mvvmlight

Xamarin Forms, the MVVMLight Toolkit and I: Command Chaining

Xamarin Forms, the MVVMLight Toolkit and I: Command Chaining

The problem

Sometimes, we want to invoke a method that is available via code for a control. Due to the abstraction of our MVVM application, the ViewModel has no access to all those methods that are available if we would access the control via code. There are several approaches to solve this problem. In one of my recent projects, I needed to invoke a method of a custom control, which should be routed into the platform renderers I wrote for such a custom control. I remembered that I have indeed read quite a few times about command chaining for such cases and tried to implement it. In the beginning, it may sound weird to do this, but the more often I see this technique, the more I like it.

Simple Demo control

For demo purposes, I created this really simple Xamarin.Forms user control:

<?xml version="1.0" encoding="UTF-8"?>
<ContentView xmlns="http://xamarin.com/schemas/2014/forms" 
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="XfMvvmLight.Controls.CommandChainingDemoControl">
  <ContentView.Content>
      <StackLayout HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand">
            <Label x:Name="LabelFilledFromBehind" Margin="12" FontSize="Large" />   
        </StackLayout>
  </ContentView.Content>
</ContentView>

As you can see, there is just a label without text. We will write the necessary code to fill this label with some text just by invoking a method in the code behind. To be able to do so, we need a BindableProperty (once again) to get our foot into the door of the control:

public static BindableProperty DemoCommandProperty = BindableProperty.Create(nameof(DemoCommand), typeof(ICommand), typeof(CommandChainingDemoControl), null, BindingMode.OneWayToSource);

public ICommand DemoCommand
{
    get => (ICommand)GetValue(DemoCommandProperty);
    set => SetValue(DemoCommandProperty, value);
}

The implementation is pretty straightforward. We have done this already during this series, so you should be familiar if you were following. One thing, however, is different. For this BindableProperty, we are using BindingMode.OneWayToSource. By doing so, we are basically making it a read-only property, which sends its changes only down to the ViewModel (the source). If we would not do this, the ViewModel could change the property, which we do not want here.

Now we have the BindableProperty in place, we need to create an instance of the Command that will be sent down to the ViewModel. We are doing this as soon as the control is instantiated in the constructor:

public CommandChainingDemoControl()
      {
          InitializeComponent();

          this.DemoCommand = new Command(() =>
          {
              FillFromBehind();
          });
      }

      private void FillFromBehind()
      {
          this.LabelFilledFromBehind.Text = "Text was empty, but we used command chaining to show this text inside a control.";
      }

That’s all we need to do in the code behind.

ViewModel

For this demo, I created a new page and a corresponding ViewModel in the demo project. Here is the very basic ViewModel code:

using System.Windows.Input;
using GalaSoft.MvvmLight.Command;

namespace XfMvvmLight.ViewModel
{
    public class CommandChainingDemoViewModel : XfNavViewModelBase
    {
        private ICommand _invokeDemoCommand;
        private RelayCommand _demo1Command;

        public CommandChainingDemoViewModel()
        {
        }

        public ICommand InvokeDemoCommand { get => _invokeDemoCommand; set => Set(ref _invokeDemoCommand, value); }

        public RelayCommand Demo1Command => _demo1Command ?? (_demo1Command = new RelayCommand(() =>
        {
            this.InvokeDemoCommand?.Execute(null);
        }));
    }
}

As you can see, the ViewModel includes two Commands. One is the pure ICommand implementation that gets its value from the OneWayToSource-Binding. We are not using MVVMLight’s RelayCommand here to avoid casting between types, which always led to an exception when I tested the implementation first. The second command is bound to a button in the CommandChainingDemoPage and will be the trigger to execute the InvokeDemoCommand.

Final steps

The final steps are just a few simple ones. We need to connect the  InvokeDemoCommand to the user control we created earlier, while we need to bind the Demo1Commandto the corresponding button in the view. This is the page’s code after doing so:

<?xml version="1.0" encoding="utf-8" ?>
<baseCtrl:XfNavContentPage
    xmlns:baseCtrl="clr-namespace:XfMvvmLight.BaseControls" xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:ctrl="clr-namespace:XfMvvmLight.Controls;assembly=XfMvvmLight"
             x:Class="XfMvvmLight.View.CommandChainingDemoPage" RegisteredPageKey="{Binding CommandChainingDemoPageKey, Source=Locator}">
    <ContentPage.BindingContext>
        <Binding Path="CommandChainingDemoVm" Source="{StaticResource Locator}" />
    </ContentPage.BindingContext>

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>

        <ctrl:CommandChainingDemoControl Grid.Row="0" DemoCommand="{Binding InvokeDemoCommand, Mode=OneWayToSource}" Margin="12"></ctrl:CommandChainingDemoControl>

        <Button Text="Execute Command Chaining" Command="{Binding Demo1Command}" Margin="12" Grid.Row="1" />

    </Grid>
</baseCtrl:XfNavContentPage>

One thing to point out is that we are also specifying the OneWayToSource binding here once again. It should work with normal binding, but it I recommend to do like I did, which makes the code easier to understand for others (and of course yourself). That’s all – we have now a working command chain that invokes a method inside the user control from our ViewModel.

Conclusion

Command chaining can be a convenient way to invoke actions on controls that are otherwise not possible due to the abstraction of layers in MVVM. Once you got the concept, they are pretty easy to implement. This technique is also usable outside of Xamarin.Forms, so do not hesitate to use it out there. Just remember the needed steps:

  • create a user control (or a derived one if you need to call a method on framework controls)
  • add a BindableProperty/DependecyProperty and set its default binding mode to OneWayToSource
  • instantiate the BindableProperty/DependecyProperty inside the constructor of the user control
  • pass the method call/code into the Action part of the newly created Command  instance
  • create the commands in the ViewModel
  • connect the Commands to your final view implementation

Like I wrote earlier, I came across this (again) when I was writing a custom Xamarin.Forms control with renderers, where I had to invoke methods inside the renderer from my ViewModel. Other techniques that I saw to solve this is using Messengers (be it the one from MVVMLight or the Xamarin.Forms Messenger implementation) or the good old Boolean switch implementation (uses also a BindableProperty/DependecyProperty). I decided to use the command chaining approach as it is pretty elegant in my eyes and not that complicated to implement.

The series’ sample project is updated and available here on Github. Like always, I hope this post is useful for some of you.

Happy coding, everyone!


all articles of this series

title image credit

Posted by msicc in Android, Dev Stories, iOS, UWP, Xamarin, 6 comments
Xamarin Forms, the MVVMLight Toolkit and I: migrating the Forms project and MVVMLight to .NET Standard

Xamarin Forms, the MVVMLight Toolkit and I: migrating the Forms project and MVVMLight to .NET Standard

When I started this series, Xamarin and Xamarin.Forms did not fully support .NET Standard. The sample project for this series has still a portable class library, and as I wanted to blog on another topic, I got reminded that I never updated the project. This post will be all about the change to .NET Standard, focusing on the Xamarin.Forms project as well as the MVVMLight library, which is also available as a .NET Standard version in the meantime.

Step-by-Step

I have done the necessary conversion steps already quite a few times, and to get you through the conversion very quickly, I will show you a set of screenshots. Where applicable, I will provide also some XML snippets that you can copy and paste.

Step 1

step1_unload project

The first step is to unload the project file. To do so, select your Xamarin Forms project in Solution Explorer and right click again on it bring up the context menu. From there, select “Unload Project“.

Step 2

step2_edit_csproj

Next, right-click the selected Xamarin.Forms project again to make the context menu visible once again. From there select “Edit [PROJECTNAME].csproj” to open up the file.

Step 3

step3_replace_all

Now that the file is open, press “CTRL + A”, followed by “DEL”. Seriously, just do it. Once the file is empty, copy these lines and paste them into your now empty file:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>

</Project>

Step 4

step4_open_packages_config

This step is not really necessary, but I want to point one thing out. The old PCL-project type has listed quite a bunch of libraries in the packages.config file. These are all NuGet packages necessary to make the app actually run (they get pulled in during the install of a NuGet package as dependencies). Now that we are converting, we are getting rid of the packages.config file. You can delete right away in the Solution Explorer. We will “install” the needed packages (marked with the arrows) in

Step 5

step5_changed_csproj

For “installing” NuGet packages into the converted project, we are adding a new ItemGroup to the XML-File. The Package System.ValueTuple is referenced – because in this series’ sample project we have some code in there that uses it. The absolute minimum you need to get it running is:

<!--
Template for Nuget Packages:
<ItemGroup>
  <PackageReference Include="" Version="" />
</ItemGroup>
-->

<ItemGroup>
  <!--MvvmLight has changed, so do we!-->
  <PackageReference Include="MvvmLightLibsStd10" Version="5.4.1" />
  <!--needed references-->
  <PackageReference Include="Xamarin.Forms" Version="3.1.0.583944" />
</ItemGroup>

If you have other NuGet packages, just add them into this item group. They will get installed in the correct version if you follow this tutorial until the end.

You might have noticed that the MVVMLight package I am inserting here is not the same as before. This is absolutely true, but for a reason. Laurent Bugnion has published the .NET Standard version quite some time ago. If you want to read his blog post, you can find it here.

The second change I want to outline is DebugTypesettings. These are also not set in that way if you create a new project that is already .NET Standard. In order to enable debugging of your Xamarin.Forms code, however, you absolutely should also add these lines:

<!--these enable debugging in the Forms project-->
<PropertyGroup Condition=" '$(Configuration)'=='Debug' ">
  <DebugType>full</DebugType>
  <DebugSymbols>true</DebugSymbols>
  <GenerateDocumentationFile>false</GenerateDocumentationFile>
</PropertyGroup>
<PropertyGroup Condition=" '$(Configuration)'=='Release' ">
  <DebugType>pdbonly</DebugType>
  <GenerateDocumentationFile>true</GenerateDocumentationFile>
</PropertyGroup>

These properties provide the debug symbols and the pdb-files a lot of analytic services are dependent on. Simple put, copy and paste them into your project. Hit the “Save”-Button and close the file.

Step 6

step6_delete_obj

Now that we have changed the project file, our already downloaded packages and created dll-files are no longer valid. To make sure we are able to compile, we need to delete the content of the obj folder in the Folder View of the Solution Explorer in Visual Studio.

Step 7

step7_delete_bin

Do the same to the content of the bin folder and switch back to the Solution View by hitting the Folder View-Button again

Step 8

step8_reload_project

Now we are finally able to reload the project as the base conversion is already done.

Step 9

step9_fix errors

Trying to be optimistic and hit the Rebuild button will result in these (and maybe even more) errors. The first one is one that we can solve very fast, while the second one is only the first in a row of errors.

Step 10

step10_delete_properties_folder

To solve the assembly attributes error, just go to the Folder View again and select the Properties folder. Bring up the context menu by right-clicking on it and select the delete option. Confirm the deletion to get rid of the folder. The new project style creates the assembly information based on the project file during build, which is causing the errors.

Step 11

step11_unneeded_reference

Now let’s face the next error. As the MVVMLight .NET Standard version does no longer rely on the CommonServiceLocatorlike before, we are able to remove this reference from our ViewModelLocator.

Step 12

step12_unneeded_instantiation

Of course, we now can also remove the instantiation call for the removed ServiceLocator.

Step 13

step13_replace_servicelocator_calls

In the ViewModel instance reference, replace ServiceLocator.Current with SimpleIoc.Default. Hit the save button again. You might have more errors to fix. Do so, and finally save your ViewModelLocator.

Step 14

step14_rebuild_succeeded

After all the work is done, we are now able to compile the .NET Standard version of our Xamarin.Forms project. If you fixed all of your errors in the step before, you should achieve a similar result like me and get a success message.

Final steps

Now that the Xamarin.Forms project is built, you might want to try to build all other projects in the solution as well.  The changed structure of the Xamarin.Forms project will have an impact also on the platform projects, that’s why I absolutely recommend deleting the contents of bin and object folders there, too. This will solve you a lot of work to get things to compile again.

As always, I hope this post is helpful for some of you. If you have any questions, feel free to ping me on my social accounts or write a comment below. The next post in this series will involve again more code, so stay tuned –  it will be out soon.

Please find the updated sample project here on GitHub.

Happy coding, everyone!

title image credit

Posted by msicc in Dev Stories, Xamarin, 1 comment