如何更改 Xamarin Forms 中控件的视觉 state？

Question

I am trying to make a tabs like view with plain xamarin forms because I don't want to use any third party plugin. For that I used two frames like below and changed its state as "Selected" & "Unselected" when tapped on that frame to make it look like that.

Style for frame:

<Style TargetType="Frame">
                <Setter Property="VisualStateManager.VisualStateGroups">
                    <VisualStateGroupList>
                        <VisualStateGroup>
                            <VisualState x:Name="Selected">
                                <VisualState.Setters>
                                    <Setter Property="BackgroundColor" Value="Orange" />
                                </VisualState.Setters>
                            </VisualState>
                            <VisualState x:Name="UnSelected">
                                <VisualState.Setters>
                                    <Setter Property="BackgroundColor" Value="White" />
                                </VisualState.Setters>
                            </VisualState>
                        </VisualStateGroup>
                    </VisualStateGroupList>
                </Setter>
            </Style>

My Frame:

<Frame x:Name="AllNewsTab" Padding="10,5,10,5" CornerRadius="3" HasShadow="False" VerticalOptions="FillAndExpand">
                    <Label Text="All" FontFamily="{StaticResource BoldFont}" TextColor="{StaticResource BodyTextColor}" FontSize="Medium" HorizontalTextAlignment="Center" VerticalTextAlignment="Center" HorizontalOptions="Center"/>
                    <Frame.GestureRecognizers>
                        <TapGestureRecognizer NumberOfTapsRequired="1" Tapped="Tab_Tapped"/>
                    </Frame.GestureRecognizers>
                </Frame>

Tapped Event:

private void Tab_Tapped(object sender, EventArgs e)
        {
            if (frameSelected != null)
                VisualStateManager.GoToState(frameSelected, "UnSelected");

            VisualStateManager.GoToState((Frame)sender, "Selected");

            frameSelected = (Frame)sender;
}

But I want one frame to look selected when the page appears for the first time. So I tried to do like this in the pages OnAppearing Method. But it doesn't work. What is the problem here?

protected override void OnAppearing()
        {
            VisualStateManager.GoToState(AllNewsTab, "Selected");

            base.OnAppearing();
        }

Answer 1

Try this, In xamarin, VisualElement have 4 states such as Normal, Disabled, Focused, Selected. And we can define our own VisualElements.

MainPage.Xml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d"
             x:Class="Xam_VS_Test.Views.MainPage">
    <ContentPage.Resources>
        <Style TargetType="Frame">
            <Setter Property="VisualStateManager.VisualStateGroups">
                <VisualStateGroupList>
                    <VisualStateGroup Name="SelectionStates">
                        <VisualState x:Name="Selected">
                            <VisualState.Setters>
                                <Setter Property="BackgroundColor" Value="Orange" />
                            </VisualState.Setters>
                        </VisualState>
                        <VisualState x:Name="UnSelected">
                            <VisualState.Setters>
                                <Setter Property="BackgroundColor" Value="White" />
                            </VisualState.Setters>
                        </VisualState>
                    </VisualStateGroup>
                </VisualStateGroupList>
            </Setter>
        </Style>
    </ContentPage.Resources>
    <ContentPage.Content>
        <StackLayout Orientation="Vertical" >
            <Frame x:Name="AllNewsTab" Padding="10,5,10,5" HeightRequest="20" WidthRequest="50" HorizontalOptions="FillAndExpand" CornerRadius="3" HasShadow="False" VerticalOptions="FillAndExpand">
                <Label Text="All" FontSize="Large" HorizontalTextAlignment="Center" VerticalTextAlignment="Center" HorizontalOptions="Center"/>
                <Frame.GestureRecognizers>
                    <TapGestureRecognizer NumberOfTapsRequired="1" Tapped="Tab_Tapped"/>
                </Frame.GestureRecognizers>
            </Frame>
            <Frame x:Name="AllNewsTab2" Padding="10,5,10,5" HeightRequest="20" WidthRequest="50" HorizontalOptions="FillAndExpand" CornerRadius="3" HasShadow="False" VerticalOptions="FillAndExpand">
                <Label Text="1" FontSize="Large" HorizontalTextAlignment="Center" VerticalTextAlignment="Center" HorizontalOptions="Center"/>
                <Frame.GestureRecognizers>
                    <TapGestureRecognizer NumberOfTapsRequired="1" Tapped="Tab_Tapped"/>
                </Frame.GestureRecognizers>
            </Frame>
            <Frame x:Name="AllNewsTab3" Padding="10,5,10,5" HeightRequest="20" WidthRequest="50" HorizontalOptions="FillAndExpand" CornerRadius="3" HasShadow="False" VerticalOptions="FillAndExpand">
                <Label Text="2" FontSize="Large" HorizontalTextAlignment="Center" VerticalTextAlignment="Center" HorizontalOptions="Center"/>
                <Frame.GestureRecognizers>
                    <TapGestureRecognizer NumberOfTapsRequired="1" Tapped="Tab_Tapped"/>
                </Frame.GestureRecognizers>
            </Frame>
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

MainPage.cs

    public partial class MainPage : ContentPage
    {
        Frame frameSelected;
       public MainPage()
       {
           InitializeComponent();
       }
       protected override void OnAppearing()
       {
         if (frameSelected == null)
         {
            VisualStateManager.GoToState(AllNewsTab, "Selected");
            frameSelected = AllNewsTab;
         }
          base.OnAppearing();
        }
        private void Tab_Tapped(object sender, EventArgs e)
        {
            if (frameSelected != null)
                VisualStateManager.GoToState(frameSelected, "UnSelected");

            VisualStateManager.GoToState((Frame)sender, "Selected");

            frameSelected = (Frame)sender;
        }
    }