iphone - 如何实现具有 1000+ 个 subview 的 UIScrollView？

Question

我正在努力编写一个应用程序的一部分，它应该像 native iphone 照片应用程序一样运行。查看了 Orielly 的 iphone sdk app development book，它给出了一个示例代码来实现这个所谓的页面闪烁。那里的代码首先创建所有 subview ，然后隐藏/取消隐藏它们。在给定的时间只有 3 个 subview 是可见的，其余的是隐藏的。经过大量努力，我让它与当时只有大约 15 页的应用程序一起使用。

当我添加 300 页后，很明显这种预分配如此多 subview 的方法存在性能/内存问题。然后我想可能是为了我的情况我应该只分配 3 个 subview 而不是隐藏/取消隐藏它们。可能我应该在运行时删除/添加 subview 。但无法弄清楚 UIScrollView 是否可以动态更新内容。例如，开始时有 3 个帧位于屏幕的不同 x 偏移量(0、320、640)，如 UIScrollView 所理解的那样。一旦用户移动到第 3 页，我如何确保我能够添加第 4 页并删除第 1 页，而 UIScrollView 不会混淆？

希望有解决此类问题的标准方法...有人可以指导吗？

Answer 1

按照上述内容，您可以仅使用有限数量的资源来显示数千个元素(是的，这确实有点像享元模式)。这里有一些代码可以帮助你做你想做的事。

UntitledViewController 类仅包含一个 UIScroll 并将其自身设置为其委托(delegate)。我们有一个 NSArray，其中包含 NSString 实例作为数据模型(其中可能有数千个 NSString)，我们希望使用水平滚动在 UILabel 中显示每个实例。当用户滚动时，我们会移动 UILabel，将一个放在左边，另一个放在右边，以便为下一个滚动事件做好一切准备。

这是界面，相当简单:

@interface UntitledViewController : UIViewController <UIScrollViewDelegate>
{
@private
    UIScrollView *_scrollView;

    NSArray *_objects;

    UILabel *_detailLabel1;
    UILabel *_detailLabel2;
    UILabel *_detailLabel3;
}

@end

下面是该类的实现:

@interface UntitledViewController ()
- (void)replaceHiddenLabels;
- (void)displayLabelsAroundIndex:(NSInteger)index;
@end

@implementation UntitledViewController

- (void)dealloc 
{
    [_objects release];
    [_scrollView release];
    [_detailLabel1 release];
    [_detailLabel2 release];
    [_detailLabel3 release];
    [super dealloc];
}

- (void)viewDidLoad 
{
    [super viewDidLoad];

    _objects = [[NSArray alloc] initWithObjects:@"first", @"second", @"third", 
                @"fourth", @"fifth", @"sixth", @"seventh", @"eight", @"ninth", @"tenth", nil];

    _scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0.0, 0.0, 320.0, 460.0)];
    _scrollView.contentSize = CGSizeMake(320.0 * [_objects count], 460.0);
    _scrollView.showsVerticalScrollIndicator = NO;
    _scrollView.showsHorizontalScrollIndicator = YES;
    _scrollView.alwaysBounceHorizontal = YES;
    _scrollView.alwaysBounceVertical = NO;
    _scrollView.pagingEnabled = YES;
    _scrollView.delegate = self;

    _detailLabel1 = [[UILabel alloc] initWithFrame:CGRectMake(0.0, 0.0, 320.0, 460.0)];
    _detailLabel1.textAlignment = UITextAlignmentCenter;
    _detailLabel1.font = [UIFont boldSystemFontOfSize:30.0];
    _detailLabel2 = [[UILabel alloc] initWithFrame:CGRectMake(320.0, 0.0, 320.0, 460.0)];
    _detailLabel2.textAlignment = UITextAlignmentCenter;
    _detailLabel2.font = [UIFont boldSystemFontOfSize:30.0];
    _detailLabel3 = [[UILabel alloc] initWithFrame:CGRectMake(640.0, 0.0, 320.0, 460.0)];
    _detailLabel3.textAlignment = UITextAlignmentCenter;
    _detailLabel3.font = [UIFont boldSystemFontOfSize:30.0];

    // We are going to show all the contents of the _objects array
    // using only these three UILabel instances, making them jump 
    // right and left, replacing them as required:
    [_scrollView addSubview:_detailLabel1];
    [_scrollView addSubview:_detailLabel2];
    [_scrollView addSubview:_detailLabel3];

    [self.view addSubview:_scrollView];
}

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
    [_scrollView flashScrollIndicators];
}

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    [self displayLabelsAroundIndex:0];
}

- (void)didReceiveMemoryWarning 
{
    // Here you could release the data source, but make sure
    // you rebuild it in a lazy-loading way as soon as you need it again...
    [super didReceiveMemoryWarning];
}

#pragma mark -
#pragma mark UIScrollViewDelegate methods

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{
    // Do some initialization here, before the scroll view starts moving!
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    [self replaceHiddenLabels];
}

- (void)displayLabelsAroundIndex:(NSInteger)index
{
    NSInteger count = [_objects count];
    if (index >= 0 && index < count)
    {
        NSString *text = [_objects objectAtIndex:index];
        _detailLabel1.frame = CGRectMake(320.0 * index, 0.0, 320.0, 460.0);
        _detailLabel1.text = text;
        [_scrollView scrollRectToVisible:CGRectMake(320.0 * index, 0.0, 320.0, 460.0) animated:NO];

        if (index < (count - 1))
        {
            text = [_objects objectAtIndex:(index + 1)];
            _detailLabel2.frame = CGRectMake(320.0 * (index + 1), 0.0, 320.0, 460.0);
            _detailLabel2.text = text;
        }

        if (index > 0)
        {
            text = [_objects objectAtIndex:(index - 1)];
            _detailLabel3.frame = CGRectMake(320.0 * (index - 1), 0.0, 320.0, 460.0);
            _detailLabel3.text = text;
        }
    }
}

- (void)replaceHiddenLabels
{
    static const double pageWidth = 320.0;
    NSInteger currentIndex = ((_scrollView.contentOffset.x - pageWidth) / pageWidth) + 1;

    UILabel *currentLabel = nil;
    UILabel *previousLabel = nil;
    UILabel *nextLabel = nil;

    if (CGRectContainsPoint(_detailLabel1.frame, _scrollView.contentOffset))
    {
        currentLabel = _detailLabel1;
        previousLabel = _detailLabel2;
        nextLabel = _detailLabel3;
    }
    else if (CGRectContainsPoint(_detailLabel2.frame, _scrollView.contentOffset))
    {
        currentLabel = _detailLabel2;
        previousLabel = _detailLabel1;
        nextLabel = _detailLabel3;
    }
    else
    {
        currentLabel = _detailLabel3;
        previousLabel = _detailLabel1;
        nextLabel = _detailLabel2;
    }

    currentLabel.frame = CGRectMake(320.0 * currentIndex, 0.0, 320.0, 460.0);
    currentLabel.text = [_objects objectAtIndex:currentIndex];

    // Now move the other ones around
    // and set them ready for the next scroll
    if (currentIndex < [_objects count] - 1)
    {
        nextLabel.frame = CGRectMake(320.0 * (currentIndex + 1), 0.0, 320.0, 460.0);
        nextLabel.text = [_objects objectAtIndex:(currentIndex + 1)];
    }

    if (currentIndex >= 1)
    {
        previousLabel.frame = CGRectMake(320.0 * (currentIndex - 1), 0.0, 320.0, 460.0);
        previousLabel.text = [_objects objectAtIndex:(currentIndex - 1)];
    }
}

@end

希望这对您有所帮助!