[SampleApp] NavigationBar isn't showing back button in sample app

[nickrandolph]

Current behavior

In the NavigationBar pages in the Sample app, the Back button isn't showing when running the WinUI target

Expected behavior

Back button should work

How to reproduce it (as minimally and precisely as possible)

Run the WinUI target for the sample app in this repository
Go to NavigationBar
Click on Show Sample button
Click Navigation to Second Page button
No back button visible but it is there if you click on where the back button should be

Environment

Nuget Package:

Package Version(s):

Affected platform(s):

• iOS
• Android
• WebAssembly
• WebAssembly renders for Xamarin.Forms
• Windows
• Build tasks

Visual Studio:

• 2017 (version: )
• 2019 (version: )
• for Mac (version: )

Relevant plugins:

• Resharper (version: )

Anything else we need to know?

[kazo0]

This looks to be expected for non-mobile platforms right now as we are not defining a MainCommand with an Icon:

uno.toolkit.ui/samples/Uno.Toolkit.Samples/Uno.Toolkit.Samples.Shared/Content/NestedSamples/M3MaterialNavigationBarSample_NestedPage2.xaml

Lines 17 to 34 in e38ebad

	<utu:NavigationBar Style="{StaticResource MaterialNavigationBarStyle}"
	AutomationProperties.AutomationId="M3Page2NavBar"
	Content="Second Page">
	<utu:NavigationBar.PrimaryCommands>
	<AppBarButton Style="{StaticResource MaterialAppBarButtonStyle}"
	Label="More" />
	<AppBarButton Style="{StaticResource MaterialAppBarButtonStyle}"
	Label="More" />
	</utu:NavigationBar.PrimaryCommands>
	<utu:NavigationBar.SecondaryCommands>
	<AppBarButton Style="{StaticResource MaterialAppBarButtonStyle}"
	Label="More" />
	<AppBarButton Style="{StaticResource MaterialAppBarButtonStyle}"
	Label="More" />
	<AppBarButton Style="{StaticResource MaterialAppBarButtonStyle}"
	Label="More" />
	</utu:NavigationBar.SecondaryCommands>
	</utu:NavigationBar>

We can simply add one for not_mobile: but we may want to look at perhaps now bundling a new PNG asset within Toolkit, I think we can now bundle assets within libraries, right @jeromelaban / @nickrandolph ?

And I need to double check but I believe I cannot just use a <not_mobile:SymbolIcon Symbol="Back" /> directly in the default NavigationBar style because of this issue: microsoft/microsoft-ui-xaml#1494 + #235

[jeromelaban]

@kazo0 yes, it's definitely possible to bundle assets now, in the same way bundling fonts is supported.

[kazo0]

We should bundle a scaled PNG asset (or can we bundle an SVG and have resizetizer do its magic?? @nickrandolph / @pictos )

[nickrandolph]

For the library we should package scaled assets to avoid a dependency on Resizetizer

[silviuo]

Currently working on #321 which should solve this as well