SQWebImage

仿SDWebImage(探究原理)
###一、代码结构分析图

###二、首先自定义NSOperation操作，我的下载操作是在main方法中进行处理的，而SDWebImage是在start方法中进行处理，自定义回调block块

@interface WebImageOperation ()
@property (copy, nonatomic) NSString *url;
@property (copy, nonatomic) void(^finishedBlock)(UIImage *img);
@end

@implementation WebImageOperation

+ (instancetype)webImageOperationWithURL:(NSString *)url finishedBlock:(void(^)(UIImage *img)) finishedBlock {
   
    WebImageOperation *webOp = [[self alloc] init];
    webOp.url = url;
    webOp.finishedBlock = finishedBlock;
   
    return webOp;
}

//执行的时候会调用main方法
- (void)main {
   
    if (self.isCancelled) return;
    NSAssert(self.finishedBlock != nil, @"没有传递block值");
   
    [NSThread sleepForTimeInterval:1.5];
    UIImage *img = [UIImage imageNamed:self.url];
    if (self.isCancelled) return;
   
    //将数据保存至沙盒
    NSData *imgData = UIImageJPEGRepresentation(img, 0.7);
    [imgData writeToFile:self.url.appendDocumentPath atomically:YES];
   
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        self.finishedBlock(img);
    }];
   
}

- (void)start {

    [super start];
}

###三、创建下载管理器(单例)

@interface WebImageManager ()

@property (strong, nonatomic) NSOperationQueue *downloadQueue;
@property (strong, nonatomic) NSMutableDictionary *opCache;//操作缓存
@property (strong, nonatomic) NSMutableDictionary *imgCache;//图片缓存

@end

@implementation WebImageManager

- (NSMutableDictionary *)imgCache {
    if (_imgCache == nil) {
        _imgCache = [[NSMutableDictionary alloc] init];
    }
    return _imgCache;
}

- (NSMutableDictionary *)opCache {
    if (_opCache == nil) {
        _opCache = [[NSMutableDictionary alloc] init];
    }
    return _opCache;
}

- (NSOperationQueue *)downloadQueue {
    if (_downloadQueue == nil) {
        _downloadQueue = [[NSOperationQueue alloc] init];
    }
    return _downloadQueue;
}


+ (instancetype)shareWebImage {
   
    static id obj;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        if (obj == nil) {
            obj = [[self alloc] init];
        }
    });
   
    return obj;
}

- (void)webImageOperationWithURL:(NSString *)url finishedBlock:(void(^)(UIImage *img)) finishedBlock {
   
    //操作重复判断
    if (self.opCache[url] != nil) {
        NSLog(@"重复操作，请等待。。。");
        return;
    }
   
    //图片缓存
    if ([self checkChche:url]) {
        finishedBlock(self.imgCache[url]);
        return;
    }
   
    //下载操作(下载结束从缓冲池中移除)
    WebImageOperation *downloadOp = [WebImageOperation webImageOperationWithURL:url finishedBlock:^(UIImage *img) {
        finishedBlock(img);
        [self.opCache removeObjectForKey:url];
    }];
   
    //执行下载
    [self.opCache setObject:downloadOp forKey:url];
    [self.downloadQueue addOperation:downloadOp];
   
   
}

//判断缓存是否存在
- (BOOL)checkChche:(NSString *)url {
   
    //图片缓存
    if (self.imgCache[url] != nil) {
        NSLog(@"从缓存中加载");
        return YES;
    }
   
    //沙盒缓存
    NSData *imgData = [NSData dataWithContentsOfFile:url.appendDocumentPath];
    if (imgData != nil) {
        NSLog(@"从沙盒中加载");
        [self.imgCache setObject:[UIImage imageWithData:imgData] forKey:url];
        return YES;
    }
   
    return NO;
}

- (void)cancelWebImageOpWithURL:(NSString *)url {
   
    WebImageOperation *downloadOp = self.opCache[url];
    if (downloadOp == nil) return;
   
    [downloadOp cancel];
    [self.opCache removeObjectForKey:url];
}


@end

###四、创建UIImageView的子类WebImageView封装下载操作

@interface WebImageView ()
@property (copy, nonatomic) NSString *currentURL;
@end

@implementation WebImageView

- (void)setWebImage:(NSString *)url {

    //一个还没下载完，就开始下载另外一个
    if (![url isEqualToString:self.currentURL]) {
        [[WebImageManager shareWebImage] cancelWebImageOpWithURL:self.currentURL];
    }
    self.currentURL = url;
   
    [[WebImageManager shareWebImage] webImageOperationWithURL:url finishedBlock:^(UIImage *img) {
        self.image = img;
    }];
}

@end

###五、在UITableViewController中通过WebImageView中的setWebImage提供的方法即可设置下载的图片

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
   
    AppInfo *app = self.appList[indexPath.row];
   
    AppCell *cell = [tableView dequeueReusableCellWithIdentifier:@"AppCell" forIndexPath:indexPath];
   
    cell.nameLabel.text = app.name;
    **cell.downloadLabel.text = app.download;//调用此方法即可。**
    [cell.iconView setWebImage:app.icon];
   
    return cell;
}