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The XAML Previewer has been deprecated in Visual Studio 2019 version 16.8 and Visual Studio for Mac version 8.8, and replaced by the XAML Hot Reload feature in Visual Studio 2019 version 16.9 and Visual Studio for Mac version 8.9. Refer : XAML Previewer for Xamarin.Forms
XAML Hot Reload plugs into your existing workflow to increase your productivity and save you time. With Hot Reload, when you save your XAML file the changes are reflected live in your running app.
Learn more about XAML Hot Reload in the documentation.
Best Regards,
Jessie Zhang
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