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Xamarin.Forms View Presenter

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View Presenter Overview

On Forms every platform has it’s own presenter that inherits from the native platform presenter. This enables us to navigate between native and Xamarin.Forms views. On top of that we have the MvxFormsPagePresenter which handles all the comon logic for Forms related navigation.

The default presenter supports every navigation pattern that Xamarin.Forms supports itself:

  • Tabs
  • MasterDetail
  • Childs
  • Etc

Also if your app needs another kind of presentation mode, you can easily extend it!

Presentation Attributes

The presenter uses a set of PresentationAttributes to define how a view will be displayed. The existing attributes are:

MvxPagePresentationAttribute

This is the base class for the other Forms presentation attributes and cannot be used directly.

Name Type Description
HostViewModelType Type Can optionally be set to make sure the Page has the correct Host type.
WrapInNavigationPage bool When you want the Page to be shown in a new NavigationPage or when the current Stack already is a NavigationPage push it inside there. This is true by default.
NoHistory bool Clears out the other pages from the navigation stack. False by default.
Animated bool Indicates if the navigation should be animated. This is true by default.
Title string The optional title of the Page
Icon string The optional Icon of a Page

MvxCarouselPagePresentationAttribute

Used to navigate forward to one or multipel pages in a Carousel.

Name Type Description
Position CarouselPosition Use this to set the position of the Page. It can either be the Root to host Pages, or as a Carousel to be a Page inside the carousel.

MvxContentPagePresentationAttribute

This is the standard Page attribute.

MvxMasterDetailPagePresentationAttribute

Name Type Description
Position MasterDetailPosition Use this to set the position of the Page. It can either be the Root to host Pages, Master to be a Page inside the Master, or Detail to be as the content of the Page.

MvxModalPresentationAttribute

Will show the Page as a Modal.

MvxNavigationPagePresentationAttribute

Used to indicate that this is a NavigationPage.

MvxTabbedPagePresentationAttribute

Name Type Description
Position TabbedPosition Use this to set the position of the Page. It can either be the Root to host Pages, or as a Tab to be a Page inside the TabbedPage.

Views without attributes: Default values

  • If a view class has no attribute over it, the presenter will check the type and try to create the correct attribute for it:

  • CarouselPage -> MvxCarouselPagePresentationAttribute
  • ContentPage -> MvxContentPagePresentationAttribute
  • MasterDetailPage -> MvxMasterDetailPagePresentationAttribute
  • NavigationPage -> MvxNavigationPagePresentationAttribute
  • TabbedPage -> MvxTabbedPagePresentationAttribute

Override a presentation attribute at runtime

To override a presentation attribute at runtime you can implement the IMvxOverridePresentationAttribute in your view and determine the presentation attribute in the PresentationAttribute method like this:

public MvxBasePresentationAttribute PresentationAttribute(MvxViewModelRequest request)
{
    return new MvxContentPagePresentationAttribute()
    {
        
    };
}

As you can see in the code snippet, you will be able to make your choice using a MvxViewModelRequest. This object will contain the PresentationValues dictionary alongside other properties.

If you return null from the PresentationAttribute method, the ViewPresenter will fallback to the attribute used to decorate the view. If the view is not decorated with any presentation attribute, then it will use the default attribute instead.

Hint: Be aware that this.ViewModel property will be null during PresentationAttribute. If you want to have the ViewModel instance available, you need to use the MvxNavigationService and cast the request parameter to MvxViewModelInstanceRequest.