect sourceTile = CGRectZero;sourceTile.size.width = sourceResolution.width;// The source tile height is dynamic. Since we specified the size// of the source tile in MB, see how many rows of pixels high it// can be given the input image width.sourceTile.size.height = (int)(kTileTotalPixels / sourceTile.size.width );sourceTile.origin.x = 0.0f;// The output tile is the same proportions as the input tile, but// scaled to image scale.//计算目标图像方块
        CGRect destTile;destTile.size.width = destResolution.width;destTile.size.height = sourceTile.size.height * imageScale;destTile.origin.x = 0.0f;// The source seem overlap is proportionate to the destination seem overlap.// this is the amount of pixels to overlap each tile as we assemble the ouput image.//计算原图像方块与方块重叠的像素大小float sourceSeemOverlap = (int)((kDestSeemOverlap/destResolution.height)*sourceResolution.height);CGImageRef sourceTileImageRef;// calculate the number of read/write operations required to assemble the// output image.//计算原图像需要被分割成多少个方块 iterationsint iterations = (int)( sourceResolution.height / sourceTile.size.height );// If tile height doesn't divide the image height evenly, add another iteration// to account for the remaining pixels.int remainder = (int)sourceResolution.height % (int)sourceTile.size.height;if(remainder) {iterations++;}// Add seem overlaps to the tiles, but save the original tile height for y coordinate calculations.//根据重叠像素计算原图方块的大小后，获取原图中该方块内的数据，把该数据写入到相对应的目标方块中float sourceTileHeightMinusOverlap = sourceTile.size.height;sourceTile.size.height += sourceSeemOverlap;destTile.size.height += kDestSeemOverlap;for( int y = 0; y < iterations; ++y ) {@autoreleasepool {sourceTile.origin.y = y * sourceTileHeightMinusOverlap + sourceSeemOverlap;destTile.origin.y = destResolution.height - (( y + 1 ) * sourceTileHeightMinusOverlap * imageScale + kDestSeemOverlap);sourceTileImageRef = CGImageCreateWithImageInRect( sourceImageRef, sourceTile );if( y == iterations - 1 && remainder ) {float dify = destTile.size.height;destTile.size.height = CGImageGetHeight( sourceTileImageRef ) * imageScale;dify -= destTile.size.height;destTile.origin.y += dify;}CGContextDrawImage( destContext, destTile, sourceTileImageRef );CGImageRelease( sourceTileImageRef );}}CGImageRef destImageRef = CGBitmapContextCreateImage(destContext);CGContextRelease(destContext);if (destImageRef == NULL) {return image;}//生成处理结束以后的图片UIImage *destImage = [UIImage imageWithCGImage:destImageRef scale:image.scale orientation:image.imageOrientation];CGImageRelease(destImageRef);if (destImage == nil) {return image;}return destImage;}
}/**图片是否能够解码@param image 图片@return 能否解码*/
+ (BOOL)shouldDecodeImage:(nullable UIImage *)image {// Prevent "CGBitmapContextCreateImage: invalid context 0x0" errorif (image == nil) {return NO;}// do not decode animated images//如果是动态图片不处理if (image.images != nil) {return NO;}CGImageRef imageRef = image.CGImage;//获取image的alpha通道。通过通道获取图片数据CGImageAlphaInfo alpha = CGImageGetAlphaInfo(imageRef);BOOL anyAlpha = (alpha == kCGImageAlphaFirst ||alpha == kCGImageAlphaLast ||alpha == kCGImageAlphaPremultipliedFirst ||alpha == kCGImageAlphaPremultipliedLast);// do not decode images with alpha//如果有alpha通道值，则不处理if (anyAlpha) {return NO;}return YES;
}/**是否需要压缩原始图片的大小(图像大于目标尺寸才需要压缩)@param image UIImage对象@return 是否支持压缩*/
+ (BOOL)shouldScaleDownImage:(nonnull UIImage *)image {BOOL shouldScaleDown = YES;CGImageRef sourceImageRef = image.CGImage;CGSize sourceResolution = CGSizeZero;sourceResolution.width = CGImageGetWidth(sourceImageRef);sourceResolution.height = CGImageGetHeight(sourceImageRef);//图片总共像素float sourceTotalPixels = sourceResolution.width * sourceResolution.height;//如果图片的总像素大于一定比例，则需要做简化处理float imageScale = kDestTotalPixels / sourceTotalPixels;if (imageScale < 1) {shouldScaleDown = YES;} else {shouldScaleDown = NO;}return shouldScaleDown;
}/**获取图片的色彩空间@param imageRef 图片@return 色彩空间*/
+ (CGColorSpaceRef)colorSpaceForImageRef:(CGImageRef)imageRef {// currentCGColorSpaceModel imageColorSpaceModel = CGColorSpaceGetModel(CGImageGetColorSpace(imageRef));CGColorSpaceRef colorspaceRef = CGImageGetColorSpace(imageRef);BOOL unsupportedColorSpace = (imageColorSpaceModel == kCGColorSpaceModelUnknown ||imageColorSpaceModel == kCGColorSpaceModelMonochrome ||imageColorSpaceModel == kCGColorSpaceModelCMYK ||imageColorSpaceModel == kCGColorSpaceModelIndexed);if (unsupportedColorSpace) {colorspaceRef = CGColorSpaceCreateDeviceRGB();CFAutorelease(colorspaceRef);}return colorspaceRef;
}

/**关联属性@return 属性值*/
- (SDOperationsDictionary *)operationDictionary {SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);if (operations) {return operations;}operations = [NSMutableDictionary dictionary];objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);return operations;
}/**关联Operation对象与key对象@param operation Operation对象@param key key*/
- (void)sd_setImageLoadOperation:(nullable id)operation forKey:(nullable NSString *)key {if (key) {[self sd_cancelImageLoadOperationWithKey:key];if (operation) {SDOperationsDictionary *operationDictionary = [self operationDictionary];operationDictionary[key] = operation;}}
}/**取消当前key对应的所有实现了SDWebImageOperation协议的Operation对象@param key Operation对应的key*/
- (void)sd_cancelImageLoadOperationWithKey:(nullable NSString *)key {// Cancel in progress downloader from queue//获取当前View对应的所有keySDOperationsDictionary *operationDictionary = [self operationDictionary];//获取对应的图片加载Operationid operations = operationDictionary[key];//取消所有当前View对应的所有Operationif (operations) {if ([operations isKindOfClass:[NSArray class]]) {for (id <SDWebImageOperation> operation in operations) {if (operation) {//SDWebImageCombinedOperation对象的cancel方法
                    [operation cancel];}}} else if ([operations conformsToProtocol:@protocol(SDWebImageOperation)]){[(id<SDWebImageOperation>) operations cancel];}[operationDictionary removeObjectForKey:key];}
}/**根据key移除相应的Operation对象@param key key*/
- (void)sd_removeImageLoadOperationWithKey:(nullable NSString *)key {if (key) {SDOperationsDictionary *operationDictionary = [self operationDictionary];[operationDictionary removeObjectForKey:key];}
}