Xamarin.Forms, a popular choice for cross-platform Xamarin apps, has two stacks. It’s common to implement navigation by pushing and popping Pages on a stack, but the two stacks are intended for very different situations and using the right one will make your code easier to understand. How do you know which stack you should use for your projects?

Today, we’re announcing a new class in the Xamarin University course schedule for Xamarin Certification! Navigation Patterns in Xamarin.Forms (XAM205) will show you how to code the stack and tab navigation patterns, including guidelines to help you decide which pattern is most appropriate for your app.

The Two Stacks

The two Xamarin.Forms stacks are the Navigation Stack and the Modal Stack. Each provides their own set of push/pop methods, as seen below:

Since the methods operate on different stacks, you must use them as pairs. For example, if you call PushModalAsync to display a page, you must use PopModalAsync to remove it, or you’ll get a runtime exception.

Stack Availability

The Navigation Stack is implemented by the NavigationPage class. This means it’s only available from pages hosted inside a NavigationPage. The methods will throw an exception if you call PushAsync and PopAsync without a NavigationPage.

The Modal Stack, by contrast, with its PushModalAsync and PopModalAsync methods, is always available from any page.

Stack Access

Every Page in Xamarin.Forms has a Navigation property of type INavigation. The INavigation interface contains methods for both stacks:


public interface INavigation
{ ...
Task PushAsync(Page page);
Task PopAsync();
Task PushModalAsync(Page page);
Task PopModalAsync();
}

The methods with Modal in their name operate on the Modal Stack. The non-modal methods work on the Navigation Stack.

What is a Modal Page?

A modal page is a page with restricted navigation options.
 The user must indicate that they are finished before moving to a different page.

The classic examples are pages like Add-New or Edit, where the user must choose whether to save their data before they leave the page. For example, the iOS Clock app uses tabs as its top-level navigation paradigm, shown below:

Notice how the tabs are hidden, so the user is forced to tap Cancel or Save to leave the page? This is the behavior we mean when we say a page is modal.

Guidance

With that background, the guidance is straightforward:

  • Use PushAsync and PopAsync when you’re implementing stack navigation using a NavigationPage.
  • Use PushModalAsync and PopModalAsync only to display a modal page. This will make your code self-documenting; other developers will know immediately that you are implementing the concept of a modal page.

This is just a sample of the material in the Xamarin University course Navigation Patterns in Xamarin.Forms (XAM205), which shows you how to code the stack and tab navigation patterns, including guidelines to help you decide which pattern is appropriate for your app.

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