2017-01-03 90 views
0

檢查this video正確理解我的問題。UICollectionView垂直流不返回中心的第一個單元格

我有一個自定義的視圖組成,其中包含一個UICollectionView,用戶可以使用兩個按鈕向上/向下滾動。這些值應該在變化時居中,一切都很好,直到我返回到比中心低得多的第一個值。請注意,對於水平滾動視圖使用相同方法我沒有任何問題。

我對從X軸居中複製的值進行居中的方法只是改變了Y軸的相關部分。

創建一個UICollectionViewFlowLayout派生類並將其設置爲UICollectionView的UICollectioViewFlowLayout。請注意,以編程方式滾動時,不會調用override方法。

[Register("CenterVerticalCellCollectionViewFlowLayout")] 
public class CenterVerticalCellCollectionViewFlowLayout : UICollectionViewFlowLayout 
{ 
    public CenterVerticalCellCollectionViewFlowLayout(IntPtr ptr) : base(ptr) 
    { 

    } 

    public override CGPoint TargetContentOffset(CGPoint proposedContentOffset, 
               CGPoint scrollingVelocity) 
    { 
     var cv = this.CollectionView; 
     var cvBounds = cv.Bounds; 
     var halfHeight = cvBounds.Height * 0.5; 
     var proposedContentOffsetCenterY = proposedContentOffset.Y + halfHeight; 
     var attributesForVisibleCells = LayoutAttributesForElementsInRect(cvBounds); 
     UICollectionViewLayoutAttributes candidateAttributes = null; 

     foreach (var attributes in attributesForVisibleCells) 
     { 
      // == Skip comparison with non-cell items (headers and footers) == // 
      if (attributes.RepresentedElementCategory != UICollectionElementCategory.Cell) 
      { 
       continue; 
      } 

      if ((attributes.Center.Y == 0) || 
       (attributes.Center.Y > (cv.ContentOffset.Y + halfHeight) && 
       scrollingVelocity.Y < 0)) 
      { 
       continue; 
      } 

      // == First time in the loop == // 
      if (candidateAttributes == null) 
      { 
       candidateAttributes = new UICollectionViewLayoutAttributes(); 
      } 
      else 
      { 
       candidateAttributes = attributes; 
       continue; 
      } 

      var a = attributes.Center.Y- proposedContentOffsetCenterY; 
      var b = candidateAttributes.Center.Y - proposedContentOffsetCenterY; 


      if (Math.Abs(a) < Math.Abs(b)) 
      { 
       candidateAttributes = attributes; 
      } 
     } 

     // Beautification step , I don't know why it works! 
     if (proposedContentOffset.Y == -(cv.ContentInset.Top)) 
     { 
      return proposedContentOffset; 
     } 

     if (candidateAttributes == null) 
     { 
      candidateAttributes = new UICollectionViewLayoutAttributes(); 
     } 

     return new CGPoint(Math.Floor(candidateAttributes.Center.Y - 
      halfHeight), proposedContentOffset.Y); 
    } 
} 

當加載自定義UIView我設置下面。

CollectionView.ContentOffset = CGPoint.Empty; 

UIViewController.ViewWillLayoutSubviews我打電話給下面的方法。

CollectionView.CollectionViewLayout.InvalidateLayout(); 

UIViewController.ViewDidLayoutSubviews我打電話給下面。

var insets = CollectionView.ContentInset; 
var value = (View.Frame.Height - (CollectionView.CollectionViewLayout as UICollectionViewFlowLayout).ItemSize.Height) * 0.5; 

insets.Top = (nfloat)value; 
insets.Bottom = (nfloat)value; 
CollectionView.ContentInset = insets; 
CollectionView.DecelerationRate = UIScrollView.DecelerationRateFast; 

最後下面的代碼以編程方式滾動上/下。

partial void UpButtonTouchUpInside(UIButton sender) 
{ 
    MoveSelectionCollectionViewCell(); 
} 

partial void DownButtonTouchUpInside(UIButton sender) 
{ 
    MoveSelectionCollectionViewCell(false); 
} 

void MoveSelectionCollectionViewCell(bool moveUp = true) 
{ 
    var firstVisibleIndexPath = CollectionView.IndexPathsForVisibleItems.FirstOrDefault(); 

    if (firstVisibleIndexPath != null) 
    { 
     var row = moveUp ? firstVisibleIndexPath.Row + 1 : firstVisibleIndexPath.Row - 1; 
     if (row > -1 && 
      row < _source.Collection.Count) 
     { 
      var scrollAtIndexPath = NSIndexPath.FromRowSection(row, 0); 
      CollectionView.ScrollToItem(
       scrollAtIndexPath, 
       UICollectionViewScrollPosition.CenteredVertically, 
       true); 
     } 
    } 
} 

查看下面的UICollectionView IB屬性。

IB Properties 1 IB Properties 2

回答

1

在審查我的問題讓我意識到這個問題!

當在UIViewController.ViewDidLayoutSubviews調用代碼時,它會考慮到View.Frame.Height這是錯誤的,因爲自定義控件View與UICollectionView的尺寸不同,因此證明了此問題。

更改此:

var value = (View.Frame.Height - (CollectionView.CollectionViewLayout as UICollectionViewFlowLayout).ItemSize.Height) * 0.5; 

要這樣:

var value = (CollectionView.Frame.Height - (CollectionView.CollectionViewLayout as UICollectionViewFlowLayout).ItemSize.Height) * 0.5; 

的伎倆。

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