Check if a Specific Instance of class already exists - ios

Question

is there anyway to check if a specific instance of a class has already been created. I feel like it is hard to check if the instance already exists when there is a chance you may not have created it yet.
Here is my code:
-(IBAction)done:(id)sender
{ //I want to figure out how to check if 'newWindow' already exists before I create another 'newWindow'
SimpleTableView *newWindow = [self.storyboard instantiateViewControllerWithIdentifier:#"SimpleTableView"];
[self.navigationController pushViewController:newWindow animated:YES];
}
Thanks for all the help guys.

Answer 1

Yes, there is a simple way to do it.
You just need to have some reference to it (for example create a property) and check whether it is nil (not initialized) or not. You can do it like this:
if(!myReference){
//if you get here it means that it hasn't been initialized yet so you have to do it
}

Answer 2

First make newWindow an ivar or a property.
Then:
if (!newWindow){
newWindow = [self.storyboard instantiateViewControllerWithIdentifier:#"SimpleTableView"];
}

Answer 3

I wrote you a method that checks all viewControllers in UINavigationController:
- (BOOL)classExistsInNavigationController:(Class)class
{
for (UIViewController *controller in self.navigationController.viewControllers)
{
if ([controller isKindOfClass:class])
{
return YES;
}
}
return NO;
}
Use it like this:
- (IBAction)done:(id)sender
{
//I want to figure out how to check if 'newWindow' already exists before I create another newWindow
if (![self classExistsInNavigationController:[SimpleTableView class]])
{
SimpleTableView *newWindow = [self.storyboard instantiateViewControllerWithIdentifier:#"SimpleTableView"];
[self.navigationController pushViewController:newWindow animated:YES];
}
}
You can also do something like this:
- (UIViewController *)classExistsInNavigationController:(Class)class
{
for (UIViewController *controller in self.navigationController.viewControllers)
{
if ([controller isKindOfClass:class])
{
return controller;
}
}
return nil;
}
And use it like this if you want to pop to the viewController that exists already:
- (IBAction)done:(id)sender
{
//I want to figure out how to check if 'newWindow' already exists before I create another newWindow
UIViewController *controller = [self classExistsInNavigationController:[SimpleTableView class]];
if (!controller)
{
SimpleTableView *newWindow = [self.storyboard instantiateViewControllerWithIdentifier:#"SimpleTableView"];
[self.navigationController pushViewController:newWindow animated:YES];
}
else
{
[self.navigationController popToViewController:controller animated:YES];
}
}

Answer 4

You can use if/else to check newWindow exists or not.
if (newWindow) { // newWindow is exist to do something
// Do something.
} else { // newWindow is not exist to do something
// Do something.
}

Answer 5

You can implement an instance counter (https://stackoverflow.com/a/30509753/4647396) in the class you want to track.
Then just check if the counter is greater than 0.
If I interpret your question correctly you just want to know wether an instance exists and dont need a reference to it.