为什么UITextField的窗口为零？

Question

I am trying to implement "previous," "next," and "done" buttons for a series of UITextField s, each of which is contained in a UITableViewCell in a grouped UITableView . I hold on to the UITextFields in an NSMutableArray , and keep an integer pointing to the UITextField that is currently active. Here are the two selectors that get fired when the Previous and Next buttons are tapped, respectively.

-(IBAction)didSelectPreviousButton:(id)sender
{    
    if ((textFieldIndex - 1) >= 0) {
        UITextField *currentField = [self.testTextFields objectAtIndex:(textFieldIndex)];
        UITextField *nextTextField = [self.testTextFields objectAtIndex:(--textFieldIndex)];
        BOOL result = [nextTextField becomeFirstResponder];
        NSLog([NSString stringWithFormat:@"currentField's window: %@", currentField.window]);
        NSLog([NSString stringWithFormat:@"nextTextField's window: %@", nextTextField.window]);
    } else {
        [self dismissKeyboard:sender];
    }
}

-(IBAction)didSelectNextButton:(id)sender
{
    if ((textFieldIndex + 1) < [self.inspectionItemSpec.numberOfTests intValue]) {
        UITextField *currentField = [self.testTextFields objectAtIndex:(textFieldIndex)];
        UITextField *nextTextField = [self.testTextFields objectAtIndex:(++textFieldIndex)];
        BOOL result = [nextTextField becomeFirstResponder];
        NSLog([NSString stringWithFormat:@"currentField's window: %@", currentField.window]);
        NSLog([NSString stringWithFormat:@"nextTextField's window: %@", nextTextField.window]);
    } else {
        [self dismissKeyboard:sender];
    }
}

As you can see, I am logging the window property of the current & next text field, and in the didSelectNextButton , everything is correct. However, in didSelectPreviousButton , nextTextField.window is always nil . Why would this be happening?

(Note that the previous button is enabled only after the user has tapped the next button once.)

Answer 1

This may be because each UITextField is in a UITableViewCell while also being referenced in self.testTextFields . Because of the way cells are re-used by tables in iOS, you could (and probably will) end up in a situation where the next text field in your array is not the text field in the next visible row in the table.

If you post your tableView:cellForRowAtIndexPath: code, that may make the problem apparent.

Answer 2

The window is the root of the view hierarchy. If the window property is nil, the view hasn't been added to the view hierarchy via something like

    [someViewInTheViewHierarchy addSubview:yourView].