SDImageCoderHelper 源码分析

前提知识补充，看我这篇文章

动图拆分为每一帧，和将所有帧组装成动图

// 构建动画图片，根据传进来的帧组进行构建
+ (UIImage *)animatedImageWithFrames:(NSArray<SDImageFrame *> *)frames {
    NSUInteger frameCount = frames.count;
    if (frameCount == 0) {
        return nil;
    }
    
    UIImage *animatedImage;
    
#if SD_UIKIT || SD_WATCH
    // 创建一个 NSUInteger 数组，不能为负数，里面存放着每一帧的播放时间
    NSUInteger durations[frameCount];
    for (size_t i = 0; i < frameCount; i++) {
        // 将每一帧的播放时间变成ms放到数组中去
        durations[i] = frames[i].duration * 1000;
    }
    // 求出最大公约数
    NSUInteger const gcd = gcdArray(frameCount, durations);
    // 总播放时间
    __block NSUInteger totalDuration = 0;
    NSMutableArray<UIImage *> *animatedImages = [NSMutableArray arrayWithCapacity:frameCount];
    [frames enumerateObjectsUsingBlock:^(SDImageFrame * _Nonnull frame, NSUInteger idx, BOOL * _Nonnull stop) {
        // 取出每一帧的详细信息
        UIImage *image = frame.image;
        NSUInteger duration = frame.duration * 1000;
        totalDuration += duration;
        NSUInteger repeatCount;
        if (gcd) {
            // 如果最大公约数大于0，那么重复次数就是时间/最大公约数
            repeatCount = duration / gcd;
        } else {
            // 否则播放一次
            repeatCount = 1;
        }
        // 根据重复次数将图片放入数组中
        for (size_t i = 0; i < repeatCount; ++i) {
            [animatedImages addObject:image];
        }
    }];
    // 创建动图
    animatedImage = [UIImage animatedImageWithImages:animatedImages duration:totalDuration / 1000.f];
    
#else
    
    NSMutableData *imageData = [NSMutableData data];
    CFStringRef imageUTType = [NSData sd_UTTypeFromImageFormat:SDImageFormatGIF];
    // Create an image destination. GIF does not support EXIF image orientation
    CGImageDestinationRef imageDestination = CGImageDestinationCreateWithData((__bridge CFMutableDataRef)imageData, imageUTType, frameCount, NULL);
    if (!imageDestination) {
        // Handle failure.
        return nil;
    }
    
    for (size_t i = 0; i < frameCount; i++) {
        @autoreleasepool {
            SDImageFrame *frame = frames[i];
            NSTimeInterval frameDuration = frame.duration;
            CGImageRef frameImageRef = frame.image.CGImage;
            NSDictionary *frameProperties = @{(__bridge NSString *)kCGImagePropertyGIFDictionary : @{(__bridge NSString *)kCGImagePropertyGIFDelayTime : @(frameDuration)}};
            CGImageDestinationAddImage(imageDestination, frameImageRef, (__bridge CFDictionaryRef)frameProperties);
        }
    }
    // Finalize the destination.
    if (CGImageDestinationFinalize(imageDestination) == NO) {
        // Handle failure.
        CFRelease(imageDestination);
        return nil;
    }
    CFRelease(imageDestination);
    CGFloat scale = MAX(frames.firstObject.image.scale, 1);
    
    SDAnimatedImageRep *imageRep = [[SDAnimatedImageRep alloc] initWithData:imageData];
    NSSize size = NSMakeSize(imageRep.pixelsWide / scale, imageRep.pixelsHigh / scale);
    imageRep.size = size;
    animatedImage = [[NSImage alloc] initWithSize:size];
    [animatedImage addRepresentation:imageRep];
#endif
    
    return animatedImage;
}
// 从动图中拆分出每一帧
+ (NSArray<SDImageFrame *> *)framesFromAnimatedImage:(UIImage *)animatedImage {
    if (!animatedImage) {
        return nil;
    }
    
    NSMutableArray<SDImageFrame *> *frames = [NSMutableArray array];
    NSUInteger frameCount = 0;
    
#if SD_UIKIT || SD_WATCH
    // 取出动图中所有帧
    NSArray<UIImage *> *animatedImages = animatedImage.images;
    frameCount = animatedImages.count;
    if (frameCount == 0) {
        return nil;
    }
    // 平均时间
    NSTimeInterval avgDuration = animatedImage.duration / frameCount;
    // 如果没有时间，默认 100ms
    if (avgDuration == 0) {
        avgDuration = 0.1; // if it's a animated image but no duration, set it to default 100ms (this do not have that 10ms limit like GIF or WebP to allow custom coder provide the limit)
    }
    // 记录不同的帧额数量
    __block NSUInteger index = 0;
    // 每一帧的重复次数
    __block NSUInteger repeatCount = 1;
    // 记录前一张图片，每次和前一张图片比较
    __block UIImage *previousImage = animatedImages.firstObject;
    [animatedImages enumerateObjectsUsingBlock:^(UIImage * _Nonnull image, NSUInteger idx, BOOL * _Nonnull stop) {
        // ignore first
        // 忽略第一张
        if (idx == 0) {
            return;
        }
        // 如果当前和前一张是一张图片
        if ([image isEqual:previousImage]) {
            // 那么这张图片额重复次数就+1
            repeatCount++;
        } else {
            // 如果不一样，那么就直接构建 SDImageFrame 对象
            // 将前一帧图片放进去，然后更新重复次数
            SDImageFrame *frame = [SDImageFrame frameWithImage:previousImage duration:avgDuration * repeatCount];
            [frames addObject:frame];
            repeatCount = 1;
            index++;
        }
        // 然后更改 pre 的值
        previousImage = image;
        // last one
        // 如果是最后一张，直接加进去
        // 最后一张和之前有可能是重复也有可能没有重复，但一定没有加进去过
        if (idx == frameCount - 1) {
            SDImageFrame *frame = [SDImageFrame frameWithImage:previousImage duration:avgDuration * repeatCount];
            [frames addObject:frame];
        }
    }];
    
#else
    
    NSRect imageRect = NSMakeRect(0, 0, animatedImage.size.width, animatedImage.size.height);
    NSImageRep *imageRep = [animatedImage bestRepresentationForRect:imageRect context:nil hints:nil];
    NSBitmapImageRep *bitmapImageRep;
    if ([imageRep isKindOfClass:[NSBitmapImageRep class]]) {
        bitmapImageRep = (NSBitmapImageRep *)imageRep;
    }
    if (!bitmapImageRep) {
        return nil;
    }
    frameCount = [[bitmapImageRep valueForProperty:NSImageFrameCount] unsignedIntegerValue];
    if (frameCount == 0) {
        return nil;
    }
    CGFloat scale = animatedImage.scale;
    
    for (size_t i = 0; i < frameCount; i++) {
        @autoreleasepool {
            // NSBitmapImageRep need to manually change frame. "Good taste" API
            [bitmapImageRep setProperty:NSImageCurrentFrame withValue:@(i)];
            NSTimeInterval frameDuration = [[bitmapImageRep valueForProperty:NSImageCurrentFrameDuration] doubleValue];
            NSImage *frameImage = [[NSImage alloc] initWithCGImage:bitmapImageRep.CGImage scale:scale orientation:kCGImagePropertyOrientationUp];
            SDImageFrame *frame = [SDImageFrame frameWithImage:frameImage duration:frameDuration];
            [frames addObject:frame];
        }
    }
#endif
    // 返回帧组
    return frames;
}

获取颜色空间和透明度

// 获取颜色空间
// 通过案例，提高性能，因为获取一次就够了
+ (CGColorSpaceRef)colorSpaceGetDeviceRGB {
#if SD_MAC
    CGColorSpaceRef screenColorSpace = NSScreen.mainScreen.colorSpace.CGColorSpace;
    if (screenColorSpace) {
        return screenColorSpace;
    }
#endif
    static CGColorSpaceRef colorSpace;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
#if SD_UIKIT
        if (@available(iOS 9.0, tvOS 9.0, *)) {
            colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceSRGB);
        } else {
            colorSpace = CGColorSpaceCreateDeviceRGB();
        }
#else
        colorSpace = CGColorSpaceCreateDeviceRGB();
#endif
    });
    return colorSpace;
}
// 获取透明度
+ (BOOL)CGImageContainsAlpha:(CGImageRef)cgImage {
    if (!cgImage) {
        return NO;
    }
    CGImageAlphaInfo alphaInfo = CGImageGetAlphaInfo(cgImage);
    BOOL hasAlpha = !(alphaInfo == kCGImageAlphaNone ||
                      alphaInfo == kCGImageAlphaNoneSkipFirst ||
                      alphaInfo == kCGImageAlphaNoneSkipLast);
    return hasAlpha;
}

解压缩图片

// 解码图片
+ (UIImage *)decodedImageWithImage:(UIImage *)image {
    if (![self shouldDecodeImage:image]) {
        return image;
    }
    
    CGImageRef imageRef = [self CGImageCreateDecoded:image.CGImage];
    if (!imageRef) {
        return image;
    }
#if SD_MAC
    UIImage *decodedImage = [[UIImage alloc] initWithCGImage:imageRef scale:image.scale orientation:kCGImagePropertyOrientationUp];
#else
    UIImage *decodedImage = [[UIImage alloc] initWithCGImage:imageRef scale:image.scale orientation:image.imageOrientation];
#endif
    CGImageRelease(imageRef);
    // 将新图片的属性 copy 一份
    SDImageCopyAssociatedObject(image, decodedImage);
    // 标记为已解码
    decodedImage.sd_isDecoded = YES;
    return decodedImage;
}

// 解压缩图片
// https://code.imerc.cc/2017/07/09/ios-image-decode/
// https://www.jianshu.com/p/cc670b4ec97e
+ (CGImageRef)CGImageCreateDecoded:(CGImageRef)cgImage {
    return [self CGImageCreateDecoded:cgImage orientation:kCGImagePropertyOrientationUp];
}

+ (CGImageRef)CGImageCreateDecoded:(CGImageRef)cgImage orientation:(CGImagePropertyOrientation)orientation {
    if (!cgImage) {
        return NULL;
    }
    size_t width = CGImageGetWidth(cgImage);
    size_t height = CGImageGetHeight(cgImage);
    if (width == 0 || height == 0) return NULL;
    size_t newWidth;
    size_t newHeight;
    switch (orientation) {
        case kCGImagePropertyOrientationLeft:
        case kCGImagePropertyOrientationLeftMirrored:
        case kCGImagePropertyOrientationRight:
        case kCGImagePropertyOrientationRightMirrored: {
            // These orientation should swap width & height
            newWidth = height;
            newHeight = width;
        }
            break;
        default: {
            newWidth = width;
            newHeight = height;
        }
            break;
    }
    
    BOOL hasAlpha = [self CGImageContainsAlpha:cgImage];
    // iOS prefer BGRA8888 (premultiplied) or BGRX8888 bitmapInfo for screen rendering, which is same as `UIGraphicsBeginImageContext()` or `- [CALayer drawInContext:]`
    // Though you can use any supported bitmapInfo (see: https://developer.apple.com/library/content/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_context/dq_context.html#//apple_ref/doc/uid/TP30001066-CH203-BCIBHHBB ) and let Core Graphics reorder it when you call `CGContextDrawImage`
    // But since our build-in coders use this bitmapInfo, this can have a little performance benefit
    CGBitmapInfo bitmapInfo = kCGBitmapByteOrder32Host;
    bitmapInfo |= hasAlpha ? kCGImageAlphaPremultipliedFirst : kCGImageAlphaNoneSkipFirst;
    
    
    // 开始解码图片
    CGContextRef context = CGBitmapContextCreate(NULL, newWidth, newHeight, 8, 0, [self colorSpaceGetDeviceRGB], bitmapInfo);
    if (!context) {
        return NULL;
    }
    
    // Apply transform
    CGAffineTransform transform = SDCGContextTransformFromOrientation(orientation, CGSizeMake(newWidth, newHeight));
    CGContextConcatCTM(context, transform);
    // 通过draw先将图片画出来
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), cgImage); // The rect is bounding box of CGImage, don't swap width & height
    // 然后拿到位图
    CGImageRef newImageRef = CGBitmapContextCreateImage(context);
    CGContextRelease(context);
    
    return newImageRef;
}

调整图片大小

// 调整原始图片的大小
+ (CGImageRef)CGImageCreateScaled:(CGImageRef)cgImage size:(CGSize)size {
    if (!cgImage) {
        return NULL;
    }
    // 原图片的宽高
    size_t width = CGImageGetWidth(cgImage);
    size_t height = CGImageGetHeight(cgImage);
    // 如果原来的宽高和要调节的宽高一样，就直接返回
    if (width == size.width && height == size.height) {
        CGImageRetain(cgImage);
        return cgImage;
    }
    // 创建 input output buffer
    __block vImage_Buffer input_buffer = {}, output_buffer = {};
    // 此处是个骚操作
    @onExit {
        if (input_buffer.data) free(input_buffer.data);
        if (output_buffer.data) free(output_buffer.data);
    };
    // 获取透明度
    BOOL hasAlpha = [self CGImageContainsAlpha:cgImage];
    // iOS display alpha info (BGRA8888/BGRX8888)
    
    CGBitmapInfo bitmapInfo = kCGBitmapByteOrder32Host;
    bitmapInfo |= hasAlpha ? kCGImageAlphaPremultipliedFirst : kCGImageAlphaNoneSkipFirst;
    vImage_CGImageFormat format = (vImage_CGImageFormat) {
        .bitsPerComponent = 8,
        .bitsPerPixel = 32,
        .colorSpace = NULL,
        .bitmapInfo = bitmapInfo,
        .version = 0,
        .decode = NULL,
        .renderingIntent = kCGRenderingIntentDefault,
    };
    // 先根据原始数据生成原始inputbuffer
    vImage_Error a_ret = vImageBuffer_InitWithCGImage(&input_buffer, &format, NULL, cgImage, kvImageNoFlags);
    if (a_ret != kvImageNoError) return NULL;
    
    // 创建 output buffer
    output_buffer.width = MAX(size.width, 0);
    output_buffer.height = MAX(size.height, 0);
    // 这里是什么意思呢?
    output_buffer.rowBytes = SDByteAlign(output_buffer.width * 4, 64);
    output_buffer.data = malloc(output_buffer.rowBytes * output_buffer.height);
    if (!output_buffer.data) return NULL;
    
    vImage_Error ret = vImageScale_ARGB8888(&input_buffer, &output_buffer, NULL, kvImageHighQualityResampling);
    if (ret != kvImageNoError) return NULL;
    
    // 根据output buffer 创建image
    CGImageRef outputImage = vImageCreateCGImageFromBuffer(&output_buffer, &format, NULL, NULL, kvImageNoFlags, &ret);
    if (ret != kvImageNoError) {
        CGImageRelease(outputImage);
        return NULL;
    }
    
    return outputImage;
}

其他工具方法

// 默认缩放大小限制
+ (NSUInteger)defaultScaleDownLimitBytes {
    return kDestImageLimitBytes;
}
// 设置缩放的最大限制
+ (void)setDefaultScaleDownLimitBytes:(NSUInteger)defaultScaleDownLimitBytes {
    if (defaultScaleDownLimitBytes < kBytesPerMB) {
        return;
    }
    kDestImageLimitBytes = defaultScaleDownLimitBytes;
}

#if SD_UIKIT || SD_WATCH
// Convert an EXIF image orientation to an iOS one.
/**
将EXIF图像方向转换为iOS版本的方向
*/
+ (UIImageOrientation)imageOrientationFromEXIFOrientation:(CGImagePropertyOrientation)exifOrientation {
    UIImageOrientation imageOrientation = UIImageOrientationUp;
    switch (exifOrientation) {
        case kCGImagePropertyOrientationUp:
            imageOrientation = UIImageOrientationUp;
            break;
        case kCGImagePropertyOrientationDown:
            imageOrientation = UIImageOrientationDown;
            break;
        case kCGImagePropertyOrientationLeft:
            imageOrientation = UIImageOrientationLeft;
            break;
        case kCGImagePropertyOrientationRight:
            imageOrientation = UIImageOrientationRight;
            break;
        case kCGImagePropertyOrientationUpMirrored:
            imageOrientation = UIImageOrientationUpMirrored;
            break;
        case kCGImagePropertyOrientationDownMirrored:
            imageOrientation = UIImageOrientationDownMirrored;
            break;
        case kCGImagePropertyOrientationLeftMirrored:
            imageOrientation = UIImageOrientationLeftMirrored;
            break;
        case kCGImagePropertyOrientationRightMirrored:
            imageOrientation = UIImageOrientationRightMirrored;
            break;
        default:
            break;
    }
    return imageOrientation;
}

// Convert an iOS orientation to an EXIF image orientation.
/**
将iOS版本的方向转换为EXIF图像方向
*/
+ (CGImagePropertyOrientation)exifOrientationFromImageOrientation:(UIImageOrientation)imageOrientation {
    CGImagePropertyOrientation exifOrientation = kCGImagePropertyOrientationUp;
    switch (imageOrientation) {
        case UIImageOrientationUp:
            exifOrientation = kCGImagePropertyOrientationUp;
            break;
        case UIImageOrientationDown:
            exifOrientation = kCGImagePropertyOrientationDown;
            break;
        case UIImageOrientationLeft:
            exifOrientation = kCGImagePropertyOrientationLeft;
            break;
        case UIImageOrientationRight:
            exifOrientation = kCGImagePropertyOrientationRight;
            break;
        case UIImageOrientationUpMirrored:
            exifOrientation = kCGImagePropertyOrientationUpMirrored;
            break;
        case UIImageOrientationDownMirrored:
            exifOrientation = kCGImagePropertyOrientationDownMirrored;
            break;
        case UIImageOrientationLeftMirrored:
            exifOrientation = kCGImagePropertyOrientationLeftMirrored;
            break;
        case UIImageOrientationRightMirrored:
            exifOrientation = kCGImagePropertyOrientationRightMirrored;
            break;
        default:
            break;
    }
    return exifOrientation;
}
#endif

#pragma mark - Helper Function
+ (BOOL)shouldDecodeImage:(nullable UIImage *)image {
    // Prevent "CGBitmapContextCreateImage: invalid context 0x0" error
    if (image == nil) {
        return NO;
    }
    // Avoid extra decode
    // 已经被解码
    if (image.sd_isDecoded) {
        return NO;
    }
    // do not decode animated images
    // 动图
    if (image.sd_isAnimated) {
        return NO;
    }
    // do not decode vector images
    // 矢量图
    if (image.sd_isVector) {
        return NO;
    }
    
    return YES;
}

+ (BOOL)shouldScaleDownImage:(nonnull UIImage *)image limitBytes:(NSUInteger)bytes {
    BOOL shouldScaleDown = YES;
    
    // 原始分辨率大小
    CGImageRef sourceImageRef = image.CGImage;
    CGSize sourceResolution = CGSizeZero;
    sourceResolution.width = CGImageGetWidth(sourceImageRef);
    sourceResolution.height = CGImageGetHeight(sourceImageRef);
    float sourceTotalPixels = sourceResolution.width * sourceResolution.height;
    if (sourceTotalPixels <= 0) {
        return NO;
    }
    // 目标总像素
    CGFloat destTotalPixels;
    // 如果设置最大限制为0，那么就用默认的60M
    if (bytes == 0) {
        bytes = kDestImageLimitBytes;
    }
    // 目标期望大小，总的像素数
    destTotalPixels = bytes / kBytesPerPixel;
    // 1M 里面的像素总数都比目标大小大或者等于，就代表设置的目标大小太小了，没法缩放
    if (destTotalPixels <= kPixelsPerMB) {
        // Too small to scale down
        return NO;
    }
    // 目标大小和原来总像素大小做一个比例
    float imageScale = destTotalPixels / sourceTotalPixels;
    if (imageScale < 1) {
        shouldScaleDown = YES;
    } else {
        shouldScaleDown = NO;
    }
    
    return shouldScaleDown;
}

static inline CGAffineTransform SDCGContextTransformFromOrientation(CGImagePropertyOrientation orientation, CGSize size) {
    // Inspiration from @libfeihu
    // We need to calculate the proper transformation to make the image upright.
    // We do it in 2 steps: Rotate if Left/Right/Down, and then flip if Mirrored.
    CGAffineTransform transform = CGAffineTransformIdentity;
    
    switch (orientation) {
        case kCGImagePropertyOrientationDown:
        case kCGImagePropertyOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, size.width, size.height);
            transform = CGAffineTransformRotate(transform, M_PI);
            break;
            
        case kCGImagePropertyOrientationLeft:
        case kCGImagePropertyOrientationLeftMirrored:
            transform = CGAffineTransformTranslate(transform, size.width, 0);
            transform = CGAffineTransformRotate(transform, M_PI_2);
            break;
            
        case kCGImagePropertyOrientationRight:
        case kCGImagePropertyOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform, 0, size.height);
            transform = CGAffineTransformRotate(transform, -M_PI_2);
            break;
        case kCGImagePropertyOrientationUp:
        case kCGImagePropertyOrientationUpMirrored:
            break;
    }
    
    switch (orientation) {
        case kCGImagePropertyOrientationUpMirrored:
        case kCGImagePropertyOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, size.width, 0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
            
        case kCGImagePropertyOrientationLeftMirrored:
        case kCGImagePropertyOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform, size.height, 0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
        case kCGImagePropertyOrientationUp:
        case kCGImagePropertyOrientationDown:
        case kCGImagePropertyOrientationLeft:
        case kCGImagePropertyOrientationRight:
            break;
    }
    
    return transform;
}

#if SD_UIKIT || SD_WATCH

// 求最大公约数
static NSUInteger gcd(NSUInteger a, NSUInteger b) {
    NSUInteger c;
    while (a != 0) {
        c = a;
        a = b % a;
        b = c;
    }
    return b;
}

static NSUInteger gcdArray(size_t const count, NSUInteger const * const values) {
    if (count == 0) {
        return 0;
    }
    NSUInteger result = values[0];
    for (size_t i = 1; i < count; ++i) {
        result = gcd(values[i], result);
    }
    return result;
}
#endif