iOS之category

category ：有翻译为分类，有翻译为类别，个人感觉这种翻译多多少少有些误导，所以我就不翻译了,直接喊英文

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category 个人觉得是oc语法中的一大亮点

不破坏老得类的结构去进行，扩充方法，扩充属性，也可以将庞大的类分门别类的存放

扩充方法比较常用也比较简答，这边主要讲一下扩充属性

还是用demo解释，效果图如下：

效果图.gif

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** 调用方式：（这么简单的调用是不是很爽？）**

#import "ViewController.h"
#import "UIViewController+Loading.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    //一句话调用
    [self showLoading];
    //在不需要loading的时候
    //[self hideLoading];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
}

@end

---

实现
这里主要用到了运行时的概念，首先我们创建UIViewController的category UIViewController+Loading

.h文件

#import <UIKit/UIKit.h>

@interface UIViewController (Loading)

- (void)showLoading;
- (void)hideLoading;

@end

.m文件

#import "UIViewController+Loading.h"
#import "UIImage+animatedGIF.h"
#import <objc/runtime.h>

const char *loadingViewKey = "loadingViewKey";

@interface UIViewController (Private)

@property (nonatomic, strong)UIImageView* loadingView;

@end

@implementation UIViewController (Loading)

- (UIImageView*)loadingView{
    UIImageView* view = objc_getAssociatedObject(self, &loadingViewKey);
    if (!view) {
        view = [[UIImageView alloc]initWithImage:[UIImage animatedImageWithAnimatedGIFData:[NSData dataWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"loading_img" ofType:@"gif"]]]];
        [view sizeToFit];
        view.center = self.view.center;
        self.loadingView = view;
    }
    return view;
}

- (void)setLoadingView:(UIImageView *)loadingView{
    objc_setAssociatedObject(self, &loadingViewKey, loadingView, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (void)showLoading{
    [self.view addSubview:self.loadingView];
}

- (void)hideLoading{
    [self.loadingView removeFromSuperview];
}

@end

** 讲解: **

1. import <objc/runtime.h>（这个正在学习中，目前停留在会用）
2. 在Loading这个category中新增loadingView属性，重写其get set方法，因为category是不允许新增属性的，但是我们知道.（点）语法无非也就是调用get set

3. objc_getAssociatedObject （请看下图官方文档）

   
   objc_getAssociatedObject.png

4. objc_setAssociatedObject

   
   objc_setAssociatedObject.png

object ： 参数作为待扩展的对象实例
key： 作为该对象实例的属性的键
value： 就是对象实例的属性的值
policy： 作为关联的策略

5. 这个key一定要保持唯一，所以我们用常量指针地址作为key

---

下面是加载gif UIImageView的 category （gitHub上的）

#import <UIKit/UIKit.h>

/**
        UIImage (animatedGIF)
        
    This category adds class methods to `UIImage` to create an animated `UIImage` from an animated GIF.
*/
@interface UIImage (animatedGIF)

/*
        UIImage *animation = [UIImage animatedImageWithAnimatedGIFData:theData];
    
    I interpret `theData` as a GIF.  I create an animated `UIImage` using the source images in the GIF.
    
    The GIF stores a separate duration for each frame, in units of centiseconds (hundredths of a second).  However, a `UIImage` only has a single, total `duration` property, which is a floating-point number.
    
    To handle this mismatch, I add each source image (from the GIF) to `animation` a varying number of times to match the ratios between the frame durations in the GIF.
    
    For example, suppose the GIF contains three frames.  Frame 0 has duration 3.  Frame 1 has duration 9.  Frame 2 has duration 15.  I divide each duration by the greatest common denominator of all the durations, which is 3, and add each frame the resulting number of times.  Thus `animation` will contain frame 0 3/3 = 1 time, then frame 1 9/3 = 3 times, then frame 2 15/3 = 5 times.  I set `animation.duration` to (3+9+15)/100 = 0.27 seconds.
*/
+ (UIImage *)animatedImageWithAnimatedGIFData:(NSData *)theData;

/*
        UIImage *image = [UIImage animatedImageWithAnimatedGIFURL:theURL];
    
    I interpret the contents of `theURL` as a GIF.  I create an animated `UIImage` using the source images in the GIF.
    
    I operate exactly like `+[UIImage animatedImageWithAnimatedGIFData:]`, except that I read the data from `theURL`.  If `theURL` is not a `file:` URL, you probably want to call me on a background thread or GCD queue to avoid blocking the main thread.
*/
+ (UIImage *)animatedImageWithAnimatedGIFURL:(NSURL *)theURL;

@end

#import "UIImage+animatedGIF.h"
#import <ImageIO/ImageIO.h>

#if __has_feature(objc_arc)
#define toCF (__bridge CFTypeRef)
#define fromCF (__bridge id)
#else
#define toCF (CFTypeRef)
#define fromCF (id)
#endif

@implementation UIImage (animatedGIF)

static int delayCentisecondsForImageAtIndex(CGImageSourceRef const source, size_t const i) {
    int delayCentiseconds = 1;
    CFDictionaryRef const properties = CGImageSourceCopyPropertiesAtIndex(source, i, NULL);
    if (properties) {
        CFDictionaryRef const gifProperties = CFDictionaryGetValue(properties, kCGImagePropertyGIFDictionary);
        if (gifProperties) {
            NSNumber *number = fromCF CFDictionaryGetValue(gifProperties, kCGImagePropertyGIFUnclampedDelayTime);
            if (number == NULL || [number doubleValue] == 0) {
                number = fromCF CFDictionaryGetValue(gifProperties, kCGImagePropertyGIFDelayTime);
            }
            if ([number doubleValue] > 0) {
                // Even though the GIF stores the delay as an integer number of centiseconds, ImageIO “helpfully” converts that to seconds for us.
                delayCentiseconds = (int)lrint([number doubleValue] * 100);
            }
        }
        CFRelease(properties);
    }
    return delayCentiseconds;
}

static void createImagesAndDelays(CGImageSourceRef source, size_t count, CGImageRef imagesOut[count], int delayCentisecondsOut[count]) {
    for (size_t i = 0; i < count; ++i) {
        imagesOut[i] = CGImageSourceCreateImageAtIndex(source, i, NULL);
        delayCentisecondsOut[i] = delayCentisecondsForImageAtIndex(source, i);
    }
}

static int sum(size_t const count, int const *const values) {
    int theSum = 0;
    for (size_t i = 0; i < count; ++i) {
        theSum += values[i];
    }
    return theSum;
}

static int pairGCD(int a, int b) {
    if (a < b)
        return pairGCD(b, a);
    while (true) {
        int const r = a % b;
        if (r == 0)
            return b;
        a = b;
        b = r;
    }
}

static int vectorGCD(size_t const count, int const *const values) {
    int gcd = values[0];
    for (size_t i = 1; i < count; ++i) {
        // Note that after I process the first few elements of the vector, `gcd` will probably be smaller than any remaining element.  By passing the smaller value as the second argument to `pairGCD`, I avoid making it swap the arguments.
        gcd = pairGCD(values[i], gcd);
    }
    return gcd;
}

static NSArray *frameArray(size_t const count, CGImageRef const images[count], int const delayCentiseconds[count], int const totalDurationCentiseconds) {
    int const gcd = vectorGCD(count, delayCentiseconds);
    size_t const frameCount = totalDurationCentiseconds / gcd;
    UIImage *frames[frameCount];
    for (size_t i = 0, f = 0; i < count; ++i) {
        UIImage *const frame = [UIImage imageWithCGImage:images[i]];
        for (size_t j = delayCentiseconds[i] / gcd; j > 0; --j) {
            frames[f++] = frame;
        }
    }
    return [NSArray arrayWithObjects:frames count:frameCount];
}

static void releaseImages(size_t const count, CGImageRef const images[count]) {
    for (size_t i = 0; i < count; ++i) {
        CGImageRelease(images[i]);
    }
}

static UIImage *animatedImageWithAnimatedGIFImageSource(CGImageSourceRef const source) {
    size_t const count = CGImageSourceGetCount(source);
    CGImageRef images[count];
    int delayCentiseconds[count]; // in centiseconds
    createImagesAndDelays(source, count, images, delayCentiseconds);
    int const totalDurationCentiseconds = sum(count, delayCentiseconds);
    NSArray *const frames = frameArray(count, images, delayCentiseconds, totalDurationCentiseconds);
    UIImage *const animation = [UIImage animatedImageWithImages:frames duration:(NSTimeInterval)totalDurationCentiseconds / 100.0];
    releaseImages(count, images);
    return animation;
}

static UIImage *animatedImageWithAnimatedGIFReleasingImageSource(CGImageSourceRef CF_RELEASES_ARGUMENT source) {
    if (source) {
        UIImage *const image = animatedImageWithAnimatedGIFImageSource(source);
        CFRelease(source);
        return image;
    } else {
        return nil;
    }
}

+ (UIImage *)animatedImageWithAnimatedGIFData:(NSData *)data {
    return animatedImageWithAnimatedGIFReleasingImageSource(CGImageSourceCreateWithData(toCF data, NULL));
}

+ (UIImage *)animatedImageWithAnimatedGIFURL:(NSURL *)url {
    return animatedImageWithAnimatedGIFReleasingImageSource(CGImageSourceCreateWithURL(toCF url, NULL));
}

@end