Nick's .NET Travels

Continually looking for the yellow brick road so I can catch me a wizard....

Adding Back Button Support to Xamarin.Forms with BuildIt.Forms

In my previous post I discussed the support I added for hardware and virtual back buttons in a Universal Windows Platform application (UWP) using BuildIt.Lifecycle. At an application level the back button (irrespective of whether it’s a hardware back button, such as on a phone, or a virtual back button, such as in desktop or tablet mode) is designed to return the user to the previous state of the application. In most cases this correlates to going to either the previous page in an application, or to the previous application, if the user is on the first page of the current application. Occasionally, there may be sub-states within a page, in which case the back button should be able to step back through those states, before causing the application to go back to the previous page.

This means that the back button should be applied at the highest level of the active window, which means passing it into StateManager of the region that correlates to the active window. In a UWP application, this is relatively straight forward as the application can subscribe to the BackRequested event, for example:

SystemNavigationManager.GetForCurrentView().BackRequested += BackRequested;

In the event handler, the StateManager for the region is queried to find out if a previous state exists. If it does, a call to GoBackToPreviousState is made.

Unfortunately Xamarin.Forms (XForms) doesn’t offer a global event for the back button. Instead it’s up to every page to override the OnBackButtonPressed method and return true if the back navigation should be cancelled. Having every page of the XForms application inherit from a custom base page isn’t an option I wanted to force upon users of this library. Luckily it’s not necessary to intercept the OnBackButtonPressed method on every page; it can instead be intercepted in the NavigationPage at the root of the application. By providing a CustomNavigationPage class, and requiring that it be used as the root of the application, it’s possible for the back button to be intercepted and applied to the active region.

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