SDWebImage之工具类
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SDWebImage
使用了很多工具类来对图片的处理,比如获取图片类型、图片放大缩小、GIF图片处理、图片解压缩处理等。下面我们来看一下这几个工具类。
1.NSData+ImageContentType
这个类提供了一个类方法sd_imageFormatForImageData
。通过这个方法传入图片的NSData数据,然后返回图片类型。
/** 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 WEBP if (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; }
2.UIImage+MultiFormat
该分类实现了NSData与UIImage对象之间的相互转换,并且是根据图片类型做转换。比如GIF的UIImage转换为GIF格式的NSData。
/** 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,jpg SDImageFormat 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_WEBP else if (imageFormat == SDImageFormatWebP) { image = [UIImage sd_imageWithWebPData:data]; } #endif else { 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.CGImage scale:image.scale orientation: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_WATCH int 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data if (usePNG) { imageData = UIImagePNGRepresentation(self); } else { imageData = UIImageJPEGRepresentation(self, (CGFloat)1.0); } #else NSBitmapImageFileType imageFileType = NSJPEGFileType; if (imageFormat == SDImageFormatGIF) { imageFileType = NSGIFFileType; } else if (imageFormat == SDImageFormatPNG) { imageFileType = NSPNGFileType; } imageData = [NSBitmapImageRep representationOfImageRepsInArray:self.representations usingType:imageFileType properties:@{}]; #endif } return imageData; }
3.UIImage+GIF
该分类实现了对GIF图片的NSData的处理,处理方式是取出GIF图片的第一张UIImage来显示。如果要显示动态图片的话,需要使用FLAnimatedImageView
来显示。
/** 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_WATCH CGFloat scale = 1; scale = [WKInterfaceDevice currentDevice].screenScale; #elif SD_UIKIT CGFloat 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_MAC staticImage = [[UIImage alloc] initWithCGImage:CGImage size:NSZeroSize]; #endif CGImageRelease(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); }
4.SDWebImageDecoder
本类实现图片的解码操作,对于太大的图片,先按照一定比例缩小然后再解码。在这里,大家可能有一个问题:为什么要解码呢?
4.1为什么要解码?
在我们实际的项目开发中,我们经常使用imageNamed:方法来加载图片,系统默认会在主线程立即进行图片的解码工作,这一过程就是把图片解码成可供控件直接使用的位图。当在主线程调用了大量的imageNamed:
方法后,就会产生卡顿。为了解决这个问题我们有两种处理方法:
不使用imageNamed:
加载图片,使用imageWithContentsOfFile:来加载图片;
自己解码图片,把这个解码过程放到子线程。
关于图片的存储方式和处理,大家可以看一下这篇文章:图片格式。
4.2SDWebImageDecoder源码分析
#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:imageRefWithoutAlpha scale:image.scale orientation: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RGB CGColorSpaceRef 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 iterations int 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" error if (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 { // current CGColorSpaceModel 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; }
5.UIView+WebCacheOperation
UIView+WebCacheOperation主要用来记录 UIView 加载 Operation 操作,大多数情况下一个 View 仅拥有一个 Operation ,默认的 key 是当前类的类名,如果设置了不同的 key,将保存不同的 Operation 。比如一个 UIButton,可以设置不同状态下的图片,那么需要记录多个 Operation ,它主要采用一个字典来保存所有的 Operation 。
/** 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key SDOperationsDictionary *operationDictionary = [self operationDictionary]; //获取对应的图片加载Operation id operations = operationDictionary[key]; //取消所有当前View对应的所有Operation if (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]; } }
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