XF Grid使用-两行两列-跟WPF不同

<?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:local="clr-namespace:App8"
             x:Class="App8.MainPage">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"></RowDefinition>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"></ColumnDefinition>
        </Grid.ColumnDefinitions>
        <BoxView Color="Red" Grid.Row="0" Grid.Column="0"></BoxView>
        <BoxView Color="Black" Grid.Row="0" Grid.Column="1"></BoxView>
        <BoxView Color="Yellow" Grid.Row="1" Grid.Column="0"></BoxView>
        <BoxView Color="Green" Grid.Row="1" Grid.Column="1"></BoxView>
    </Grid>

</ContentPage>