为什么这个视图控制器不能设置下一个视图控制器的文本字段？

Question

I have a view controller that has navigation bar with a done button, and two text fields. When the done button is pressed, the method postInfo is called. Here is the implementation:

- (void)postInfo{
ListingViewController* lvc = [[ListingViewController alloc] init];

NSString* listingName = listingNameField.text;
NSString* listingPrice = listingPriceField.text;

NSLog(@"%@", listingName);
NSLog(@"%@", listingPrice); 

[lvc.titleLabel setText:listingName];
[lvc.priceLabel setText:listingPrice];

[self.navigationController pushViewController:lvc animated:YES]; 

}

Here ListingViewController.h :

#import <UIKit/UIKit.h>

@interface ListingViewController : UIViewController

@property (weak, nonatomic) IBOutlet UILabel *priceLabel;
@property (weak, nonatomic) IBOutlet UILabel *titleLabel;

@end

The UILabel's are set through a xib file and are empty. Will post whatever other code is needed upon request.

Answer 1

At the time you do that push, the next controller's view hasn't been loaded yet, so you can't access its views. You need to create NSString properties in ListingViewController and pass a string to those in your postnfo method. Then in ListingViewController's viewDidLoad method, use those properties to populate the labels (which will have been loaded by the time viewDidLoad runs).

Answer 2

Change your code to this.

- (void)postInfo
{
  ListingViewController* lvc = [[ListingViewController alloc] init];
  [lvc view];    // loads the view 

  NSString* listingName = listingNameField.text;
  NSString* listingPrice = listingPriceField.text;

  NSLog(@"%@", listingName);
  NSLog(@"%@", listingPrice); 

  [lvc.titleLabel setText:listingName];
  [lvc.priceLabel setText:listingPrice];

  [self.navigationController pushViewController:lvc animated:YES]; 
}

This happens because until the view property of the view controller is accessed, the view will not be loaded and all of the subviews are nil. They can be configured after viewDidLoad is called on the view controller being pushed. Calling [lvc view] loads the view immediately.

Answer 3

I usually get this too (in the example of when using segues). I get around it by setting an NSString property instead of an IBOutlet during prepareForSegue of the destinationController . Then during viewDidLoad of the next View Controller, I take the value of the property and assign it to the UILabel .

You may find the explanation in this answer useful: https://stackoverflow.com/a/8094146/2358334

If you set a breakpoint just after the line you call

testViewController *viewController = segue.destinationViewController;

when you build and run the project, you will find that the UITextField property in the destinationViewController is not allocated and initiated (memory is 0x0) at the breakpoint. Meanwhile the NSString property is already allocated and initialised (so you can set its value).

Answer 4

Try to do this

first you need to load the view (push viewcontroller) and then you can access the properties (because you have created views by IBOutlet

- (void)postInfo{
    ListingViewController* lvc = [[ListingViewController alloc] init];

    NSString* listingName = listingNameField.text;
    NSString* listingPrice = listingPriceField.text;

    NSLog(@"%@", listingName);
    NSLog(@"%@", listingPrice); 

    [self.navigationController pushViewController:lvc animated:YES];

    [lvc.titleLabel setText:listingName];
    [lvc.priceLabel setText:listingPrice]; 

}