Gif图片如越来越受欢迎，移动端对它的支持也是有些知识点的，主要是加载图片性能优化
- Gif图片的生成
Gif图片是一个图片数组，iOS的imageWithNamed方法只会取图片的第一帧，并不会显示动图，需要用到UIImage的animatedImageWithImages:duration:方法，第一参数传包含Gif每一帧图片的数组，第二个参数传Gif图片一次播放的时间，
CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);
通过图片data获取source；
size_t count = CGImageSourceGetCount(source);
if (count <= 1)
animatedImage = [[UIImage alloc] initWithData:data];
如果count就是一张，就直接initWithData返回；否则，
NSMutableArray *images = [NSMutableArray array];
NSTimeInterval duration = 0.0f;
for (size_t i = 0; i < count; i++) {
CGImageRef imageRef = CGImageSourceCreateImageAtIndex(source, i, NULL);
if (!imageRef) {
continue;
}
float frameDuration = 0.1f;
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];
}
}
CFRelease(cfFrameProperties);
duration += frameDuration;
[images addObject:[UIImage imageWithCGImage:imageRef scale:[UIScreen mainScreen].scale orientation:UIImageOrientationUp]];
CGImageRelease(imageRef);
}
if (!duration) {
duration = (1.0f / 10.0f) * count;
}
animatedImage = [UIImage animatedImageWithImages:images duration:duration];
CFRelease(source);
至此，Gif的动画图片animatedImage就生成了
- GIf图片展示性能优化
由于iOS在处理图片需要对它进行解压缩，这个过程默认在主线程，而且也是非常耗时间，耗资源，内存占用会增加，那么当需要加载的图片比较多时，比如Gif图片，就会对我们应用的响应性造成严重的影响，特别是在滑动的tableView上，这个问题会表现得很明显，所以在返回图片的时候最好将它解压缩好。
参考SDWebImage中的解压缩代码，只需要将上述imageRef在转化UIImage之前进行处理一下就好了，
@autoreleasepool {
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);
}
size_t width = CGImageGetWidth(imageRef);
size_t height = CGImageGetHeight(imageRef);
size_t bytesPerRow = 4 * width;
CGContextRef context = CGBitmapContextCreate(NULL,width,height,8,bytesPerRow,colorspaceRef,kCGBitmapByteOrderDefault|kCGImageAlphaNoneSkipLast);
if (context != NULL) {
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGImageRef imageRefWithoutAlpha = CGBitmapContextCreateImage(context);
CGContextRelease(context);
CGImageRelease(imageRef);
imageRef = imageRefWithoutAlpha;
}
}
这样获取的imageRef就是解压缩好的图片了。
- 通过以上两步结合就可以比较好的支持Gif图片的展示了，如下；
- (UIImage *)gifImageWithData:(NSData *)data{
CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);
size_t count = CGImageSourceGetCount(source);
if (count <= 1) {
return [[UIImage alloc] initWithData:data];
}
NSMutableArray *images = [NSMutableArray array];
NSTimeInterval duration = 0.0f;
for (size_t i = 0; i < count; i++) {
CGImageRef imageRef = CGImageSourceCreateImageAtIndex(source, i, NULL);
if (!imageRef) {
continue;
}
@autoreleasepool {
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);
}
size_t width = CGImageGetWidth(imageRef);
size_t height = CGImageGetHeight(imageRef);
size_t bytesPerRow = 4 * width;
CGContextRef context = CGBitmapContextCreate(NULL,width,height,8,bytesPerRow,colorspaceRef,kCGBitmapByteOrderDefault|kCGImageAlphaNoneSkipLast);
if (context != NULL) {
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGImageRef imageRefWithoutAlpha = CGBitmapContextCreateImage(context);
CGContextRelease(context);
CGImageRelease(imageRef);
imageRef = imageRefWithoutAlpha;
}
}
float frameDuration = 0.1f;
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];
}
}
CFRelease(cfFrameProperties);
duration += frameDuration;
[images addObject:[UIImage imageWithCGImage:imageRef scale:[UIScreen mainScreen].scale orientation:UIImageOrientationUp]];
CGImageRelease(imageRef);
}
if (!duration) {
duration = (1.0f / 10.0f) * count;
}
UIImage *animatedImage = [UIImage animatedImageWithImages:images duration:duration];
CFRelease(source);
return animatedImage;
}