How to tell a viewcontroller to update its UI from another class

Question

I am trying to understand how updating of a viewController that is currently visible works. I have ViewControllerA and ClassA. I want to tell ViewControllerA to reloadData on the tableview from ClassA. What is the best way to go about doing this?

I have found this question and answer but I dont think this will work in my situation or I am not understanding it correctly.

what is your classA inherited from?? – Vish Jan 08 '14 at 18:16 — Vish, Jan 08 '14 at 18:16

score 3 · Accepted Answer · answered Jan 08 '14 at 18:16

The easiest way without knowing your setup would be to use NSNotificationCenter. Here is what you could do:

In ViewControllerA add in hooks for NSNotificationCenter:

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];

    //Register for notification setting the observer to your table and the UITableViewMethod reloadData. So when this NSNotification is received, it tells your UITableView to reloadData
    [[NSNotificationCenter defaultCenter] addObserver:self.table selector:@selector(reloadData) name:@"ViewControllerAReloadData" object:nil];
}

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];

    //Need to remove the listener so it doesn't get notifications when the view isn't visible or unloaded.
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

Then in ClassA when you want to tell ViewControllerA to reload data just post the NSNotification.

- (void)someMethod {
    [[NSNotificationCenter defaultCenter] postNotificationName:@"ViewControllerAReloadData" object:nil];
}

Thats exactly what I was looking for, Thank you for such a simple informative answer! — BluGeni, Jan 08 '14 at 18:21
Not a problem! Glad I could help. Just make sure to accept it when you can. — random, Jan 08 '14 at 18:22

score 2 · Answer 2 · edited May 23 '17 at 11:59

Answers here about using NSNotificationCenter are good. Be aware of a few other approaches:

A common one is the delegate pattern (see here and here).

Another is that the view controller observes the model change using KVO. (see here and here).

Another good one, often overlooked, which can probably be used in your case is the "do almost nothing" pattern. Just reloadData on your table view when viewWillAppear.

score 1 · Answer 3 · edited Apr 01 '15 at 19:23

Key value Coding , NSNotificationCentre and Delegates are preferred. But NSNotificationCentre is easiest in your case.

The UIViewController that contains UITableView must add observer like this : Init :

[[NSNotificationCenter defaultCenter] addObserver:self
    selector:@selector(methodToReloadTable:) 
    name:@"TABLE_RELOAD"
    object:nil];

In delloc method :

  [[NSNotificationCenter defaultCenter] removeObserver:self];

Post it from any XYZ class like on any UIButton action :

[[NSNotificationCenter defaultCenter] 
        postNotificationName:@"TABLE_RELOAD" 
        object:self];

Advantage of NSNotificationCentre is that they can add observers in multiple classes ..

How to tell a viewcontroller to update its UI from another class

3 Answers3