UICollectionView：激活，滚动到第一响应者并将其分配给远程单元

Question

I've got a text field inside of a UICollectionViewCell that can receive first-responder status. The cell currently isn't visible on-screen, and I want to scroll to the cell based off of a button hit from a UISegmentedControl . There's two segments to this control… and a hit to the second segment should scroll to the first cell in the 2nd section of the UICollectionView . After this happens, the cell should get selected programatically, and then the text field inside of that cell is supposed to get first responder status and bring up the keyboard.

What's happening now (inside my action method from a value change from the segmented control) is that a call to -[UICollectionView selectItemAtIndexPath:animated:scrollPosition:] isn't scrolling to it at all (and I'm using UICollectionViewScrollPositionTop ; may as well be " …None "). If I thumb down the list manually, the cell is indeed selected (it gets a darker background color in that state), but the text field certainly doesn't have first responder status.

To fix the scroll problem, I've been able to ascertain the position of the cell in the list, and scroll to the cell's content offset (I've also used scrollRectToVisible here). Then I manually select it (as well as telling the delegate to fire its appropriate method as well, where the cell's text field gains first responder status).

- (void)directionSegmentedControlChanged:(UISegmentedControl *)sender {
    NSIndexPath *path = [NSIndexPath indexPathForItem:0 inSection:sender.selectedSegmentIndex];
    UICollectionViewLayoutAttributes *attributes = [self.collectionView layoutAttributesForItemAtIndexPath:path];
    [self.collectionView setContentOffset:attributes.frame.origin animated:YES];
    [self.collectionView selectItemAtIndexPath:path animated:NO scrollPosition:UICollectionViewScrollPositionNone];
    [self.collectionView.delegate collectionView:self.collectionView didSelectItemAtIndexPath:path];
}

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
    BDKCollectionViewCell *cell = (BDKCollectionViewCell *)[collectionView cellForItemAtIndexPath:indexPath];
    [cell.textField becomeFirstResponder];
}

The problem here is that the cell as it's seen in -[collectionView:didSelectItemAtIndexPath:] is nil, because it's not in the visible cell set of the collection view when the method gets fired.

What's the best way to solve this? I've tried tossing my scrolling code inside of a [UIView animateWithDuration:animations:completion:] block, and assigned first responder upon completion there, but manually animating the collection view in this manner neglects to load any of the cells that should be scrolled past. Any ideas?

---

Update : many thanks to @Esker, who suggested I simply perform the "focus selection" action after a delay using Grand Central Dispatch. My solution ended up looking like this.

- (void)directionSegmentedControlChanged:(UISegmentedControl *)sender {
    NSIndexPath *path = [NSIndexPath indexPathForItem:0  inSection:sender.selectedSegmentIndex];

    UICollectionViewLayoutAttributes *attributes = [self.collectionView layoutAttributesForItemAtIndexPath:path];
    [self.collectionView setContentOffset:attributes.frame.origin animated:YES];

    dispatch_time_t startAfter = dispatch_time(DISPATCH_TIME_NOW, 0.28 * NSEC_PER_SEC);
    dispatch_after(startAfter, dispatch_get_main_queue(), ^{
        [self.collectionView selectItemAtIndexPath:path animated:NO scrollPosition:UICollectionViewScrollPositionNone];
        [self collectionView:self.collectionView didSelectItemAtIndexPath:path];
    });
}

Answer 1

I had a similar challenge with a UITableView : scrolling to a cell that was not yet visible, and assigning first responder to a UITextField within the target cell once it was visible. Here's a simplified description of how I handle this. I imagine this approach could work with a UICollectionView , but I don't have much experience with collection views.

1. If the desired cell/text field is currently visible, immediately send it becomeFirstResponder , and scroll to the cell if desired.
2. Otherwise, set a property in your view controller or a similar class that indicates that a text field needs focus, and which one needs focus
3. Tell the table view/collection view to scroll to the desired index path
4. In collectionView:cellForItemAtIndexPath: , you could try to check that property to see if a text field at the given indexPath needs to get focus, and if so, send it becomeFirstResponder immediately, but I found this won't work if the cell is scrolling into view, presumably because at this point, when you're configuring the new cell, it's not yet actually in the view hierarchy. So I added a check, if becomeFirstResponder returns NO at this point, I try again after a delay:

dispatch_after(someDelay, dispatch_get_main_queue(), ^(void){
    [self getFocus:textField];
});

The getFocus method will both send becomeFirstResponder to the text field and clear that property that tracks which text field needs focus.

My actual implementation is somewhat specialized for the view model associated with my table view, and encapsulated in a couple of classes and using some KVO, but I wanted to avoid that and focus on the minimum required logic in the description above.