从零开始打造一个iOS图片加载框架（二）

淩亂°似流年 2022-10-01 07:50 262阅读 0赞

*  [JImage][]
 *  [从零开始打造一个iOS图片加载框架（一）][iOS]
 *  [从零开始打造一个iOS图片加载框架（三）][iOS 1]

### 一、前言 ###

> 上一章节主要讲解了图片的简单加载、内存/磁盘缓存等内容，但目前该框架还不支持GIF图片的加载。而GIF图片在我们日常开发中是非常常见的。因此，本章节将着手实现对GIF图片的加载。

### 二、加载GIF图片 ###

#### 1. 加载本地GIF图片 ####

> `UIImageView`本身是支持对GIF图片的加载的，将GIF图片加入到`animationImages`属性中，并通过`startAnimating`和`stopAnimating`来启动/停止动画。

UIImageView* animatedImageView = [[UIImageView alloc] initWithFrame:self.view.bounds];
    animatedImageView.animationImages = [NSArray arrayWithObjects:    
                                   [UIImage imageNamed:@"image1.gif"],
                                   [UIImage imageNamed:@"image2.gif"],
                                   [UIImage imageNamed:@"image3.gif"],
                                   [UIImage imageNamed:@"image4.gif"], nil];
    animatedImageView.animationDuration = 1.0f;
    animatedImageView.animationRepeatCount = 0;
    [animatedImageView startAnimating];
    [self.view addSubview: animatedImageView];
    复制代码

#### 2.加载网络GIF图片 ####

> 与本地加载的不同之处在于我们通过网络获取到的是`NSData`类型，如果只是简单地通过`initImageWithData:`方法转化为image，那么往往只能获取到GIF中的第一张图片。我们知道GIF图片其实就是由于多张图片组合而成。因此，我们这里最重要是如何从`NSData`中解析转化为`images`。

*  `JImageCoder`:我们定义一个类转化用于图像的解析

@interface JImageCoder : NSObject
    + (instancetype)shareCoder;
    - (UIImage *)decodeImageWithData:(NSData *)data;
    @end
    复制代码

我们知道通过网络请求下载之后返回的是`NSData`数据

NSURLSessionDataTask *dataTask = [self.session dataTaskWithURL:URL completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        //do something: data->image
     }];
    复制代码

对于PNG和JPEG格式我们可以直接使用`initImageWithData`方法来转化为`image`，但对于GIF图片，我们则需要特殊处理。那么处理之前，我们就需要根据`NSData`来判断图片对应的格式。

*  根据`NSData`数据判断图片格式：这里参考了`SDWebImage`的实现，根据数据的第一个字节来判断。

- (JImageFormat)imageFormatWithData:(NSData *)data {
        if (!data) {
            return JImageFormatUndefined;
        }
        uint8_t c;
        [data getBytes:&c length:1];
        switch (c) {
            case 0xFF:
                return JImageFormatJPEG;
            case 0x89:
                return JImageFormatPNG;
            case 0x47:
                return JImageFormatGIF;
            default:
                return JImageFormatUndefined;
        }
    }
    复制代码

获取到图片的格式之后，我们就可以根据不同的格式来分别进行处理

- (UIImage *)decodeImageWithData:(NSData *)data {
        JImageFormat format = [self imageFormatWithData:data];
        switch (format) {
            case JImageFormatJPEG:
            case JImageFormatPNG:{
                UIImage *image = [[UIImage alloc] initWithData:data];
                image.imageFormat = format;
                return image;
            }
            case JImageFormatGIF:
                return [self decodeGIFWithData:data];
            default:
                return nil;
        }
    }
    复制代码

针对GIF图片中的每张图片的获取，我们可以使用`ImageIO`中的相关方法来提取。要注意的是对于一些对象，使用完之后要及时释放，否则会造成内存泄漏。

- (UIImage *)decodeGIFWithData:(NSData *)data {
        CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);
        if (!source) {
            return nil;
        }
        size_t count = CGImageSourceGetCount(source);
        UIImage *animatedImage;
        if (count <= 1) {
            animatedImage = [[UIImage alloc] initWithData:data];
            animatedImage.imageFormat = JImageFormatGIF;
        } else {
            NSMutableArray<UIImage *> *imageArray = [NSMutableArray array];
            for (size_t i = 0; i < count; i ++) {
                CGImageRef imageRef = CGImageSourceCreateImageAtIndex(source, i, NULL);
                if (!imageRef) {
                    continue;
                }
                UIImage *image = [[UIImage alloc] initWithCGImage:imageRef];
                [imageArray addObject:image];
                CGImageRelease(imageRef);
            }
            animatedImage = [[UIImage alloc] init];
            animatedImage.imageFormat = JImageFormatGIF;
            animatedImage.images = [imageArray copy];
        }
        CFRelease(source);
        return animatedImage;
    }
    复制代码

为了使得`UIImage`对象可以存储图片的格式和GIF中的`images`，这里实现了一个`UIImage`的分类

typedef NS_ENUM(NSInteger, JImageFormat) {
        JImageFormatUndefined = -1,
        JImageFormatJPEG = 0,
        JImageFormatPNG = 1,
        JImageFormatGIF = 2
    };
    @interface UIImage (JImageFormat)
    @property (nonatomic, assign) JImageFormat imageFormat;
    @property (nonatomic, copy) NSArray *images;
    @end
    
    @implementation UIImage (JImageFormat)
    - (void)setImages:(NSArray *)images {
        objc_setAssociatedObject(self, @selector(images), images, OBJC_ASSOCIATION_COPY_NONATOMIC);
    }
    - (NSArray *)images {
        NSArray *images = objc_getAssociatedObject(self, @selector(images));
        if ([images isKindOfClass:[NSArray class]]) {
            return images;
        }
        return nil;
    }
    - (void)setImageFormat:(JImageFormat)imageFormat {
        objc_setAssociatedObject(self, @selector(imageFormat), @(imageFormat), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    - (JImageFormat)imageFormat {
        JImageFormat imageFormat = JImageFormatUndefined;
        NSNumber *value = objc_getAssociatedObject(self, @selector(imageFormat));
        if ([value isKindOfClass:[NSNumber class]]) {
            imageFormat = value.integerValue;
            return imageFormat;
        }
        return imageFormat;
    }
    @end
    复制代码

使用`JImageCoder`将`NSData`类型的数据解析为`images`之后，便可以像本地加载GIF一样使用了。

static NSString *gifUrl = @"https://user-gold-cdn.xitu.io/2019/3/27/169bce612ee4dc21";
    - (void)downloadImage {
        __weak typeof(self) weakSelf = self;
        [[JImageDownloader shareInstance] fetchImageWithURL:gifUrl completion:^(UIImage * _Nullable image, NSError * _Nullable error) {
            __strong typeof (weakSelf) strongSelf = weakSelf;
            if (strongSelf && image) {
                if (image.imageFormat == JImageFormatGIF) {
                    strongSelf.imageView.animationImages = image.images;
                    [strongSelf.imageView startAnimating];
                } else {
                    strongSelf.imageView.image = image;
                }
            }
        }];
    }
    复制代码

#### 3.实现效果 ####

和`YYAnimatedImage`和`FLAnimatedImage`分别进行了对比，会发现自定义框架加载的GIF播放会更快些。我们回到`UIImageView`的GIF本地加载中，会发现遗漏了两个重要的属性：

@property (nonatomic) NSTimeInterval animationDuration;         // for one cycle of images. default is number of images * 1/30th of a second (i.e. 30 fps)
    @property (nonatomic) NSInteger      animationRepeatCount;      // 0 means infinite (default is 0)
    复制代码

`animatedDuration`定义了动画的周期，由于我们没有给它设置GIF的周期，所以这里使用的默认周期。接下来我们将回到GIF图片的解析过程中，增加这两个相关属性。

#### 4.GIF的`animationDuration`和`animationRepeatCount`属性 ####

*  `animationRepeatCount`：动画执行的次数

CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);
    ....
    NSInteger loopCount = 0;
    CFDictionaryRef properties = CGImageSourceCopyProperties(source, NULL);
    if (properties) {
        CFDictionaryRef gif = CFDictionaryGetValue(properties, kCGImagePropertyGIFDictionary);
        if (gif) {
            CFTypeRef loop = CFDictionaryGetValue(gif, kCGImagePropertyGIFLoopCount);
            if (loop) {
                CFNumberGetValue(loop, kCFNumberNSIntegerType, &loopCount);
            }
        }
        CFRelease(properties); //注意使用完需要释放
    }
    复制代码

*  `animationDuration`：动画执行周期

> 我们分别获取到GIF中每张图片对应的delayTime（显示时间），最后求它们的和，便可以作为GIF动画的一个完整周期。而图片的delayTime可以通过`ImageSource`中的`kCGImagePropertyGIFUnclampedDelayTime`或`kCGImagePropertyGIFDelayTime`属性获取。

NSTimeInterval duration = 0;
    for (size_t i = 0; i < count; i ++) {
        CGImageRef imageRef = CGImageSourceCreateImageAtIndex(source, i, NULL);
        if (!imageRef) {
            continue;
        }
        ....
        float delayTime = kJAnimatedImageDefaultDelayTimeInterval;
        CFDictionaryRef properties = CGImageSourceCopyPropertiesAtIndex(source, i, NULL);
        if (properties) {
            CFDictionaryRef gif = CFDictionaryGetValue(properties, kCGImagePropertyGIFDictionary);
            if (gif) {
                CFTypeRef value = CFDictionaryGetValue(gif, kCGImagePropertyGIFUnclampedDelayTime);
                if (!value) {
                    value = CFDictionaryGetValue(gif, kCGImagePropertyGIFDelayTime);
                }
                if (value) {
                    CFNumberGetValue(value, kCFNumberFloatType, &delayTime);
                }
            }
            CFRelease(properties);
        }
        duration += delayTime;
    }
    复制代码

获取之后加入到UIImage属性中:

animatedImage = [[UIImage alloc] init];
    animatedImage.imageFormat = JImageFormatGIF;
    animatedImage.images = [imageArray copy];
    animatedImage.loopCount = loopCount;
    animatedImage.totalTimes = duration;
    
    - (void)downloadImage {
        __weak typeof(self) weakSelf = self;
        [[JImageDownloader shareInstance] fetchImageWithURL:gifUrl completion:^(UIImage * _Nullable image, NSError * _Nullable error) {
            __strong typeof (weakSelf) strongSelf = weakSelf;
            if (strongSelf && image) {
                if (image.imageFormat == JImageFormatGIF) {
                    strongSelf.imageView.animationImages = image.images;
                    strongSelf.imageView.animationDuration = image.totalTimes;
                    strongSelf.imageView.animationRepeatCount = image.loopCount;
                    [strongSelf.imageView startAnimating];
                } else {
                    strongSelf.imageView.image = image;
                }
            }
        }];
    }
    复制代码

实现效果如下：

发现通过设置动画周期和次数之后，动画加载的更快了！！！为了解决这个问题，重新阅读了`YYAnimatedImage`和`FLAnimatedImage`的源码，发现它们在获取GIF图片的delayTime时，都会有一个小小的细节。

FLAnimatedImage.m
    const NSTimeInterval kFLAnimatedImageDelayTimeIntervalMinimum = 0.02;
    const NSTimeInterval kDelayTimeIntervalDefault = 0.1;
    // Support frame delays as low as `kFLAnimatedImageDelayTimeIntervalMinimum`, with anything below being rounded up to `kDelayTimeIntervalDefault` for legacy compatibility.
    // To support the minimum even when rounding errors occur, use an epsilon when comparing. We downcast to float because that's what we get for delayTime from ImageIO.
    if ([delayTime floatValue] < ((float)kFLAnimatedImageDelayTimeIntervalMinimum - FLT_EPSILON)) {
        FLLog(FLLogLevelInfo, @"Rounding frame %zu's `delayTime` from %f up to default %f (minimum supported: %f).", i, [delayTime floatValue], kDelayTimeIntervalDefault, kFLAnimatedImageDelayTimeIntervalMinimum);
        delayTime = @(kDelayTimeIntervalDefault);
    }
    
    UIImage+YYWebImage.m
    static NSTimeInterval _yy_CGImageSourceGetGIFFrameDelayAtIndex(CGImageSourceRef source, size_t index) {
        NSTimeInterval delay = 0;
        CFDictionaryRef dic = CGImageSourceCopyPropertiesAtIndex(source, index, NULL);
        if (dic) {
            CFDictionaryRef dicGIF = CFDictionaryGetValue(dic, kCGImagePropertyGIFDictionary);
            if (dicGIF) {
                NSNumber *num = CFDictionaryGetValue(dicGIF, kCGImagePropertyGIFUnclampedDelayTime);
                if (num.doubleValue <= __FLT_EPSILON__) {
                    num = CFDictionaryGetValue(dicGIF, kCGImagePropertyGIFDelayTime);
                }
                delay = num.doubleValue;
            }
            CFRelease(dic);
        }
        // http://nullsleep.tumblr.com/post/16524517190/animated-gif-minimum-frame-delay-browser-compatibility
        if (delay < 0.02) delay = 0.1;
        return delay;
    }
    复制代码

如上所示，`YYAnimatedImage`和`FLAnimatedImage`对于delayTime小于0.02的情况下，都会设置为默认值0.1。这么处理的主要目的是为了更好兼容更低级的设备，具体可以查看[这里][Link 1]。

static const NSTimeInterval kJAnimatedImageDelayTimeIntervalMinimum = 0.02;
    static const NSTimeInterval kJAnimatedImageDefaultDelayTimeInterval = 0.1;
    float delayTime = kJAnimatedImageDefaultDelayTimeInterval;
    CFDictionaryRef properties = CGImageSourceCopyPropertiesAtIndex(source, i, NULL);
    if (properties) {
        CFDictionaryRef gif = CFDictionaryGetValue(properties, kCGImagePropertyGIFDictionary);
        if (gif) {
            CFTypeRef value = CFDictionaryGetValue(gif, kCGImagePropertyGIFUnclampedDelayTime);
            if (!value) {
                value = CFDictionaryGetValue(gif, kCGImagePropertyGIFDelayTime);
            }
            if (value) {
                CFNumberGetValue(value, kCFNumberFloatType, &delayTime);
                if (delayTime < ((float)kJAnimatedImageDelayTimeIntervalMinimum - FLT_EPSILON)) {
                    delayTime = kJAnimatedImageDefaultDelayTimeInterval;
                }
            }
        }
        CFRelease(properties);
    }
    duration += delayTime;
    复制代码

为了让动画效果更接近`YYAnimatedImage`和`FLAnimatedImage`，我们同样在获取delayTime时增加条件判断。具体效果如下：

### 三、总结 ###

> 本小节主要实现了图片框架对GIF图片的加载功能。重点主要集中在通过`ImageIO`中的相关方法来获取到GIF中的每张图片，以及图片对应的周期和执行次数等。在最后结尾处也提及到了在获取图片delayTime时的一个小细节。通过这个细节也可以体现出自己动手打造框架的好处，因为如果只是简单地去阅读相关源码，往往很容易忽略很多细节。

#### 参考资料 ####

*  [GIF动画加载框架-FLAnimatedImage解析][GIF_-FLAnimatedImage]

转载于:https://juejin.im/post/5c9b3e3d518825303c705dd2

[JImage]: https://link.juejin.im?target=https%3A%2F%2Fgithub.com%2Fjiangleligejiang%2FJImage
[iOS]: https://juejin.im/post/5c9a0d2d518825573578d5d2
[iOS 1]: https://juejin.im/post/5ca5eb7be51d456fb44cf33a
[Link 1]: https://link.juejin.im?target=https%3A%2F%2Fgithub.com%2FFlipboard%2FFLAnimatedImage%2Fissues%2F98
[GIF_-FLAnimatedImage]: https://link.juejin.im?target=https%3A%2F%2Fwww.jianshu.com%2Fp%2F10644979f01c