Скачать или смотреть How to Call a Function in Another View Controller in Xamarin.iOS

Learn how to effectively call functions across different view controllers in `Xamarin.iOS`. This guide provides clear steps and solutions for common issues you may encounter!
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This video is based on the question https://stackoverflow.com/q/64695550/ asked by the user 'Taher El-hares' ( https://stackoverflow.com/u/11306390/ ) and on the answer https://stackoverflow.com/a/64698614/ provided by the user 'Jai Gupta' ( https://stackoverflow.com/u/5141991/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Call a Function in Another View Controller in Xamarin.iOS

In Xamarin.iOS application development, it's common to need to interact with multiple view controllers. A typical scenario is when you have a side menu (like a sidebar or a pop-up), and you want to trigger actions in the main view controller when certain UI elements are interacted with. This guide explores how to properly call functions in another view controller, specifically focusing on resolving a common error you might encounter along the way.

The Problem

Let's consider a situation where you have a controller named LeftViewController that acts as a side menu for your main controller, ViewController. You want to update a button's title in ViewController when a button in LeftViewController is clicked. The current implementation is running into an error: "Object reference not set to an instance of an object."

Example Setup

Here's what your initial setup might look like:

ViewController: Responsible for the main user interface, which includes a button (s2) that needs its title updated.

LeftViewController: The side menu that contains the button triggering the update.

Here’s a snippet that might resemble your code structure:

[[See Video to Reveal this Text or Code Snippet]]

In LeftViewController, you might have tried to set it all up like this:

[[See Video to Reveal this Text or Code Snippet]]

Unfortunately, you are hitting that frustrating error, and it’s time to dissect why this happens and how to fix it.

Understanding the Error

The error message "Object reference not set to an instance of an object" typically means that you're trying to access a member variable or method of an object that hasn't been initialized. In your case, this can be linked back to two potential pointers of failure:

Uninitialized Button (s2): If s2 isn't properly instantiated or linked in your code, trying to call s2.SetTitle will lead to this error.

Improper Reference to CommonClass: If CommonClass.value is being called without CommonClass being instantiated, it'll also throw an error.

The Solution

To properly solve the problem and allow for calling functions between view controllers, follow these clear steps:

Step 1: Pass the Correct Instance

Instead of creating a new instance of ViewController in refresh_all, you should pass the existing instance of ViewController to the refresh_all class. Here’s how you can modify your refresh_all class:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Adjust Your Event Handler

Now that refresh_all requires a ViewController instance, you need to pass this when instantiating refresh_all in your LeftViewController:

[[See Video to Reveal this Text or Code Snippet]]

Tip: Ensure that ParentViewController returns the actual ViewController instance.

Step 3: Verify Instantiation of UI Components

Double-check that the button s2 is properly linked in your ViewController. If s2 is either null or not linked, ensure that it is initialized in the ViewDidLoad method or through storyboard connections.

Step 4: Check the CommonClass Instance

As for CommonClass, make sure that CommonClass is a valid instance and has the value property properly set up. You might need to look into using static properties or methods if you need global access to CommonClass.

Conclusion

By following the outlined steps, you should be able to effectively call functions between your view controllers in Xamarin.iOS. The key is to manage instances properly and ensure that all UI elements are instantiated before being accessed. Happy coding, and don’t hesitate to troubleshoot any issues along the way!