How to open the MainPage.axml from a Xamarin Project in Designer mode?

App.xaml ("design") e App.cs (code behind) is just a file say 'start' in Xamarin-Forms projects.

The file ( App.xaml ) does not contain (directly) anything related to the part of UI , I say directly because it can contain design style settings, font color for example, etc. In short this view does not have the function of interacting with the user.

One of the functions of the file ( App.cs ) is to define the application home page, among others, for more details see: App Class

Design mode still does not exist which makes development tiresome since to check change in the UI it is necessary to run application, however, I know two tools that make this task less painful:

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Responding to your question / quote:

Yes, in short, in Xamarin.Forms you write the layout (View) of your screens and the application logic only once (in your case within App1) and the native layout for the other platforms is generated (compiled ) automatically. You can also individually change the layout and modify the individual behavior of each platform if you want something more specific.

Now let's answer your question:

There is no Designer available yet for XAML Forms XAML pages. You must do everything in the "hand". To see the result you must run the application on the desired emulator or on your mobile.

This information can be found on XAML's Xamarin.Forms basics page: Xamarin. Forms XAML

Translating what is written there:

  

There is not yet a visual editor for XAML generated in Xamarin.Forms applications, so all XAML should be handwritten.