Drawing Grid on Canvas and Retaining during Navigation

Question

I hope some one can help.

I am working on a project were I can draw a grid on a Canvas but when you navigate away then back the grid has been removed.

The navigation process does not result in new views.

How can I retain the grid between navigation or at least redraw when navigating back. The drawing process relies upon Behaviors therefore I cannot use such Events as Loaded as the Canvas Behavior reference is null.

I have included a link to a demonstration project.

text

Thankyou for your assistance.

Answer 1

Hi,
one of possible solution can be this: XAML of OneView without UserControl.DataContext:

<UserControl x:Class="NavigationTest.Views.OneView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:vm="clr-namespace:NavigationTest.ViewModels"
             xmlns:behaviors="http://schemas.microsoft.com/xaml/behaviors"
             xmlns:bh="clr-namespace:NavigationTest.Behaviors"
             mc:Ignorable="d" 
             d:DesignHeight="450" d:DesignWidth="800">

    <!--<UserControl.DataContext>
        <vm:OneViewModel/>
    </UserControl.DataContext>-->
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="52"/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <StackPanel Orientation="Horizontal">
            <Label Grid.Row="0" FontSize="36" HorizontalAlignment="Center" VerticalAlignment="Center" Content="Home"/>
            <CheckBox Content="Draw Grid" Margin="10,5,10,5" IsChecked="{Binding DrawGrid}" VerticalAlignment="Center"/>
            <TextBlock Text="Grid Spacing" VerticalAlignment="Center" Margin="10"/>
            <TextBox Width="50" Text="{Binding GridSpacing}" Margin="10"/>
        </StackPanel>
        <Grid Grid.Row="1">
            <Canvas x:Name="BackgroundCanvas" Width="800" Height="450">
                <behaviors:Interaction.Behaviors>
                    <bh:BackgroundCanvasBehavior/>
                </behaviors:Interaction.Behaviors>
            </Canvas>
        </Grid>
    </Grid>
</UserControl>

An changed code for OneViewModel:

using NavigationTest.Helpers;
using System.Windows.Controls;
using System.Windows.Media;

namespace NavigationTest.ViewModels
{
	public class OneViewModel : BaseViewModel
	{
		private Canvas _backgroundCanvas;

		public Canvas BackgroundCanvas
		{
			get => this._backgroundCanvas;
			set
			{
				this._backgroundCanvas = value;
				if (_DrawGrid) GoDrawGrid();
			}
		}

		private bool _DrawGrid;
		public bool DrawGrid
		{
			get { return _DrawGrid; }
			set { if (_DrawGrid != value) _DrawGrid = value; OnPropertyChanged(nameof(DrawGrid)); GoDrawGrid(); }
		}

		private int _GridSpacing = 20;
		public int GridSpacing
		{
			get { return _GridSpacing; }
			set { if (_GridSpacing != value) _GridSpacing = value; OnPropertyChanged(nameof(GridSpacing)); }
		}

		public OneViewModel()
		{

		}

		private void GoDrawGrid()
		{
			BackgroundCanvas.Children.Clear();
			if (DrawGrid == true)
			{
				SolidColorBrush gridColorBrush = new SolidColorBrush(Colors.Red);
				int SelectedScale = 1;//Temp
				bool AutoScaleGrid = true;//Temp
				ShowGrid showGrid = new ShowGrid(BackgroundCanvas, gridColorBrush);
				showGrid.CreateGrid(GridSpacing, SelectedScale, AutoScaleGrid);
			}

		}
	}
}