Order of events when loading a View in iOS

Question

This one is driving me a little batty and my eyes are glossing over.

I have an app with a navigation controller.

View A has some input fields and a "Continue" button which loads View B

When I tap the "Back" button on the upper left of the navigation controller its resulting in events being fired in an order I'm not expecting/understanding

My tracing reveals ...

View B: viewWillDisappear
View A: viewWillAppear
View B: textFieldShouldEndEditing

EDIT -- more detail/code explaining my previously vague question

Conceptually the following approach has been working fine, and passed several rounds of QA testing.

In summary, I'm using the textFieldShouldEndEditing to validate textfields. If they aren't valid, I retain focus on the field and show them a message of whats wrong. All is good and validations work as the user attempts to go from field to field.

The condition that is problematic with the code below is if someone enters a partial value and then clicks BACK. All of the UITextFields in the entire app Freeze up (don't allow input) and in some cases the app crashes.

The approach I'm attempting which led me to post the initial Question was to create a private: BOOL isDisappearing;

Which I could check in viewWillDisappear (which in most cases fires PRIOR to textFieldShouldEndEditing), and if its YES I would short circuit the problematic code that is firing and freezing the UITextFields/app.

This is working in several views fine, but in 1 case where 'VIEW A:viewWillAppear' event fires before the textFieldShouldEndEditing below (VIEW B) - the isDisappearing gets set to 'NO' somehow and the problematic code is firing in textFieldShouldEndEditing

I hope this helps and you can follow. I find it hard to explain without code – but I've tried to trim it down to just what is relevant. I hope this is appropriate here – I'm pretty new to the community.

Code for VIEW B:

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)rangereplacementString:(NSString *)string
{   
    // Enforce max lengths
    // The return key from the keyboard counts as a character, so we have to exempt it
if (textField.tag == ROUTING_NUMBER_TAG)
{
    NSUInteger newLength = [textField.text length] + [string length] - range.length; 
    return (newLength > 9 && ![string isEqualToString:@"\n"]) ? NO : YES;           
}
else if (textField.tag == ACCOUNT_NUMBER_TAG)
{
    NSUInteger newLength = [textField.text length] + [string length] - range.length; 
    return (newLength > 17 && ![string isEqualToString:@"\n"]) ? NO : YES;           
}
return YES;
}

// textField validation
- (BOOL)textFieldShouldEndEditing:(UITextField *)textField
{
if (isDisappearing)

    return YES;

//run fields through validators and display validation messages.
//IF THEY DON’T PASS VALIDATION IM "HOLDING THEM HOSTAGE" BY KEEPING THE FOCUS ON THE UITEXTFIELD (returning NO)

if (textField.tag == ROUTING_NUMBER_TAG )
{
    if ([Utility isValidRoutingNumber:textField.text]== NO)
    {
        [[iToast makeText:NSLocalizedString(@"Enter valid routing number", @"")] show];
        return NO;
    }
    else
    {   //save it
         extension.payment.routingNumber = routingNumber.text;
    }
}
else if (textField.tag == ACCOUNT_NUMBER_TAG)
{
    if ([Utility isValidAccountNumber:textField.text] == NO)
    {
        [[iToast makeText:NSLocalizedString(@"Enter valid account number", @"")] show];
        return NO;
    }
    else
    {   //save it
        extension.payment.accountNumber = accountNumber.text;
    }
}
return YES;
}


- (void)textFieldDidEndEditing:(UITextField *)textField
{

//I do nothing here except nulling out the 'activefield' var I use to autoscroll the uiscrollview as the user taps around from field to field

}



- (void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];

//reset the bool so that when they come back we're back to the 'normal' state and validation will again be checked in textFieldShouldEndEditing

isDisappearing = NO;
}

- (void)viewDidAppear:(BOOL)animated
{
[super viewDidAppear:animated];
}


-(void)viewWillDisappear:(BOOL)animated
{
TRC_ENTRY
//set the bool to bypass validations in textFieldShouldEndEditing

isDisappearing = YES;
[super viewWillDisappear:animated];
}

Answer 1

Other than cases where the order of messages is clearly defined or strongly implied by names, you should avoid depending on any particular order. For example, you can reasonably expect -viewWillAppear to be called before -viewDidAppear for any given view, but don't expect one view's -viewWillAppear to be called in any particular order with respect to any message sent to a different view.

If you need help figuring out how to implement a particular feature without depending on order, please ask. But again, unless the order of invocation is documented or blatantly obvious from the method names, don't expect a particular order.

Update: I don't see exactly what's going wrong in the code you added, but perhaps a few suggestions will help:

1. Is your isDisappearing variable an instance variable of your view controller, a global variable, or what? If it's an instance variable, figure out how it's being changed. If it's a global variable, well... don't do that.

2. Be sure that you're heeding the warning in the docs to the effect that -textViewShouldEndEditing: is only advisory, and that the view may stop editing no matter what you return.

3. Try temporarily removing the iToast stuff. If the crash still happens, at least you've eliminated that as a source of problems. If it stops happening, you'll have narrowed your search.

4. Identify the cause of the crash. (This should really be first on the list.) Crashes don't just happen mysteriously -- there's a reason that it happens. Find that reason, and you're 85% done. Start by examining the stack trace when the crash occurs. If that doesn't provide enough clues, place a breakpoint somewhere before the line that causes the crash and start stepping until you crash. If all else fails, start logging messages to trace execution and monitor your assumptions.

5. What exactly is your view controller A doing in its -viewWillAppear method? Could that be part of the problem? Could you move that code to, say, -viewDidAppear instead?

Answer 2

Try doing [self.youTextField resignFirstResponder] on view B viewWillDisappear.

Answer 3

This might not be directly relevant to your question, but if you feel you don't fully understand lifetime of views, I suggest take a look here: What is the process of a UIViewController birth (which method follows which)?

It's a great explanation.