SDWebImage源码阅读-第三篇

+ (NSString *)sd_contentTypeForImageData:(NSData *)data {
    uint8_t c;
    [data getBytes:&c length:1];
    switch (c) {
        case 0xFF:
            return @"image/jpeg";
        case 0x89:
            return @"image/png";
        case 0x47:
            return @"image/gif";
        case 0x49:
        case 0x4D:
            return @"image/tiff";
        case 0x52:
            if ([data length] < 12) {
                return nil;
            }

            NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(0, 12)] encoding:NSASCIIStringEncoding];
            if ([testString hasPrefix:@"RIFF"] && [testString hasSuffix:@"WEBP"]) {
                return @"image/webp";
            }

            return nil;
    }
    return nil;
}

// 指定在main bundle中gif的文件名，读取文件的二进制，然后调用下面的方法
+ (UIImage *)sd_animatedGIFNamed:(NSString *)name; 

// 将gif文件的二进制转为animatedImage
+ (UIImage *)sd_animatedGIFWithData:(NSData *)data; 

// 将self.images数组中的图片按照指定的尺寸缩放，返回一个animatedImage，一次播放的时间是self.duration
- (UIImage *)sd_animatedImageByScalingAndCroppingToSize:(CGSize)size; 

+ (UIImage *)sd_animatedGIFWithData:(NSData *)data {
    if (!data) {
        return nil;
    }
    CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);
    // 获取图片数量(如果传入的是gif图的二进制，那么获取的是图片帧数)
    size_t count = CGImageSourceGetCount(source);
    UIImage *animatedImage;
    if (count <= 1) {
        animatedImage = [[UIImage alloc] initWithData:data];
    }
    else {
        NSMutableArray *images = [NSMutableArray array];
        NSTimeInterval duration = 0.0f;
        // 遍历每张图，通过`sd_frameDurationAtIndex:source:`获取每张图需要的播放时间，用duration累加，将图到出为UIImage，依次放到数组imges中
        for (size_t i = 0; i < count; i++) {
            CGImageRef image = CGImageSourceCreateImageAtIndex(source, i, NULL);
            if (!image) {
                continue;
            }
            duration += [self sd_frameDurationAtIndex:i source:source];
            [images addObject:[UIImage imageWithCGImage:image scale:[UIScreen mainScreen].scale orientation:UIImageOrientationUp]];
            CGImageRelease(image);
        }
        // 如果上面的计算播放时间方法没有成功，就按照下面方法计算
        // 计算一次播放的总时间：每张图播放1/10秒 * 图片总数
        if (!duration) {
            duration = (1.0f / 10.0f) * count;
        }        
        animatedImage = [UIImage animatedImageWithImages:images duration:duration];
    }
    CFRelease(source);
    return animatedImage;
}

+ (float)sd_frameDurationAtIndex:(NSUInteger)index source:(CGImageSourceRef)source {
    float frameDuration = 0.1f;
    // 获取这一帧的属性字典
    CFDictionaryRef cfFrameProperties = CGImageSourceCopyPropertiesAtIndex(source, index, nil);
    NSDictionary *frameProperties = (__bridge NSDictionary *)cfFrameProperties;
    NSDictionary *gifProperties = frameProperties[(NSString *)kCGImagePropertyGIFDictionary];
    // 从字典中获取这一帧持续的时间
    NSNumber *delayTimeUnclampedProp = gifProperties[(NSString *)kCGImagePropertyGIFUnclampedDelayTime];
    if (delayTimeUnclampedProp) {
        frameDuration = [delayTimeUnclampedProp floatValue];
    } else {
        NSNumber *delayTimeProp = gifProperties[(NSString *)kCGImagePropertyGIFDelayTime];
        if (delayTimeProp) {
            frameDuration = [delayTimeProp floatValue];
        }
    }    
    // 许多烦人的广告指定duration为0来让图像尽可能快地闪过。
    // 我们遵循Firefox的做法：对于指定duration小于<= 10 ms的帧设置duration值为100ms
    // 详见<rdar://problem/7689300>和<http://webkit.org/b/36082>
    if (frameDuration < 0.011f) {
        frameDuration = 0.100f;
    }
    CFRelease(cfFrameProperties);
    return frameDuration;
}

+ (UIImage *)sd_imageWithWebPData:(NSData *)data {
    WebPDecoderConfig config;
    // 初始化解码器结构体(WebPDecoderConfig类型)变量config
    if (!WebPInitDecoderConfig(&config)) {
        return nil;
    }
    // 将WebP图片的二进制数据传递给config
    if (WebPGetFeatures(data.bytes, data.length, &config.input) != VP8_STATUS_OK) {
        return nil;
    }

    config.output.colorspace = config.input.has_alpha ? MODE_rgbA : MODE_RGB;
    config.options.use_threads = 1;

    // 将WebP图片数据解码为RGBA值数组，保存在config中
    if (WebPDecode(data.bytes, data.length, &config) != VP8_STATUS_OK) {
        return nil;
    }
    // 从config中读取出图片的宽高信息
    int width = config.input.width;
    int height = config.input.height;
    if (config.options.use_scaling) {
        width = config.options.scaled_width;
        height = config.options.scaled_height;
    }

    // 根据解码后的RGBA值数组创建UIImage.
    // 1.创建数据提供者，参数指定了RGBA值数组的开始地址`config.output.u.RGBA.rgba`和长度`config.output.u.RGBA.size`，用于释放数据的回调`FreeImageData`
    CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, config.output.u.RGBA.rgba, config.output.u.RGBA.size, FreeImageData);
    CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
    CGBitmapInfo bitmapInfo = config.input.has_alpha ? kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast : 0;
    size_t components = config.input.has_alpha ? 4 : 3;
    CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
    // 2.使用数据提供者和其他信息创建CGImageRef
    CGImageRef imageRef = CGImageCreate(width, height, 8, components * 8, components * width, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);

    CGColorSpaceRelease(colorSpaceRef);
    CGDataProviderRelease(provider);
    // 3.将CGImageRef转为UIImage
    UIImage *image = [[UIImage alloc] initWithCGImage:imageRef];
    CGImageRelease(imageRef);

    return image;
}

+ (UIImage *)sd_imageWithData:(NSData *)data {
    if (!data) {
        return nil;
    }
    
    UIImage *image;
    NSString *imageContentType = [NSData sd_contentTypeForImageData:data];
    if ([imageContentType isEqualToString:@"image/gif"]) {
        // 1.如果是gif，使用gif的data转UIImage方法
        image = [UIImage sd_animatedGIFWithData:data];
    }
#ifdef SD_WEBP
    else if ([imageContentType isEqualToString:@"image/webp"]) {
        // 2.如果是WebP，使用WebP的data转UIImage方法
        image = [UIImage sd_imageWithWebPData:data];
    }
#endif
    else {// 3.其他情况
        image = [[UIImage alloc] initWithData:data];
        // 获取图片的方向
        UIImageOrientation orientation = [self sd_imageOrientationFromImageData:data];
        // 如果方向不是向上，则使用方向重新创建图片
        if (orientation != UIImageOrientationUp) {
            image = [UIImage imageWithCGImage:image.CGImage
                                        scale:image.scale
                                  orientation:orientation];
        }
    }
    return image;
}