SDWebImage源码解读 之 UIImage+GIF

SDWebImage源码解读之UIImage+GIF前言本篇是和GIF相关的一个UIImage的分类。

主要提供了三个方法：+ (UIImage *)sd_animatedGIFNamed:(NSString *)name ----- 根据名称获取图片+ (UIImage *)sd_animatedGIFWithData:(NSData *)data ----- 根据NSData获取图片- (UIImage *)sd_animatedImageByScalingAndCroppingToSize:(CGSize)size ----- 修改图片到指定的尺寸UIImage的size，scale属性我们先不管图片的更高级的知识，我们简单的对size和scale这两个属性做一下介绍。

注意：如果要获取一个图片的尺寸，不是直接使用image.size，而是使用image.size*image.scale。

当然，这是伪代码。

原因就是我们在获取size的时候。

使用的是Point坐标，而图片的尺寸是以像素为参照的。

系统为我们处理了这两种坐标系的转换工作。

我们用一个例子来演示上边的内容：UIImage *image = [UIImage imageNamed:@"photo_delete"];NSLog(@"-----尺寸:(%f %f)", image.size.width, image.size.height);打印结果为：-----尺寸:(18.000000 18.000000)可以看出来。

使用size这个属性是不对的。

该图片的实际尺寸为：那我们修改下代码：UIImage *image = [UIImage imageNamed:@"photo_delete"];NSLog(@"-----尺寸:(%f %f)", image.size.width * image.scale, image.size.height * image.scale);打印结果如下：-----尺寸:(36.000000 36.000000)修改图片到指定的尺寸- (UIImage *)sd_animatedImageByScalingAndCroppingToSize:(CGSize)size {if (CGSizeEqualToSize(self.size, size) || CGSizeEqualToSize(size, CGSizeZero)) {return self;}CGSize scaledSize = size;CGPoint thumbnailPoint = CGPointZero;CGFloat widthFactor = size.width / self.size.width;CGFloat heightFactor = size.height / self.size.height;scaledSize.width = self.size.width * scaleFactor;scaledSize.height = self.size.height * scaleFactor;if (widthFactor > heightFactor) {thumbnailPoint.y = (size.height - scaledSize.height) * 0.5;}else if (widthFactor < heightFactor) {thumbnailPoint.x = (size.width - scaledSize.width) * 0.5;}NSMutableArray *scaledImages = [NSMutableArray array];for (UIImage *image in self.images) {UIGraphicsBeginImageContextWithOptions(size, NO, 0.0);[image drawInRect:CGRectMake(thumbnailPoint.x, thumbnailPoint.y, scaledSize.width, scaledSize.height)];UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();[scaledImages addObject:newImage];UIGraphicsEndImageContext();}return [UIImage animatedImageWithImages:scaledImages duration:self.duration];}上边的方法能够实现把图片的尺寸修剪为size，剪裁的前提是根据图片原来的比例。

具体的实现，在这里就不举例说明了。

和数学原理有点关系。

+ (float)sd_frameDurationAtIndex:(NSUInteger)index source:(CGImageSourceRef)source一个Image Sources抽象出来了图片数据，通过raw memory buffer减轻开发人员对数据的处理。

Image Sources包含不止一个图像，缩略图，各个图像的特征和图片文件。

通过CGImageSource实现。

可以这么说：CGImageSourceRef就是对图像数据的一层封装。

+ (float)sd_frameDurationAtIndex:(NSUInteger)index source:(CGImageSourceRef)source {float frameDuration = 0.1f;CFDictionaryRef cfFrameProperties = CGImageSourceCopyPropertiesAtIndex(source, index, nil);NSDictionary *frameProperties = (__bridge NSDictionary *)cfFrameProperties;NSDictionary *gifProperties = frameProperties[(NSString *)kCGImagePropertyGIFDictionary];NSNumber *delayTimeUnclampedProp = gifProperties[(NSString *)kCGImagePropertyGIFUnclampedDelayTime];if (delayTimeUnclampedProp) {frameDuration = [delayTimeUnclampedProp floatValue];}NSNumber *delayTimeProp = gifProperties[(NSString *)kCGImagePropertyGIFDelayTime];if (delayTimeProp) {frameDuration = [delayTimeProp floatValue];}}// Many annoying ads specify a 0 duration to make an image flash as quickly as possible.// We follow Firefox's behavior and use a duration of 100 ms for any frames that specify// a duration of <= 10 ms. See <rdar://problem/7689300> and </b/36082>// for more information.if (frameDuration < 0.011f) {frameDuration = 0.100f;}CFRelease(cfFrameProperties);return frameDuration;}+ (UIImage )sd_animatedGIFWithData:(NSData )data当我们由NSData => UIImage 的时候，我们应该考虑更多一点。

如果NSData中不止一张图片，应该怎么办？获取NSData中的图片数量CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);size_t count = Count(source);如果图片数量小于或者等于1，直接转换if (count <= 1) {animatedImage = [[UIImage alloc] initWithData:data];}数量大于1的情况取出每一个图片计算总的duration生成UIImage代码如下：+ (UIImage *)sd_animatedGIFWithData:(NSData *)data {if (!data) {return nil;}size_t count = CGImageSourceGetCount(source);UIImage *animatedImage;if (count <= 1) {animatedImage = [[UIImage alloc] initWithData:data];}else {NSMutableArray *images = [NSMutableArray array];NSTimeInterval duration = 0.0f;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);}if (!duration) {duration = (1.0f / 10.0f) * count;}animatedImage = [UIImage animatedImageWithImages:images duration:duration];}CFRelease(source);return animatedImage;}+ (UIImage )sd_animatedGIFNamed:(NSString )name+ (UIImage *)sd_animatedGIFNamed:(NSString *)name {CGFloat scale = [UIScreen mainScreen].scale;if (scale > 1.0f) {NSString *retinaPath = [[NSBundle mainBundle] pathForResource:[name stringByAppendingString:@"@2x"] ofType:@"gif"];NSData *data = [NSData dataWithContentsOfFile:retinaPath];if (data) {return [UIImage sd_animatedGIFWithData:data];}NSString *path = [[NSBundle mainBundle] pathForResource:name ofType:@"gif"];data = [NSData dataWithContentsOfFile:path];if (data) {return [UIImage sd_animatedGIFWithData:data];}return [UIImage imageNamed:name];}else {NSString *path = [[NSBundle mainBundle] pathForResource:name ofType:@"gif"];NSData *data = [NSData hContentsOfFile:path];if (data) {return [UIImage sd_animatedGIFWithData:data];}return [UIImage imageNamed:name];}}补充在这里补充一点实现渐进式图片加载的步骤。