[iOS]异步加载UIImageView----AsyImageView

能够异步加载图片的UIImageview，通过调用方法loadImageWithUrl:与loadImageWithUrl:andDefaultImage:来进行异步加载。用到了NSCache、文件缓存、NSOperation、NSQueue来完成。
首先是头文件的定义

定义缓存地址的宏

#define kCIVCache [NSHomeDirectory() stringByAppendingString:@"/Library/Caches/CIVCache.txt"]

定义加载Operation接口

@protocol AsyImageLoadOperation 
- (void)cancel;
@end

定义单例队列Manager

@interface QueueManager : NSObject
+(id)sharedManager;
@property (nonatomic,retain) NSOperationQueue *queue;
@end

定义缓存模型

@interface CIVImageCache : NSObject
@property (strong,nonatomic) NSURL *url;
@property (strong,nonatomic) NSData *imageData;
@end

定义图片加载Operation

@interface ImageLoadOperation : NSOperation
-(id)initWithUrl:(NSURL *)url;
@property (nonatomic,strong) NSURL *url;
@property (nonatomic,strong) CIVImageCache *resultCache;
@end

定义AsyImageView

@interface AsyImageView : UIImageView
-(void)loadImageWithUrl:(NSURL *)url;
-(void)loadImageWithUrl:(NSURL *)url andDefultImage:(UIImage *)image;
@end

接着就是实现头文件，.m文件中需要引入

#import "objc/runtime.h"
#include 

定义静态的operationKey字符变量

static char operationKey;

QueueManager的实现

@implementation QueueManager

unsigned int countOfCores() {
    unsigned int ncpu;
    size_t len = sizeof(ncpu);
    sysctlbyname("hw.ncpu", &ncpu, &len, NULL, 0);
    return ncpu;
}

static QueueManager *instance;
+(id)sharedManager{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance=[[QueueManager alloc] init];
    });
    return instance;
}

-(id)init{
    self=[super init];
    if (self) {
        _queue=[[NSOperationQueue alloc] init];
        _queue.maxConcurrentOperationCount=countOfCores();
    }
    return self;
}

countOfCores方法用来获取当前机器的CPU核数。在init初始化中初始化一个NSOperationQueue，并为这个队列指定最大并发操作数（最好是CPU有多少核就设置为多少的最大并发数）。单例的实现使用GCD的dispatch_once。

实现缓存模型对象

@implementation CIVImageCache 

-(id)init{
    self=[super init];
    if (self){
        _imageData=[[NSData alloc] init];
        _url=[[NSURL alloc] init];
    }
    return self;
}

-(id)initWithCoder:(NSCoder *)aDecoder{
    self=[super init];
    if (self){
        _imageData  =   [aDecoder decodeObjectForKey:@"_imageData"];
        _url        =   [aDecoder decodeObjectForKey:@"_url"];
    }
    return self;
}

-(void)encodeWithCoder:(NSCoder *)aCoder{
    [aCoder encodeObject:_imageData forKey:@"_imageData"];
    [aCoder encodeObject:_url forKey:@"_url"];
}

-(id)copyWithZone:(NSZone *)zone{
    CIVImageCache *uObject=[[[self class] allocWithZone:zone] init];
    uObject.imageData=self.imageData;
    uObject.url=self.url;
    return uObject;
}

@end

这个没有什么好说的，这个对象主要用来在加载完成后传递数据以及将数据保存在本地。 接下来实现图片加载操作（ImageLoadOperation）

@implementation ImageLoadOperation

-(id)initWithUrl:(NSURL *)url{
    self=[super init];
    if (self) {
        _url=url;
    }
    return self;
}

-(void)main{
    NSData *cacheData=[NSData dataWithContentsOfFile:kCIVCache];
    NSDictionary *cacheDic=[NSKeyedUnarchiver unarchiveObjectWithData:cacheData];
    if (cacheDic!=nil) {
        if ([[cacheDic allKeys] containsObject:_url.description]) {
            CIVImageCache *cache=[[CIVImageCache alloc] init];
            cache.url=_url;
            cache.imageData=[cacheDic objectForKey:_url.description];
            _resultCache=cache;
        }else{
            [self loadFromInternet];
        }
    }else{
        [self loadFromInternet];
    }
}

-(void)loadFromInternet{
    NSData *imageData=[NSData dataWithContentsOfURL:_url];
    UIImage *image=[UIImage imageWithData:imageData];
    imageData = UIImageJPEGRepresentation(image, 0.0000001);
    CIVImageCache *cache=[[CIVImageCache alloc] init];
    cache.url=_url;
    cache.imageData=imageData;
    _resultCache=cache;
}
@end

main函数中为主要的加载操作，首先从本地缓存中获取数据，判断是否已经存在URL的请求缓存，如果没有调用loadFromInternet方法从网络下载图片。

最后来实现异步

@interface AsyImageView ()
@property (nonatomic,weak) NSOperation *lastOperation;
@property (strong, nonatomic) NSCache *memCache;
@property (assign, nonatomic) dispatch_queue_t ioQueue;
@end

定义AsyImageView的“私有”变量，lastOperation用来关联operation的顺序（这个最后好像没有用到）memCache则用来进行缓存，ioQueue是用来缓存文件的操作队列。

AsyImageView使用两种初始化方法来初始化：

-(id)init{
    self=[super init];
    if (self) {
        _ioQueue = dispatch_queue_create("com.noez.AsyImageCache", DISPATCH_QUEUE_SERIAL);
        NSString *fullNamespace = [@"com.noez.AsyImageCache." stringByAppendingString:@"1"];
        _memCache = [[NSCache alloc] init];
        _memCache.name = fullNamespace;
    }
    return self;
}

-(id)initWithFrame:(CGRect)frame{
    self=[super initWithFrame:frame];
    if (self) {
        _ioQueue = dispatch_queue_create("com.noez.AsyImageCache", DISPATCH_QUEUE_SERIAL);
        NSString *fullNamespace = [@"com.noez.AsyImageCache." stringByAppendingString:@"1"];
        _memCache = [[NSCache alloc] init];
        _memCache.name = fullNamespace;
    }
    return self;
}

在AsyImageView中的layoutSubviews中也需要初始化ioQueue与memCache，如果是直接在XIB中直接设置AsyImageView的话并不会调用两个初始化方法。

loadImageWithUrl方法中开始异步的加载过程：

-(void)loadImageWithUrl:(NSURL *)url{
    [self cancelCurrentImageLoad];
    NSOperationQueue *queue=[[QueueManager sharedManager] queue];
    __weak AsyImageView *weakSelf=self;
    id operation=[self downloadWithURL:url queue:queue completed:^(UIImage *image, NSData *data, NSError *error, BOOL isFinished) {
        void (^block)(void) = ^{
            __strong AsyImageView *sself = weakSelf;
            if (!sself) return;
            if (image){
                sself.image = image;
                [sself setNeedsLayout];
            }
        };
        if ([NSThread isMainThread]){
            block();
        }
        else{
            dispatch_sync(dispatch_get_main_queue(), block);
        }
    }];
    objc_setAssociatedObject(self, &operationKey, operation, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

在这个方法中首先调用cancelCurrentImageLoad来取消当前的加载操作：

- (void)cancelCurrentImageLoad
{
    // Cancel in progress downloader from queue
    id operation = objc_getAssociatedObject(self, &operationKey);
    if (operation)
    {
        [operation cancel];
        objc_setAssociatedObject(self, &operationKey, nil, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
}

之后获取单例的操作队列后调用downloadWithURL方法开始异步加载，加载完之后通过block将图片赋值给image属性。

-(id)downloadWithURL:(NSURL *)url queue:(NSOperationQueue *)queue completed:(void (^)(UIImage *image, NSData *data, NSError *error, BOOL isFinished))completedBlock{
    ImageLoadOperation *op=[[ImageLoadOperation alloc] init];
    [self queryDiskCacheForKey:url.description done:^(UIImage *image) {
        if (image==nil) {
            op.url=url;
            __weak ImageLoadOperation *weakOp=op;
            op.completionBlock=^{
                CIVImageCache *cache=weakOp.resultCache;
                UIImage *dimage=[UIImage imageWithData:cache.imageData];
                completedBlock(dimage,nil,nil,YES);
                [self storeImage:dimage imageData:cache.imageData forKey:cache.url.description toDisk:YES];
            };
            [self.lastOperation addDependency:op];//待定
            self.lastOperation=op;
            [queue addOperation:op];
        }else{
            completedBlock(image,nil,nil,YES);
        }
    }];
    return op;
}

在加载前首先调用queryDiskCacheForKey方法从缓存中获取图片，如果缓存中没有图片，则使用图片加载操作加载图片，在操作完成时使用block保存图片并调用completedBlock显示图片。如果缓存中有图片则直接调用completedBlock显示图片。一下分别是保存图片与从缓存获取图片的方法:

- (void)storeImage:(UIImage *)image imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk{
    if (!image || !key){
        return;
    }

    [self.memCache setObject:image forKey:key cost:image.size.height * image.size.width * image.scale];

    if (toDisk){
        dispatch_async(self.ioQueue, ^{
                           NSData *data = imageData;

                           if (!data){
                               if (image){
                                   data = UIImageJPEGRepresentation(image, (CGFloat)1.0);
                               }
                           }
                           if (data){
                               NSData *cacheData=[NSData dataWithContentsOfFile:kCIVCache];
                               if (cacheData==nil) {
                                   cacheData=[[NSData alloc] init];
                               }
                               NSMutableDictionary *dic=[NSKeyedUnarchiver unarchiveObjectWithData:cacheData];
                               if (dic==nil) {
                                   dic=[[NSMutableDictionary alloc] init];
                               }
                               if (![[dic allKeys] containsObject:key]) {
                                   [dic setObject:data forKey:key];
                               }
                               NSData *data=[NSKeyedArchiver archivedDataWithRootObject:dic];
                               [data writeToFile:kCIVCache atomically:YES];
                           }
                       });
    }
}

- (void)queryDiskCacheForKey:(NSString *)key done:(void (^)(UIImage *image))doneBlock{
    if (!doneBlock) return;
    if (!key){
        doneBlock(nil);
        return;
    }

    UIImage *image = [self imageFromMemoryCacheForKey:key];
    if (image){
        doneBlock(image);
        return;
    }
    if (_ioQueue==nil) {
        _ioQueue = dispatch_queue_create("com.noez.AsyImageCache", DISPATCH_QUEUE_SERIAL);
    }
    dispatch_async(self.ioQueue, ^{
                       @autoreleasepool{
                           UIImage *diskImage = [self diskImageForKey:key];
                           if (diskImage){
                               CGFloat cost = diskImage.size.height * diskImage.size.width * diskImage.scale;
                               [self.memCache setObject:diskImage forKey:key cost:cost];
                           }

                           dispatch_async(dispatch_get_main_queue(), ^{
                                              doneBlock(diskImage);
                                          });
                       }
                   });
}