NSThread 延迟加载图片列表

从Documents目录下读取图片列表时，有加载延迟，界面有一个空白期。我想办法默认显示空图片占位（显示一个默认灰边图片），然后使用NSThread加载真正的图片

UIImageView 添加在UIScrollView中 _myScrollView，3步走:

for(int i=0;i<10;i++){
　　UIImageView *iv = [[UIImageViewalloc] initWithFrame:CGRectMake(0,i*110,320,300)];
　　iv.image = [UIImageimageNamed:@"defautlImage"];
　　[_myScrollView addSubview:iv];
　　//生成线程并启用它
　　[NSThreaddetachNewThreadSelector:@selector(loadPictures:) toTarget:selfwithObject:nil];
}

//加载图片
-(void)loadPictures:(id)sender{
    int len = [_myScrollView.subviews count];
    int i=0;
    for (;i<len;i++) {
        UIView *subview = [_myScrollView.subviews objectAtIndex:i];
        if ([subview isKindOfClass:[UIImageView class]]) {
            [self performSelectorOnMainThread:@selector(show:) withObject:[NSNumber numberWithInt:i] waitUntilDone:YES];
        }
    }
}

//更新图片
-(void)show:(NSNumber*)index{
    UIImageView *iv = (UIImageView*)[_myScrollView.subviews objectAtIndex:[index intValue]];
    UIImage *image = [UIImage imageWithContentsOfFile:------];//------是图片路径
    iv.image = image;
    iv.frame = ------;//-----重新绘制UIImageView的frame
    iv.userInteractionEnabled = YES;
    //单击imageview
    UITapGestureRecognizer *singleTouch = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(singleTouch:)];
    [iv addGestureRecognizer:singleTouch];
}