我想滚动到 viewWillAppear
中 UICollectionView
的某个项目
- (void)viewWillAppear:(BOOL)animated {
[super viewWillAppear:animated];
[collectionView_ scrollToItemAtIndexPath:[NSIndexPath indexPathForRow:selectedIndex_ inSection:0]
atScrollPosition:UICollectionViewScrollPositionLeft
animated:NO];
}
在 iOS 6 上,此代码使应用程序崩溃并返回
*** Assertion failure in -[UICollectionViewData layoutAttributesForItemAtIndexPath:], /SourceCache/UIKit_Sim/UIKit-2372/UICollectionViewData.m:485
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'must return a UICollectionViewLayoutAttributes instance from -layoutAttributesForItemAtIndexPath: for path <NSIndexPath 0x13894e70> 2 indexes [0, 2]'
在 iOS7 上它不会崩溃,只是什么都不做。
滚动到正确的项目仅在 viewDidAppear
中有效,但我想在屏幕上显示正确项目中的集合。我试图在 viewDidLayoutSubviews
中滚动它,但它也崩溃了。将调用包装在 try-catch
中可以避免崩溃,但它仍然无法正常工作。
这有什么意义呢?是否无法在出现时显示正确的项目?
非常感谢。
编辑 1
我在 viewWillAppear
和 viewDidLayoutSubviews
上打印了这个(selectedIndex_
是 2,集合有 10 个项目):
UICollectionViewLayoutAttributes *test = [collectionView_ layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForRow:selectedIndex_ inSection:0]];
两处结果都是这样
<UICollectionViewLayoutAttributes: 0x11b9ff20> index path: (<NSIndexPath: 0x11b9c450> {length = 2, path = 0 - 2}); frame = (0 0; 0 0);
编辑 2
这是我打印集合的 contentSize
的痕迹
2013-12-09 08:56:59.300 - didLoad {0, 0}
2013-12-09 08:56:59.315 - willAppear {0, 0}
2013-12-09 08:56:59.350 - viewDidLayoutSubviews {0, 0}
2013-12-09 08:56:59.781 - viewDidLayoutSubviews {3200, 223}
2013-12-09 08:56:59.879 - didAppear {3200, 223}
2013-12-09 08:56:59.882 - viewDidLayoutSubviews {3200, 223}
Collection View 是在 viewDidLoad
中以编程方式创建的
UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
[layout setScrollDirection:UICollectionViewScrollDirectionHorizontal];
collectionView_ = [[UICollectionView alloc] initWithFrame:CGRectZero collectionViewLayout:layout];
[collectionView_ setTranslatesAutoresizingMaskIntoConstraints:NO];
[collectionView_ setDelegate:self];
[collectionView_ setDataSource:self];
[collectionView_ setShowsHorizontalScrollIndicator:NO];
[collectionView_ setPagingEnabled:YES];
[collectionView_ setBackgroundColor:[UIColor whiteColor]];
[collectionView_ registerClass:[MyCollectionViewCell class] forCellWithReuseIdentifier:[MyCollectionViewCell collectionCellIdentifier]];
[scrollView_ addSubview:collectionView_];
scrollView_
是通过 XIB 创建的(XIB 中唯一的控件。我需要另一个滚动条以将其他控件放在水平集合下方)。此方法的约束设置在updateViewConstraints
- (void)updateViewConstraints {
[super updateViewConstraints];
NSDictionary *views = [self viewsDictionary];
NSDictionary *metrics = @{ @"bigMargin" : @12, @"collectionViewHeight" : @(collectionViewHeight_) };
NSMutableString *verticalConstraints = [NSMutableString stringWithString:@"V:|[collectionView_(==collectionViewHeight)]"];
[scrollView_ addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[collectionView_(==scrollView_)]|"
options:0
metrics:nil
views:views]];
if (extendedInformationView_) {
[scrollView_ addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[extendedInformationView_(==scrollView_)]|"
options:0
metrics:nil
views:views]];
[verticalConstraints appendFormat:@"-bigMargin-[extendedInformationView_]"];
}
if (actionListView_) {
[scrollView_ addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[actionListView_(==scrollView_)]|"
options:0
metrics:nil
views:views]];
[verticalConstraints appendFormat:@"-bigMargin-[actionListView_]"];
}
[verticalConstraints appendString:@"-bigMargin-|"];
[scrollView_ addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:verticalConstraints
options:0
metrics:metrics
views:views]];
}
MyCollectionViewCell
在其 initWithFrame
方法中创建其所有控件,这里是返回单元格的方法。
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
MyCollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:[MyCollectionViewCell collectionCellIdentifier]
forIndexPath:indexPath];
// Data filling
return cell;
}
最佳答案
我遇到了同样的问题,可以解决。首先,当您创建 UICollectionView 时,您必须指定一个框架及其宽度,无论高度如何,但宽度对于滚动到正确的项目非常重要。
UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
[layout setScrollDirection:UICollectionViewScrollDirectionHorizontal];
collectionView_ = [[UICollectionView alloc] initWithFrame:CGRectMake(0.0f, 0.0f, CGRectGetWidth([scrollView_ frame]), 0.0f)
collectionViewLayout:layout];
[collectionView_ setDelegate:self];
[collectionView_ setDataSource:self];
[collectionView_ setBackgroundColor:[UIColor clearColor]];
[collectionView_ setTranslatesAutoresizingMaskIntoConstraints:NO];
[collectionView_ setShowsHorizontalScrollIndicator:NO];
[collectionView_ setPagingEnabled:YES];
[scrollView_ addSubview:collectionView_];
创建 UICollectionView 后,您必须告诉 View 需要更新其约束,因为在 iOS6 中您必须强制执行它,因此调用 updateViewConstraints:
[self updateViewConstraints]
重写方法updateViewConstraints,并在这里设置所有 View 约束。请记住在调用 super 之前删除 View 的所有约束(在您的代码中您没有删除它们),并在指标字典上设置 UICollectionView 的 width 并且不要使用 [ collectionView_(==scrollView_)] 因为有时会失败,主要是在 iOS6 中。
- (void)updateViewConstraints {
[scrollView_ removeConstraints:[scrollView_ constraints]];
[super updateViewConstraints];
NSDictionary *views = [self viewsDictionary];
NSDictionary *metrics = @{ @"bigMargin" : @12, @"collectionViewHeight" : @(collectionViewHeight_), @"viewWidth" : @(CGRectGetWidth([scrollView_ frame]) };
NSMutableString *verticalConstraints = [NSMutableString stringWithString:@"V:|[collectionView_(==collectionViewHeight)]"];
[scrollView_ addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[collectionView_(==viewWidth)]|"
options:0
metrics:nil
views:views]];
if (extendedInformationView_) {
[scrollView_ addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[extendedInformationView_(==scrollView_)]|"
options:0
metrics:nil
views:views]];
[verticalConstraints appendFormat:@"-bigMargin-[extendedInformationView_]"];
}
if (actionListView_) {
[scrollView_ addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[actionListView_(==scrollView_)]|"
options:0
metrics:nil
views:views]];
[verticalConstraints appendFormat:@"-bigMargin-[actionListView_]"];
}
[verticalConstraints appendString:@"-bigMargin-|"];
[scrollView_ addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:verticalConstraints
options:0
metrics:metrics
views:views]];
}
最后,要将 UICollectionView 滚动到正确的项目,请在 viewWillLayoutSubviews 上执行此操作,并且不要忘记检查 UICollectionView 的大小是否不为零以避免应用程序崩溃:
- (void)viewWillLayoutSubviews {
[super viewWillLayoutSubviews];
if (!CGSizeEqualToSize([collectionView_ frame].size, CGSizeZero)) {
[collectionView_ scrollToItemAtIndexPath:_selectedRowItem_ inSection:0]
atScrollPosition:UICollectionViewScrollPositionLeft
animated:NO];
}
}
就是这样。希望对您有所帮助!
关于ios - 滚动到 Collection View 中的项目会使应用程序崩溃,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/20355713/