ios - 延迟加载 UIScrollView - 如何使滚动更平滑

Question

我已经在 ScrollView 中完成了延迟加载，如下所示:

-(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];
            }

        }
    }
}

基本上，如果 View 距离屏幕上有+/- 2“页”，则加载 View 。这大大减少了我使用的内存量(而不是一次加载 20 个以上的 ItemView)，这很好。然而，所有的加载/卸载确实会使滚动有点断断续续，尤其是在速度较慢的设备上。以下是 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;
}

再说一遍，我该怎么做才能使加载/卸载更快，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转移到后台线程。尝试一下，看看是否更快。

请参见此处:loading images from disk in iPhone app is slow

编辑:图像加载滞后也可能是由于 UIImages 处理延迟造成的

请参见此处:Setting image property of UIImageView causes major lag