UICollectionView - didDeselectItemAtIndexPath not

问题:

The first thing I do is to set the cell selected.

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
    cell.selected = YES;
    return cell;
}

And the cell is successfully selected. If the user touches the selected cell, than should the cell be deselected and the delegates be called. But this never happen.

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

- (void)collectionView:(UICollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath {
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

I know that the delegates are not called if I set the selection programmatically. The delegate and datasource are set.

However, this delegate gets called:

- (BOOL)collectionView:(UICollectionView *)collectionView shouldHighlightItemAtIndexPath:(NSIndexPath *)indexPath {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    return YES;
}

If I remove the cell.selected = YES than everything is working. Is there any one who can me explain this behaviour?

回答1:

The issue is, that the cell is selected, but the UICollectionView does not know anything about it. An extra call for the UICollectionView solves the problem:

[collectionView selectItemAtIndexPath:indexPath animated:NO scrollPosition:UICollectionViewScrollPositionNone]; 

Full code:

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
    cell.selected = YES;
    [collectionView selectItemAtIndexPath:indexPath animated:NO scrollPosition:UICollectionViewScrollPositionNone];
    return cell;
}

I keep the question to help someone who may face the same problem.

回答2:

I think the solution @zeiteisen provided is a way around solution. The actual thing lies in the selection mode. There is nothing to be set the in property cell.selected.

There are two properties of the UICollectionView name allowsMultipleSelection and allowsSelection.

allowsMultipleSelection is NO by default and allowsSelection is YES.

If you want to select only one cell then the code @the initialization look like

yourCollectionView.allowsMultipleSelection = NO;
yourCollectionView.allowsSelection = YES; //this is set by default

Then the settings will allow you select only one cell at a time, not less not more. You must have to select at least one cell. The didDeselectItemAtIndexPath wont be called unless you select another cell. Tapping an already selected UICollectionViewCell won't make it to Deselect. This is the policy behind the implementation.

If you want to select multiple cell then the code @the initialization should look like the following:

yourCollectionView.allowsMultipleSelection = YES;
yourCollectionView.allowsSelection = YES; //this is set by default

Then the settings will allow to select multiple cell. This settings allow none or multiple UICollectionViewCell to be selected. Number of cell selected will be

0 <= number_selected_cell <= total_number_of_cell

This is the policy behind the multiple cell selection.

If you are intended to use any of the above you are welcome to use apples api without extra headache, otherwise you got to find a way around to sneak into your goal using your own code or apple's api.

回答3:

I had the same issue. I pre-selected cells upon table load yet I had to double tap a cell to deselect it. @zeiteisen answer helped me. However, I'm working in Swift 3 so I thought I'd give an answer in Swift.

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! Cell

    if /*Test for Selection*/ {
        cell.isSelected = true
        collectionView.selectItem(at: indexPath, animated: false, scrollPosition: .left)
    }        
    return cell
}

回答4:

Add this line to your didSelectItemAtIndexPath method

[collectionView selectItemAtIndexPath:indexPath animated:NO scrollPosition:nil]

回答5:

Reloading the cell was solution for me. What i need was changing image in cell for a brief moment. I assign image to imageView in cellForItemAtIndexPath and when user touches the cell i change image and run mycode then reload the cell.

 func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {
        let cell = collectionView.cellForItemAtIndexPath(indexPath) as! MenuCollectionViewCell
        cell.backgroundImageView.image = UIImage(named: "selected")
        // handle tap events
        collectionView.reloadItemsAtIndexPaths([indexPath])
    }

Hope it helps someone.

回答6:

In my case it was caused by using same logic for

func collectionView(_ collectionView: UICollectionView, shouldHighlightItemAt indexPath: IndexPath) -> Bool

as for

func collectionView(_ collectionView: UICollectionView, shouldSelectItemAt indexPath: IndexPath) -> Bool

And when cell was selected shouldHighlightItemAt was returning false - and this prevented me to deselect that cell.

Hope this will save someone's time :)