在和別人聊天是我們會想發一些特別搞笑的Gif表情，一個圖片可以代表一切。以前一直以為想Gif作為一種常用格式，各大平台應該都會支持，直到前幾天公司項目中要用到一個Gif動畫作為過渡效果，我查了半天幫助文檔，發現IOS中居然沒有現成的支持加載和播放Gif的類。

參考網址如下：

      三種方式顯示GIF圖片

　　andrei512 / GIFLoader.h

這兩種方法都是采用ImageIO庫，對Gif文件進行解析，然後自己做動畫顯示，至此顯示Gif動畫的思路就已經比較明確了。

我們知道Gif是由一陣陣畫面組成的，而且每一幀畫面播放的時常可能會不相等，觀察上面兩個例子，發現他們都沒有對Gif中每一幀的顯示時常做處理，這樣的結果就是整個Gif中每一幀畫面都是以固定的速度向前播放，很顯然這並不總會符合需求。

於是自己寫一個解析Gif的工具類，解決每一幀畫面並遵循每一幀所對應的顯示時間進行播放。

程序的思路如下：

1、首先使用ImageIO庫中的CGImageSource家在Gif文件。

2、通過CGImageSource獲取到Gif文件中的總的幀數，以及每一幀的顯示時間。

3、通過CAKeyframeAnimation來完成Gif動畫的播放。

下面直接上我寫的解析和播放Gif的工具類的代碼：

//
//  SvGifView.h
//  SvGifSample
//
//  Created by maple on 3/28/15.
//  Copyright (c) 2015 smileEvday. All rights reserved.
//


#import <UIKit/UIKit.h>

@interface SvGifView : UIView


/*
 * @brief desingated initializer
 */
- (id)initWithCenter:(CGPoint)center fileURL:(NSURL*)fileURL;

/*
 * @brief start Gif Animation
 */
- (void)startGif;

/*
 * @brief stop Gif Animation
 */
- (void)stopGif;

/*
 * @brief get frames image(CGImageRef) in Gif
 */
+ (NSArray*)framesInGif:(NSURL*)fileURL;


@end

//
//  SvGifView.m
//  SvGifSample
//
//  Created by maple on 3/28/15.
//  Copyright (c) 2015 smileEvday. All rights reserved.
//

#import "SvGifView.h"
#import <ImageIO/ImageIO.h>
#import <QuartzCore/CoreAnimation.h>

/*
 * @brief resolving gif information
 */
void getFrameInfo(CFURLRef url, NSMutableArray *frames, NSMutableArray *delayTimes, CGFloat *totalTime,CGFloat *gifWidth, CGFloat *gifHeight)
{
    CGImageSourceRef gifSource = CGImageSourceCreateWithURL(url, NULL);
    
    // get frame count
    size_t frameCount = CGImageSourceGetCount(gifSource);
    for (size_t i = 0; i < frameCount; ++i) {
        // get each frame
        CGImageRef frame = CGImageSourceCreateImageAtIndex(gifSource, i, NULL);
        [frames addObject:(id)frame];
        CGImageRelease(frame);
        
        // get gif info with each frame
        NSDictionary *dict = (NSDictionary*)CGImageSourceCopyPropertiesAtIndex(gifSource, i, NULL);
        NSLog(@"kCGImagePropertyGIFDictionary %@", [dict valueForKey:(NSString*)kCGImagePropertyGIFDictionary]);
        
        // get gif size
        if (gifWidth != NULL && gifHeight != NULL) {
            *gifWidth = [[dict valueForKey:(NSString*)kCGImagePropertyPixelWidth] floatValue];
            *gifHeight = [[dict valueForKey:(NSString*)kCGImagePropertyPixelHeight] floatValue];
        }
        
        // kCGImagePropertyGIFDictionary中kCGImagePropertyGIFDelayTime，kCGImagePropertyGIFUnclampedDelayTime值是一樣的
        NSDictionary *gifDict = [dict valueForKey:(NSString*)kCGImagePropertyGIFDictionary];
        [delayTimes addObject:[gifDict valueForKey:(NSString*)kCGImagePropertyGIFDelayTime]];
        
        if (totalTime) {
            *totalTime = *totalTime + [[gifDict valueForKey:(NSString*)kCGImagePropertyGIFDelayTime] floatValue];
        }
    }
}

@interface SvGifView() {
    NSMutableArray *_frames;
    NSMutableArray *_frameDelayTimes;
    
    CGFloat _totalTime;         // seconds
    CGFloat _width;
    CGFloat _height;
}

@end

@implementation SvGifView


- (id)initWithCenter:(CGPoint)center fileURL:(NSURL*)fileURL;
{
    self = [super initWithFrame:CGRectZero];
    if (self) {
        
        _frames = [[NSMutableArray alloc] init];
        _frameDelayTimes = [[NSMutableArray alloc] init];
        
        _width = 0;
        _height = 0;
        
        if (fileURL) {
            getFrameInfo((CFURLRef)fileURL, _frames, _frameDelayTimes, &_totalTime, &_width, &_height);
        }
        
        self.frame = CGRectMake(0, 0, _width, _height);
        self.center = center;
    }
    
    return self;
}

+ (NSArray*)framesInGif:(NSURL *)fileURL
{
    NSMutableArray *frames = [NSMutableArray arrayWithCapacity:3];
    NSMutableArray *delays = [NSMutableArray arrayWithCapacity:3];
    
    getFrameInfo((CFURLRef)fileURL, frames, delays, NULL, NULL, NULL);
    
    return frames;
}

- (void)startGif
{
    CAKeyframeAnimation *animation = [CAKeyframeAnimation animationWithKeyPath:@"contents"];
    
    NSMutableArray *times = [NSMutableArray arrayWithCapacity:3];
    CGFloat currentTime = 0;
    int count = _frameDelayTimes.count;
    for (int i = 0; i < count; ++i) {
        [times addObject:[NSNumber numberWithFloat:(currentTime / _totalTime)]];
        currentTime += [[_frameDelayTimes objectAtIndex:i] floatValue];
    }
    [animation setKeyTimes:times];
    
    NSMutableArray *images = [NSMutableArray arrayWithCapacity:3];
    for (int i = 0; i < count; ++i) {
        [images addObject:[_frames objectAtIndex:i]];
    }
    
    [animation setValues:images];
    [animation setTimingFunction:[CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionLinear]];
    animation.duration = _totalTime;
    animation.delegate = self;
    animation.repeatCount = 5;
    
    [self.layer addAnimation:animation forKey:@"gifAnimation"];
}

- (void)stopGif
{
    [self.layer removeAllAnimations];
}

// remove contents when animation end
- (void)animationDidStop:(CAAnimation *)anim finished:(BOOL)flag
{
    self.layer.contents = nil;
}

// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect
{
    // Drawing code
}


@end

代碼很短也比較容易，就不一一解釋了。最開始的那個C函數主要就是用來解析Gif的，之所以用C函數是因為我要返回多個信息，而Objective-c只能返回一個參數，而且Objective-c和C語言可以很方便的混合編程。

測試程序Github地址：https://github.com/smileEvday/SvGifView/