Question

This maybe a simple mistake but i just can’t seem to find out what is wrong with the error Unknown type name 'TransportViewController'. I am trying to pass xCoor and yCoor which is 2 double value to my 2nd view which is the TransportViewController. Here is my codes:

TransportViewController *xCoor;
TransportViewController *yCoor;
@property (retain, nonatomic) TransportViewController *xCoor;
@property (retain, nonatomic) TransportViewController *yCoor;

This 4 line is giving me the error

MapViewController.h file

#import "TransportViewController.h"
@interface MapViewController : UIViewController{
    TransportViewController *xCoor;
    TransportViewController *yCoor;
}
@property (retain, nonatomic) TransportViewController *xCoor;
@property (retain, nonatomic) TransportViewController *yCoor;

MapViewController.m file

#import "TransportViewController.h"
@implementation MapViewController
@synthesize xCoor;
@synthesize yCoor;
.
.
.
- (IBAction) publicTransportAction:(id)sender{
    TransportViewController *view = [[TransportViewController alloc] initWithNibName:nil bundle:nil];
    self.xCoor = view;
    self.yCoor = view;
    xCoor.xGPSCoordinate = self.mapView.gps.currentPoint.x;
    yCoor.xGPSCoordinate = self.mapView.gps.currentPoint.y;
    [self presentModalViewController:view animated:NO];
}

TransportViewController.h file

#import "MapViewController.h"
@interface TransportViewController : UIViewController<UITextFieldDelegate>
{
    double xGPSCoordinate;
    double yGPSCoordinate;
}
@property(nonatomic)double xGPSCoordinate;
@property(nonatomic)double yGPSCoordinate;
@end

Answer 1

You have a circular dependency. In short, you have directed the compiler:

• MapViewController.h needs TransportViewController.h
• and TransportViewController.h needs MapViewController.h

In actuality – neither are necessary in the headers. You can use Forward Declarations in both cases.

MapViewController.h

@class TransportViewController; // << forward declaration instead of inclusion

@interface MapViewController : UIViewController {
    TransportViewController *xCoor;
    TransportViewController *yCoor;
}
@property (retain, nonatomic) TransportViewController *xCoor;
@property (retain, nonatomic) TransportViewController *yCoor;
@end

TransportViewController.h

@class MapViewController; // << not even needed, as MapViewController
                          //    does not exist in this header

@interface TransportViewController : UIViewController<UITextFieldDelegate>
{
    double xGPSCoordinate;
    double yGPSCoordinate;
}
@property(nonatomic)double xGPSCoordinate;
@property(nonatomic)double yGPSCoordinate;
@end

then your #imports can go in the *.m file where needed.

you should read up on forward declarations. you can’t use them everywhere, but you can use them very often in headers instead of #import, and the can really reduce your build times.