延迟加载 UIScrollView - 如何使滚动更流畅

Question

我已经在这样的滚动视图中完成了延迟加载：

-(void)scrollViewDidScroll:(UIScrollView *)myScrollView {

    int currentPage = (1 + myScrollView.contentOffset.x / kXItemSpacingIphone);
    for (ItemView* itemView in [self.itemRow subviews]){
        if (itemView.tag >= currentPage-2 && itemView.tag <= currentPage+2)
        {
            //keep it visible
            if (!itemView.isLoaded) {
                [itemView layoutWithData:[self.items objectAtIndex:itemView.tag-1]];
            }
        }
        else
        {
            //hide it
            if (itemView.isLoaded) {
                [itemView unloadData];
            }

        }
    }
}

如果视图距离屏幕显示 +/- 2 个“页面”，则基本上会加载该视图。这大大减少了我正在使用的内存量（而不是一次加载 20 多个 ItemView），这很好。但是，所有加载/卸载确实使滚动有点不稳定，尤其是在速度较慢的设备上。这是 ItemView 加载时实际发生的情况：

- (void)layoutWithData:(Item*)_data {
    self.data = _data;

//grab the image from the bundle
    UIImage *img;
    NSString *filePath = [[NSBundle mainBundle] pathForResource:_data.image ofType:@"jpg"];
        if(filePath.length > 0 && filePath != (id)[NSNull null]) {
            img = [UIImage imageWithContentsOfFile:filePath];
        }

    UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
    [btn setImage:img forState:UIControlStateNormal];
    [btn addTarget:self action:@selector(tapDetected:) forControlEvents:UIControlEventTouchUpInside];
    btn.frame = CGRectMake(0, 0, kItemPosterWidthIphone, kItemPosterHeightIphone);
    [self addSubview:btn];

    self.isLoaded = YES;

}

以及ItemView的卸载：

- (void)unloadData{
    for(UIView *subview in [self subviews]) {
        [subview removeFromSuperview];
    }
    self.data = nil;
    self.isLoaded = NO;
}

再次，我可以做些什么来加快加载/卸载速度，从而使 UIScrollView 更流畅？

---

尝试异步：

- (void)layoutWithData:(Item*)_data {
    self.data = _data;
    self.isLoaded = YES;

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        UIImage *img;
        NSString *filePath = [[NSBundle mainBundle] pathForResource:_data.image ofType:@"jpg"];
            if(filePath.length > 0 && filePath != (id)[NSNull null]) {
                img = [UIImage imageWithContentsOfFile:filePath];
            }

        dispatch_async(dispatch_get_main_queue(), ^{
            UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
            [btn setImage:img forState:UIControlStateNormal];
            [btn addTarget:self action:@selector(tapDetected:) forControlEvents:UIControlEventTouchUpInside];
            btn.frame = CGRectMake(0, 0, kItemPosterWidthIphone, kItemPosterHeightIphone);
            self.imageView = btn;
            [self addSubview:btn];


        });

    });
}

Answer 1

不久前我遇到了这个问题，我将加载图像从磁盘移动到后台线程。试试看是否更快。

请看这里：loading images from disk in iPhone app is slow

编辑：图像加载滞后也可能是由于 UIImages 的延迟处理造成的

请看这里：Setting image property of UIImageView causes major lag