我试图滚动到UITableView的底部后，它完成执行[自我。tableView reloadData]。
我原本有
[self.tableView reloadData]
NSIndexPath* indexPath = [NSIndexPath indexPathForRow: ([self.tableView numberOfRowsInSection:([self.tableView numberOfSections]-1)]-1) inSection: ([self.tableView numberOfSections]-1)];
[self.tableView scrollToRowAtIndexPath:indexPath atScrollPosition:UITableViewScrollPositionBottom animated:YES];
但后来我读到reloadData是异步的，所以自self以来滚动就没有发生。表视图,自我。tableView numberOfSections和[self.]tableView numberOfRowsinSection都是0。
很奇怪，我用的是:
[self.tableView reloadData];
NSLog(@"Number of Sections %d", [self.tableView numberOfSections]);
NSLog(@"Number of Rows %d", [self.tableView numberOfRowsInSection:([self.tableView numberOfSections]-1)]-1);
在控制台中，它返回Sections = 1, Row = -1;
当我在cellForRowAtIndexPath做完全相同的nslog时，我得到Sections = 1和Row = 8;(8是正确的)
创建CATransaction的可重用扩展:
public extension CATransaction {
static func perform(method: () -> Void, completion: @escaping () -> Void) {
begin()
setCompletionBlock {
completion()
}
method()
commit()
}
}
现在创建一个UITableView的扩展，使用CATransaction的扩展方法:
public extension UITableView {
func reloadData(completion: @escaping (() -> Void)) {
CATransaction.perform(method: {
reloadData()
}, completion: completion)
}
}
用法:
tableView.reloadData(completion: {
//Do the stuff
})
和一个UICollectionView版本，基于kolaworld的答案。
它需要测试。到目前为止，它在iOS 9.2和Xcode 9.2 beta 2上工作，将collectionView滚动到一个索引，作为一个闭包。
extension UICollectionView
{
/// Calls reloadsData() on self, and ensures that the given closure is
/// called after reloadData() has been completed.
///
/// Discussion: reloadData() appears to be asynchronous. i.e. the
/// reloading actually happens during the next layout pass. So, doing
/// things like scrolling the collectionView immediately after a
/// call to reloadData() can cause trouble.
///
/// This method uses CATransaction to schedule the closure.
func reloadDataThenPerform(_ closure: @escaping (() -> Void))
{
CATransaction.begin()
CATransaction.setCompletionBlock(closure)
self.reloadData()
CATransaction.commit()
}
}
用法:
myCollectionView.reloadDataThenPerform {
myCollectionView.scrollToItem(at: indexPath,
at: .centeredVertically,
animated: true)
}
只是提供另一种方法，基于补全是发送到cellForRow的“最后可见”单元格的思想。
// Will be set when reload is called
var lastIndexPathToDisplay: IndexPath?
typealias ReloadCompletion = ()->Void
var reloadCompletion: ReloadCompletion?
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
// Setup cell
if indexPath == self.lastIndexPathToDisplay {
self.lastIndexPathToDisplay = nil
self.reloadCompletion?()
self.reloadCompletion = nil
}
// Return cell
...
func reloadData(completion: @escaping ReloadCompletion) {
self.reloadCompletion = completion
self.mainTable.reloadData()
self.lastIndexPathToDisplay = self.mainTable.indexPathsForVisibleRows?.last
}
一个可能的问题是:如果reloadData()在lastIndexPathToDisplay被设置之前已经完成，“最后可见”单元格将在lastIndexPathToDisplay被设置之前显示，补全将不会被调用(并且将处于“等待”状态):
self.mainTable.reloadData()
// cellForRowAt could be finished here, before setting `lastIndexPathToDisplay`
self.lastIndexPathToDisplay = self.mainTable.indexPathsForVisibleRows?.last
如果反转，可能会在reloadData()之前通过滚动触发补全。
self.lastIndexPathToDisplay = self.mainTable.indexPathsForVisibleRows?.last
// cellForRowAt could trigger the completion by scrolling here since we arm 'lastIndexPathToDisplay' before 'reloadData()'
self.mainTable.reloadData()
