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 <NSObject>
- (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<AsyImageLoadOperation>
-(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 <sys/sysctl.h>
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<AsyImageLoadOperation> 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<AsyImageLoadOperation> 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<AsyImageLoadOperation>)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);
});
}
});
}
