Ios – Getting “Duplicate Interface Definition” error, definitely has to #import ing header files

iosobjective cxcode

I'm helping on an iOS project with lots of methods and definitions common to many different classes in the AppDelegate. So, in each of those classes, in the .h file, I use #import "AppDelegate.h". This works fine until I need access to one of those classes that already imports the AppDelegate into another class that imports AppDelegate. At this point, I get a Duplicate Interface Definition error for AppDelegate.

Ok, so that seems fair. I'm already importing AppDelegate into a file that I'm importing, so AppDelegate is getting imported from two different places. So I remove the AppDelegate line, and everything is fine.

But what happens when I need to import two classes that both need to import AppDelegate?

I have a very specific problem that I'm trying to wrap my head around, and I know it is being caused by something that has to do with this, but I'm not sure what. So I'm hoping if I figure out how I'm supposed to be handling this sort of importing, and sort everything else out, and hope that this solves my problem. So to put this in more concrete terms:

I have ClassA.h, ClassB.h, and ClassC.h. All have #import "AppDelegate.h". When I need to use #import "ClassB.h" in ClassA, I remove the #import "AppDelegate.h" line from ClassA. Everything works smoothly. But what happens if I also need to #import "ClassC.h" into ClassA, and but ClassB and ClassC NEED to have the #import "AppDelegate.h"?

EDIT:

I tried the exact scenario I described above in a clean project, and it built fine, so there is something else at play. But what I can say with certainty is that when this issue came up previously with this project, it was a duplicate interface definition of AppDelegate, and when I removed the #import "AppDelegate.h" line, the error went away, and I still had access to the AppDelegate.h methods and enums through other imported files.

Best Answer

The best prevention and cure for this is to follow some guidelines on when to import from a header file. As a general rule, never import from an Objective-C header except in these cases:

You need to extend a class declared in another header.
You need to declare conformity with a protocol declared in another header.
You need to refer to a non-class, non-protocol type defined in another header in the public methods and / or properties. To refer to protocols and classes, forward declare them with @class or @protocol, like @class ClassFromOtherHeader;

Every other #import should go in your implementation. My recommendation is to start moving all your #import statements out of headers and into the implementation files according to these rules. Start with the files you think are at the root of the problem and move outward. This will fix your problem and give you the side benefit of clearer code and faster build times.

Related Solutions

Ios – How to change the name of an iOS app

Go to Targets in Xcode.
Build Settings on your project's target (your current development name).
Search for Product Name under Packaging. Change its value to what you want your new project name to be.

Ios – How to make a UITextField move up when the keyboard is present – on starting to edit

You will only need a ScrollView if the contents you have now do not fit in the iPhone screen. (If you are adding the ScrollView as the superview of the components just to make the TextField scroll up when keyboard comes up, then it's not needed.)
The standard way to prevent the TextFields from being covered by the keyboard is to move the view up/down whenever the keyboard is shown.

Here is some sample code:

#define kOFFSET_FOR_KEYBOARD 80.0

-(void)keyboardWillShow {
    // Animate the current view out of the way
    if (self.view.frame.origin.y >= 0)
    {
        [self setViewMovedUp:YES];
    }
    else if (self.view.frame.origin.y < 0)
    {
        [self setViewMovedUp:NO];
    }
}

-(void)keyboardWillHide {
    if (self.view.frame.origin.y >= 0)
    {
        [self setViewMovedUp:YES];
    }
    else if (self.view.frame.origin.y < 0)
    {
        [self setViewMovedUp:NO];
    }
}

-(void)textFieldDidBeginEditing:(UITextField *)sender
{
    if ([sender isEqual:mailTf])
    {
        //move the main view, so that the keyboard does not hide it.
        if  (self.view.frame.origin.y >= 0)
        {
            [self setViewMovedUp:YES];
        }
    }
}

//method to move the view up/down whenever the keyboard is shown/dismissed
-(void)setViewMovedUp:(BOOL)movedUp
{
    [UIView beginAnimations:nil context:NULL];
    [UIView setAnimationDuration:0.3]; // if you want to slide up the view

    CGRect rect = self.view.frame;
    if (movedUp)
    {
        // 1. move the view's origin up so that the text field that will be hidden come above the keyboard 
        // 2. increase the size of the view so that the area behind the keyboard is covered up.
        rect.origin.y -= kOFFSET_FOR_KEYBOARD;
        rect.size.height += kOFFSET_FOR_KEYBOARD;
    }
    else
    {
        // revert back to the normal state.
        rect.origin.y += kOFFSET_FOR_KEYBOARD;
        rect.size.height -= kOFFSET_FOR_KEYBOARD;
    }
    self.view.frame = rect;

    [UIView commitAnimations];
}


- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    // register for keyboard notifications
    [[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(keyboardWillShow)
                                             name:UIKeyboardWillShowNotification
                                           object:nil];

    [[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(keyboardWillHide)
                                             name:UIKeyboardWillHideNotification
                                           object:nil];
}

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    // unregister for keyboard notifications while not visible.
    [[NSNotificationCenter defaultCenter] removeObserver:self
                                             name:UIKeyboardWillShowNotification
                                           object:nil];

    [[NSNotificationCenter defaultCenter] removeObserver:self
                                             name:UIKeyboardWillHideNotification
                                           object:nil];
}

Best Answer

Related Solutions

Ios – How to change the name of an iOS app

Ios – How to make a UITextField move up when the keyboard is present – on starting to edit

Related Topic