我如何知道UICollectionView已被完全加載？

Question

每當UICollectionView被完全加載時，我必須做一些操作，即當時應調用所有UICollectionView的數據源/佈局方法。我怎麼知道？有沒有任何委託方法知道UICollectionView加載狀態？

Answer 1

// In viewDidLoad 
[self.collectionView addObserver:self forKeyPath:@"contentSize" options:NSKeyValueObservingOptionOld context:NULL]; 

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context 
{ 
    // You will get here when the reloadData finished 
} 

- (void)dealloc 
{ 
    [self.collectionView removeObserver:self forKeyPath:@"contentSize" context:NULL]; 
} 

Answer 2

你可以這樣做......

- (void)reloadMyCollectionView{ 

     [myCollectionView reload]; 
     [self performSelector:@selector(myStuff) withObject:nil afterDelay:0.0]; 

    } 

    - (void)myStuff{ 
    // Do your stuff here. This will method will get called once your collection view get loaded. 

    } 

Answer 3

試試這個：

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section 
{ 
    return _Items.count; 
} 

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath 
{ 
    UICollectionViewCell *cell; 
    //Some cell stuff here... 

    if(indexPath.row == _Items.count-1){ 
     //THIS IS THE LAST CELL, SO TABLE IS LOADED! DO STUFF! 
    } 

    return cell; 
} 

Answer 4

這其實相當簡單。

當例如調用UICollectionView的reloadData方法或它的佈局的的invalidateLayout方法，你做到以下幾點：

dispatch_async(dispatch_get_main_queue(), ^{ 
    [self.collectionView reloadData]; 
}); 

dispatch_async(dispatch_get_main_queue(), ^{ 
    //your stuff happens here 
    //after the reloadData/invalidateLayout finishes executing 
}); 

爲什麼這個工程：

主線程（這是我們應該做所有的UI更新）包含主隊列，這是串行的，即它以FIFO方式工作。所以在上面的例子中，第一個塊被調用，它調用了我們的reloadData方法，接着是第二個塊中的其他任何東西。

現在主線程也被阻塞了。所以如果你是reloadData需要3秒執行，那麼第二個塊的處理將被那些3秒推遲。

Answer 5

這爲我工作：

[self.collectionView reloadData]; 
[self.collectionView performBatchUpdates:^{} 
           completion:^(BOOL finished) { 
            /// collection-view finished reload 
           }]; 

Answer 6

這個工作對我來說：

__weak typeof(self) wself= self; 
[self.contentCollectionView performBatchUpdates:^{ 
    [wself.contentCollectionView reloadData]; 
} completion:^(BOOL finished) { 
    [wself pageViewCurrentIndexDidChanged:self.contentCollectionView]; 
}]; 

Answer 7

做這樣的：

 UIView.animateWithDuration(0.0, animations: { [weak self] in 
       guard let strongSelf = self else { return } 

       strongSelf.collectionView.reloadData() 

      }, completion: { [weak self] (finished) in 
       guard let strongSelf = self else { return } 

       // Do whatever is needed, reload is finished here 
       // e.g. scrollToItemAtIndexPath 
       let newIndexPath = NSIndexPath(forItem: 1, inSection: 0) 
       strongSelf.collectionView.scrollToItemAtIndexPath(newIndexPath, atScrollPosition: UICollectionViewScrollPosition.Left, animated: false) 
     }) 

Answer 8

這是我如何解決問題Swift 3.0：

override func viewDidAppear(_ animated: Bool) { 
    super.viewDidAppear(animated) 

    if !self.collectionView.visibleCells.isEmpty { 
     // stuff 
    } 
} 

Answer 9

我想補充一個偉大@dezinezync答案：

斯威夫特3+

collectionView.collectionViewLayout.invalidateLayout() // or reloadData() 
DispatchQueue.main.async { 
    // your stuff here executing after collectionView has been layouted 
} 

Answer 10

防守做到這一點：

//Subclass UICollectionView 
class MyCollectionView: UICollectionView { 

    //Store a completion block as a property 
    var completion: (() -> Void)? 

    //Make a custom funciton to reload data with a completion handle 
    func reloadData(completion: @escaping() -> Void) { 
     //Set the completion handle to the stored property 
     self.completion = completion 
     //Call super 
     super.reloadData() 
    } 

    //Override layoutSubviews 
    override func layoutSubviews() { 
     //Call super 
     super.layoutSubviews() 
     //Call the completion 
     self.completion?() 
     //Set the completion to nil so it is reset and doesn't keep gettign called 
     self.completion = nil 
    } 

} 

然後調用你這樣的VC

內

let collection = MyCollectionView() 

self.collection.reloadData(completion: { 

}) 

確保您使用的是子類！