目标C-在代码中创建的对象在切换视图控制器后消失

Question

I have a simple app with two views in the storyboard, with two segues connecting them together. in the viewDidLoad of each controller I programmatically create one button, and the action for that button is to switch to the other view. The main code for one of the view controllers is shown here The code for the second one is identical except for the view controller and button names.

UIButton*           button;

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    if (!button) {
        button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
        button.frame = CGRectMake(0, 0, 50, 20);
        button.center = CGPointMake(200, 200);
        button.titleLabel.font = [UIFont systemFontOfSize:15];
        [button setTitle:@"View 2" forState:UIControlStateNormal];
        [button setBackgroundColor: [UIColor blueColor]];
        [button setTitleColor: [UIColor whiteColor] forState:UIControlStateNormal];
        button.tag = 7;
        [button addTarget:self action:@selector(view2Action:) forControlEvents:UIControlEventTouchUpInside];
        [self.view addSubview:button];
    } else {
        NSLog(@"View 1 already created. Button Tag: %d", button.tag);
    }
}

- (void)view2Action:(id)sender {
    [self performSegueWithIdentifier:@"Segue2" sender:self];
}

@end

The first view appears correctly creates the button and shows it. Clicking the button takes me to the second view, and the corresponding button there is also created and shown. And when I click this button, I do go back to the first view. But, the problem is now the button in the first view is not visible anymore. The code "if (!button)" recognizes the existence of the button, and prints the message "View 1 already created. Button Tag: 7". It seems that all the button properties are there except its graphic.

One point, if instead of creating button in code I place the button provided in the storyboard, this problem does not occur. What do I need to make the button stay displayed?

Any help will be appreciated.

Behzad

Answer 1

Your UIButton* button; variable declaration is in the wrong place. It's not syntactically illegal, but it's in the global scope of the file and of anything that imports that file. It's not technically an instance variable, as it is not within the scope of the class (neither in its @interface nor in its @implementation ). It should be declared as either an instance variable or a property.

Without seeing more of your code, I can't entirely explain the behaviour you're seeing. As a global variable it should still remain available. However, perhaps your two UIViewController classes are both accessing the same global variable, and overwriting the button properties, and the second one has stolen it and added it to it's view (removing it from the view of the first view controller). You could test this by logging it's memory address and see if it is the same in both instances, or by testing its superview property and comparing that with the view of both your view controllers.

To declare it as an instance variable or as a property, it should be within either the @interface / @end section or the @implementation / @end section. If declaring it as a property, then it should then be preceded by @property (...) . Eg:

@implementation ViewController

@property (nonatomic, strong) UIButton *button;

- (void)viewDidLoad {
    [super viewDidLoad];
    ...
    ...

Note that this is normally done in the @interface , but it is permissible in the @implementation .

(This answer is based largely on the comments from both @Gellert_Lee and yourself.)

Answer 2

Problem resolved after multiple tries at different things. And Thanks to help from Gellert Lee, and Son of the Beach. If you need to know how, please read through the comments in the answer above.