[转]如何异步加载图片等数据

大致思路为两步：(1) 加载tableView时读取图片缓存，如果存在则显示，如果不存在则异步下载；(2) 下载图片信息成功，更新图片缓存，重新加载tableView的数据

 

 - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    /* 其他代码 */

 

    // 用户头像

    UIImage *userPic = [self cachedImageForUrl:[NSURL URLWithString:url]];

    cell.userPicView.image = userPic;

 

    /* 其他代码 */

}

 

- (UIImage *)cachedImageForUrl:(NSURL *)url {

    id cachedObject = [self.cachedImage objectForKey:url];

    if (cachedObject == nil) {

        //防止reloadData时重复提交请求，添加占位符

        [self.cachedImage setObject:@"Loading..." forKey:url];

        //异步下载图片

        ASIHTTPRequest *picRequest = [ASIHTTPRequest requestWithURL:url];

        picRequest.delegate = self;

        picRequest.didFinishSelector = @selector(didFinishRequestImage:);

        picRequest.didFailSelector   = @selector(didFailRequestImage:);

        [self.queue addOperation:picRequest];

        // 更新状态栏的网络标示

        [UIApplication sharedApplication].networkActivityIndicatorVisible = YES;

    }else if ( ![cachedObject isKindOfClass:[UIImage class]] ) {

        cachedObject = nil;

    }

    return cachedObject;

}

 

- (void)didFinishRequestImage:(ASIHTTPRequest *)request {

    NSData *imageData = [request responseData];

    UIImage *image = [UIImage imageWithData:imageData];

    if (image != nil) {

        [self.cachedImage setObject:image forKey:request.url];

        [self.tableView reloadData];

    }

    [UIApplication sharedApplication].networkActivityIndicatorVisible = NO;

}

 

- (void)didFailRequestImage:(ASIHTTPRequest *)request {

    NSLog(@"Error Download Image: %@", request.error);

    [UIApplication sharedApplication].networkActivityIndicatorVisible = NO;

}