UICollectionView单元选择和单元重用

Question

选择细胞后，我想处理更改细胞外观。我想到了委托方法collectionView:didSelectItemAtIndexPath:＆amp; collectionView:didDeselectItemAtIndexPath:是我应该编辑单元格的地方。

-(void)collectionView:(UICollectionView *)collectionView 
       didSelectItemAtIndexPath:(NSIndexPath *)indexPath {

    DatasetCell *datasetCell = 
      (DatasetCell *)[collectionView cellForItemAtIndexPath:indexPath];

    [datasetCell replaceHeaderGradientWith:[UIColor skyBlueHeaderGradient]];
    datasetCell.backgroundColor = [UIColor skyBlueColor];
}

和

-(void)collectionView:(UICollectionView *)collectionView 
       didDeselectItemAtIndexPath:(NSIndexPath *)indexPath {

    DatasetCell *datasetCell = 
      (DatasetCell *)[collectionView cellForItemAtIndexPath:indexPath];

    [datasetCell replaceHeaderGradientWith:[UIColor grayGradient]];
    datasetCell.backgroundColor = [UIColor myDarkGrayColor];
}

这种方法很好，除非重复使用单元格。如果我在索引（0,0）处选择单元格，它会改变外观，但是当我向下滚动时，会有另一个单元格处于选定状态。

我相信我应该使用UICollectionViewCell方法-(void)prepareForReuse来准备细胞以便重复使用（例如，将细胞外观设置为非选择状态）但它给我带来了困难。

-(void)prepareForReuse {
    if ( self.selected ) {
        [self replaceHeaderGradientWith:[UIColor skyBlueHeaderGradient]];
        self.backgroundColor = [UIColor skyBlueColor];
    } else {
        [self replaceHeaderGradientWith:[UIColor grayGradient]];
        self.backgroundColor = [UIColor myDarkGrayColor];
    }
}

当我滚动回到顶部时，索引（0,0）处的单元格处于取消选择状态。

当我刚刚使用cell.backgroundView属性时，为了防止这种情况发生在：

-(void)prepareForReuse {
    self.selected = FALSE;
}

并且选择状态按预期工作。

有什么想法吗？

Answer 1

你的观察是正确的。由于重用单元格，这种情况正在发生。但是你不必对 prepareForReuse 做任何事情。而是检查 cellForItem 并相应地设置属性。有点像......

 - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"cvCell" forIndexPath:indexPath];


if (cell.selected) {
     cell.backgroundColor = [UIColor blueColor]; // highlight selection 
}
else
{
     cell.backgroundColor = [UIColor redColor]; // Default color
}
return cell;
}

-(void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath  {

    UICollectionViewCell *datasetCell =[collectionView cellForItemAtIndexPath:indexPath];
    datasetCell.backgroundColor = [UIColor blueColor]; // highlight selection
 }  

 -(void)collectionView:(UICollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath {

UICollectionViewCell *datasetCell =[collectionView cellForItemAtIndexPath:indexPath]; 
datasetCell.backgroundColor = [UIColor redColor]; // Default color
}

Answer 2

设置单元格backgroundView和selectedBackgroundView后，

框架将处理为您切换视图，请参阅Managing the Visual State for Selections and Highlights中的示例：

UIView* backgroundView = [[UIView alloc] initWithFrame:self.bounds];
backgroundView.backgroundColor = [UIColor redColor];
self.backgroundView = backgroundView;

UIView* selectedBGView = [[UIView alloc] initWithFrame:self.bounds];
selectedBGView.backgroundColor = [UIColor whiteColor];
self.selectedBackgroundView = selectedBGView;

您只需要在您的班级中实现UICollectionViewDelegate启用单元格，并按以下方式选择：

- (BOOL)collectionView:(UICollectionView *)collectionView
        shouldHighlightItemAtIndexPath:(NSIndexPath *)indexPath
{
    return YES;
}

- (BOOL)collectionView:(UICollectionView *)collectionView
        shouldSelectItemAtIndexPath:(NSIndexPath *)indexPath;
{
    return YES;
}

这对我有用。

Answer 3

UICollectionView在iOS 10中发生了变化，为上述解决方案带来了一些问题。

这是一个很好的指南： https://littlebitesofcocoa.com/241-uicollectionview-cell-pre-fetching

离开屏幕后，细胞现在可以保持一段时间。这意味着有时我们可能无法控制didDeselectItemAt indexPath中的单元格来调整它。然后它可以在屏幕上显示未更新和未回收。 prepareForReuse无法帮助解决这个问题。

最简单的解决方案是通过将isPrefetchingEnabled设置为false来禁用新滚动。有了这个，管理单元格的显示     cellForItemAt didSelect didDeselect按原样运作。

但是，如果您要保持新的平滑滚动行为，最好使用willDisplay：

func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
    let customCell = cell as! CustomCell
    if customCell.isSelected {
        customCell.select()
    } else {
        customCell.unselect()
    }
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "CustomCell", for: indexPath) as! CustomCell
    //Don't even need to set selection-specific things here as recycled cells will also go through willDisplay
    return cell
}

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    let cell = collectionView.cellForItem(at: indexPath) as? CustomCell
    cell?.select()
}

func collectionView(_ collectionView: UICollectionView, didDeselectItemAt indexPath: IndexPath) {
    let cell = collectionView.cellForItem(at: indexPath) as? CustomCell
    cell?.unselect() // <----- this can be null here, and the cell can still come back on screen!
}

通过上述方法，您可以在选择单元格时对其进行控制，在屏幕上取消选择，回收并重新显示。

Answer 4

Anil走在正确的轨道上（他的解决方案看起来应该可行，我开发了独立于他的解决方案）。我仍然使用prepareForReuse:方法将单元格的selected设置为FALSE，然后在cellForItemAtIndexPath中检查单元格的索引是否在`collectionView.indexPathsForSelectedItems'中，如果是这样，请突出显示。

在自定义单元格中：

-(void)prepareForReuse {
    self.selected = FALSE;
}

在cellForItemAtIndexPath:中处理突出显示和取消高亮显示重用单元格：

if ([collectionView.indexPathsForSelectedItems containsObject:indexPath]) {
    [collectionView selectItemAtIndexPath:indexPath animated:FALSE scrollPosition:UICollectionViewScrollPositionNone];
    // Select Cell
}
else {
    // Set cell to non-highlight
}

然后在didDeselectItemAtIndexPath:和didSelectItemAtIndexPath:

中处理单元格突出显示和取消高亮显示

这对我来说就像是一种魅力。

Answer 5

我有一个水平滚动集合视图（我在Tableview中使用集合视图），我也遇到了单元重用的问题，每当我选择一个项目并向右滚动时，下一个可见集合中的其他单元格会自动选择。尝试使用任何自定义单元格属性（如“已选择”，突出显示等）解决此问题并没有帮助我，所以我提出了以下解决方案，这对我有用。

步骤1：

在collectionView中创建一个变量来存储所选索引，这里我使用了一个名为selectedIndex的类级变量

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath

{

    MyCVCell *cell = (MyCVCell*)[collectionView dequeueReusableCellWithReuseIdentifier:@"MyCVCell" forIndexPath:indexPath];    

// When scrolling happens, set the selection status only if the index matches the selected Index

if (selectedIndex == indexPath.row) {

        cell.layer.borderWidth = 1.0;

        cell.layer.borderColor = [[UIColor redColor] CGColor];

    }
    else
    {
        // Turn off the selection
        cell.layer.borderWidth = 0.0;

    }
    return cell;

}
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath

{
    MyCVCell *cell = (MyCVCell *)[collectionView cellForItemAtIndexPath:indexPath];
    // Set the index once user taps on a cell
    selectedIndex = indexPath.row;
    // Set the selection here so that selection of cell is shown to ur user immediately
    cell.layer.borderWidth = 1.0;
    cell.layer.borderColor = [[UIColor redColor] CGColor];
    [cell setNeedsDisplay];
}

- (void)collectionView:(UICollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath

{

    MyCVCell *cell = (MyCVCell *)[collectionView cellForItemAtIndexPath:indexPath];

    // Set the index to an invalid value so that the cells get deselected
    selectedIndex = -1;
    cell.layer.borderWidth = 0.0;
    [cell setNeedsDisplay];

}

-anoop

Answer 6

在自定义单元格中创建公共方法：

- (void)showSelection:(BOOL)selection
{
    self.contentView.backgroundColor = selection ? [UIColor blueColor] : [UIColor white];
}

还要编写-prepareForReuse单元格方法的重新授权：

- (void)prepareForReuse
{
    [self showSelection:NO];
    [super prepareForReuse];
}

在ViewController中你应该有_selectedIndexPath变量，该变量在-didSelectItemAtIndexPath中定义并在-didDeselectItemAtIndexPath

中无效

NSIndexPath *_selectedIndexPath;

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *cellIdentifier = @"Cell";
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];

    if (_selectedIndexPath) {
        [cell showSelection:[indexPath isEqual:_selectedIndexPath]];
    }
}

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewCell *cell = [collectionView cellForItemAtIndexPath:indexPath];
    [cell showSelection:![indexPath isEqual:_selectedIndexPath]];// on/off selection
    _selectedIndexPath = [indexPath isEqual:_selectedIndexPath] ? nil : indexPath;
}

- (void)collectionView:(UICollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewCell *cell = [collectionView cellForItemAtIndexPath:indexPath];
    [cell showSelection:NO];
    _selectedIndexPath = nil;
}

Answer 7

只有@ stefanB解决方案适用于iOS 9.3

这里我需要更改 for Swift 2

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {

        //prepare your cell here..

        //Add background view for normal cell
        let backgroundView: UIView = UIView(frame: cell!.bounds)
        backgroundView.backgroundColor = UIColor.lightGrayColor()
        cell!.backgroundView = backgroundView

        //Add background view for selected cell
        let selectedBGView: UIView = UIView(frame: cell!.bounds)
        selectedBGView.backgroundColor = UIColor.redColor()
        cell!.selectedBackgroundView = selectedBGView

        return cell!
 }

 func collectionView(collectionView: UICollectionView, shouldHighlightItemAtIndexPath indexPath: NSIndexPath) -> Bool {
        return true
 }

 func collectionView(collectionView: UICollectionView, shouldSelectItemAtIndexPath indexPath: NSIndexPath) -> Bool {
        return true
 }

Answer 8

我要解决的问题是在自定义单元格中进行更改。您在其类中有一个名为 DataSetCell 的自定义单元格，您可以执行以下操作（代码在swift中）

override var isSelected: Bool {
    didSet {
        if isSelected {
            changeStuff
        } else {
            changeOtherStuff
        }
    }
}

这样做的是，每次从可重用队列中选择，取消选择，初始化或调用单元格时，该代码将运行并进行更改。希望这会对你有所帮助。

Answer 9

您遇到的问题来自缺乏对super.prepareForReuse()的呼叫。

上面的其他一些解决方案建议从委托人的功能更新单元的UI，这导致了一个有缺陷的设计，该单元的行为逻辑超出了其类。此外，还可以通过调用super.prepareForReuse()来简单地修复这些额外的代码。例如：

class myCell: UICollectionViewCell {

    // defined in interface builder
    @IBOutlet weak var viewSelection : UIView!

    override var isSelected: Bool {
        didSet {
            self.viewSelection.alpha = isSelected ? 1 : 0
        }
    }

    override func prepareForReuse() {
        // Do whatever you want here, but don't forget this :
        super.prepareForReuse()
        // You don't need to do `self.viewSelection.alpha = 0` here 
        // because `super.prepareForReuse()` will update the property `isSelected`

    }


    override func awakeFromNib() {
        super.awakeFromNib()
        // Initialization code
        self.viewSelection.alpha = 0
    }

}

采用这种设计，您甚至可以将委托人的函数collectionView:didSelectItemAt: / collectionView:didDeselectItemAt:保留为空，选择过程将被完全处理，并随着单元格的回收而正常运行。

Answer 10

您可以将单元格的selectedBackgroundView设置为backgroundColor = x。

现在，只要您点击单元格，他所选择的模式就会自动更改，并且会变为背景颜色以更改为x。

Answer 11

感谢您的回答@ RDC。

以下代码适用于 Swift 3

// MARK: - UICollectionViewDataSource protocol
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

    //prepare your cell here..
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath as IndexPath) as! MyCell
    cell.myLabel.text =  "my text"

    //Add background view for normal cell
    let backgroundView: UIView = UIView(frame: cell.bounds)
    backgroundView.backgroundColor = UIColor.lightGray
    cell.backgroundView = backgroundView

    //Add background view for selected cell
    let selectedBGView: UIView = UIView(frame: cell.bounds)
    selectedBGView.backgroundColor = UIColor.green
    cell.selectedBackgroundView = selectedBGView

    return cell
}

// MARK: - UICollectionViewDelegate protocol
func collectionView(_ collectionView: UICollectionView, shouldHighlightItemAt indexPath: IndexPath) -> Bool {
    return true
}

func collectionView(_ collectionView: UICollectionView, shouldSelectItemAt indexPath: IndexPath) -> Bool {
    return true
}

Answer 12

更改单元格属性（如单元格的背景颜色）不应该在UICollectionViewController本身上完成，它应该在CollectionViewCell类中完成。不要使用didSelect和didDeselect，只需使用它：

class MyCollectionViewCell: UICollectionViewCell 
{
     override var isSelected: Bool
     {
         didSet
         {
            // Your code
         }
     } 
}