Chinook.DataLoader.Abstractions 1.1.0

Chinook.DataLoader

Customizable async data loading recipes for MVVM development.

🎥 Here we can see the loading, error, and data visual states.

Cornerstones

• Highly Extensible
  • Everything is interface-based to easily allow more implementations.
  • A single framework can't cover everything. Our architecture is designed in a way that allows you to integrate your favorites tools easily.
• Highly Flexible
  • "Data loading" is a broad topic. We aim for our recipes to be flexible enough to support as many use-cases as possible.

Getting Started

1. Install the latest version of Chinook.DataLoader.Uno or Chinook.DataLoader.Uno.WinUI in your project.

2. Add a IDataLoader to your ViewModel.

   public MainPageViewModel()
   {
     MyAsyncValue = DataLoaderBuilder<int>
       .Empty
       .WithName("MyAsyncValue")
       .WithLoadMethod(LoadMyAsyncValue)
       .Build();
   }
   
   public IDataLoader<int> MyAsyncValue { get; }
   
   private async Task<int> LoadMyAsyncValue(CancellationToken ct, IDataLoaderRequest request)
   {
      // Your async operation here.
      await Task.Delay(1000, ct);
      return default(int);
   }
   

   💡 If you use Chinook.DynamicMvvm, you can simplify the declaration. More on that here.

3. Add a DataLoaderView style in your xaml. <details> <summary>Expand to see code sample.</summary> <p>

   You can add the following DataTemplates and Style to you Page.Resources or to a merged dictionary in you App.xaml. Feel free to copy and modify this code.

   <DataTemplate x:Key="DataLoaderViewEmptyTemplate">
        <Grid>
            <TextBlock Text="Nothing to show here"
                        HorizontalAlignment="Center"
                        VerticalAlignment="Top"
                        Margin="0,44,0,0" />
        </Grid>
    </DataTemplate>
   
    <DataTemplate x:Key="DataLoaderViewErrorNotificationTemplate">
        <Grid Background="#570000"
              VerticalAlignment="Bottom"
              Padding="8">
   
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*" />
                <ColumnDefinition Width="Auto" />
            </Grid.ColumnDefinitions>
   
            <TextBlock Foreground="White"
                        HorizontalAlignment="Left"
                        VerticalAlignment="Center">
    <Run Text="Something went wrong." />
    <Run Text="{Binding Error.Message}" />
            </TextBlock>
   
   
            <Button Content="Try Again"
                    Background="#990000"
                    HorizontalAlignment="Center"
                    Command="{Binding View.RefreshCommand}"
                    Grid.Column="1" />
        </Grid>
    </DataTemplate>
   
   
    <DataTemplate x:Key="DataLoaderViewErrorTemplate">
        <Grid>
            <StackPanel VerticalAlignment="Center"
                        HorizontalAlignment="Stretch">
   
   
                <TextBlock Text="Something went wrong"
                            TextAlignment="Center"
                            Margin="0,0,0,8" />
   
   
                <TextBlock Text="Please try again."
                            TextAlignment="Center" />
   
   
                <Button Content="Reload content"
                        HorizontalAlignment="Stretch"
                        Command="{Binding View.RefreshCommand}"
                        Margin="22,24,22,0" />
            </StackPanel>
        </Grid>
    </DataTemplate>
   
    <Style x:Key="DefaultDataLoaderViewStyle"
            TargetType="dl:DataLoaderView">
        <Setter Property="IsTabStop"
                Value="False" />
        <Setter Property="HorizontalAlignment"
                Value="Stretch" />
        <Setter Property="VerticalAlignment"
                Value="Stretch" />
        <Setter Property="HorizontalContentAlignment"
                Value="Stretch" />
        <Setter Property="VerticalContentAlignment"
                Value="Stretch" />
   
        <Setter Property="EmptyTemplate"
                Value="{StaticResource DataLoaderViewEmptyTemplate}" />
        <Setter Property="ErrorTemplate"
                Value="{StaticResource DataLoaderViewErrorTemplate}" />
        <Setter Property="ErrorNotificationTemplate"
                Value="{StaticResource DataLoaderViewErrorNotificationTemplate}" />
   
   
        <Setter Property="StateMinimumDuration"
                Value="0:0:1.5" />
   
   
        <Setter Property="StateChangingThrottleDelay"
                Value="0:0:0.100" />
   
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="dl:DataLoaderView">
                    <Grid>
                        <VisualStateManager.VisualStateGroups>
   
                            <VisualStateGroup x:Name="DataStates">
                                <VisualState x:Name="Initial" />
                                <VisualState x:Name="Data">
                                    <VisualState.Setters>
                                        <Setter Target="ContentPresenter.Visibility"
                                                Value="Visible" />
                                    </VisualState.Setters>
                                </VisualState>
                                <VisualState x:Name="Empty" />
                            </VisualStateGroup>
   
                            <VisualStateGroup x:Name="ErrorStates">
                                <VisualState x:Name="NoError" />
                                <VisualState x:Name="Error" />
                            </VisualStateGroup>
   
                            <VisualStateGroup x:Name="LoadingStates">
                                <VisualState x:Name="NotLoading" />
                                <VisualState x:Name="Loading" >
                                    <VisualState.Setters>
                                        <Setter Target="LoadingIndicator.Visibility"
                                                Value="Visible" />
                                        <Setter Target="LoadingIndicator.IsActive"
                                                Value="True" />
                                    </VisualState.Setters>
                                </VisualState>
                            </VisualStateGroup>
   
                            <VisualStateGroup x:Name="CombinedStates">
                                <VisualState x:Name="Initial_NoError_NotLoading" />
                                <VisualState x:Name="Initial_NoError_Loading" />
   
                                <VisualState x:Name="Initial_Error_NotLoading">
                                    <VisualState.Setters>
                                        <Setter Target="ErrorPresenter.Visibility"
                                                Value="Visible" />
                                    </VisualState.Setters>
                                </VisualState>
                                <VisualState x:Name="Initial_Error_Loading">
                                    <VisualState.Setters>
                                        <Setter Target="ErrorPresenter.Visibility"
                                                Value="Visible" />
                                    </VisualState.Setters>
                                </VisualState>
   
                                <VisualState x:Name="Data_NoError_NotLoading" />
                                <VisualState x:Name="Data_NoError_Loading" />
   
                                <VisualState x:Name="Data_Error_NotLoading">
                                    <VisualState.Setters>
                                        <Setter Target="ErrorNotificationPresenter.Visibility"
                                                Value="Visible" />
                                    </VisualState.Setters>
                                </VisualState>
                                <VisualState x:Name="Data_Error_Loading">
                                    <VisualState.Setters>
                                        <Setter Target="ErrorNotificationPresenter.Visibility"
                                                Value="Visible" />
                                    </VisualState.Setters>
                                </VisualState>
   
                                <VisualState x:Name="Empty_NoError_NotLoading">
                                    <VisualState.Setters>
                                        <Setter Target="EmptyPresenter.Visibility"
                                                Value="Visible" />
                                    </VisualState.Setters>
                                </VisualState>
                                <VisualState x:Name="Empty_NoError_Loading" />
                                <VisualState x:Name="Empty_Error_NotLoading">
                                    <VisualState.Setters>
                                        <Setter Target="ErrorNotificationPresenter.Visibility"
                                                Value="Visible" />
                                    </VisualState.Setters>
                                </VisualState>
                                <VisualState x:Name="Empty_Error_Loading">
                                    <VisualState.Setters>
                                        <Setter Target="ErrorNotificationPresenter.Visibility"
                                                Value="Visible" />
                                    </VisualState.Setters>
                                </VisualState>
                            </VisualStateGroup>
                        </VisualStateManager.VisualStateGroups>
   
                        <ContentPresenter x:Name="ContentPresenter"
                                          Content="{TemplateBinding State}"
                                          ContentTemplate="{TemplateBinding ContentTemplate}"
                                          HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                          Visibility="Collapsed"
                                          Padding="{TemplateBinding Padding}" />
   
                        <ContentPresenter x:Name="EmptyPresenter"
                                          Content="{TemplateBinding State}"
                                          ContentTemplate="{TemplateBinding EmptyTemplate}"
                                          HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                          Visibility="Collapsed"
                                          Padding="{TemplateBinding Padding}" />
   
                        <ContentPresenter x:Name="ErrorPresenter"
                                          Content="{TemplateBinding State}"
                                          ContentTemplate="{TemplateBinding ErrorTemplate}"
                                          HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                          Visibility="Collapsed"
                                          Padding="{TemplateBinding Padding}" />
   
                        <ContentPresenter x:Name="ErrorNotificationPresenter"
                                          Content="{TemplateBinding State}"
                                          ContentTemplate="{TemplateBinding ErrorNotificationTemplate}"
                                          HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                          Visibility="Collapsed"
                                          Padding="{TemplateBinding Padding}" />
   
                        <ProgressRing x:Name="LoadingIndicator"
                                      Foreground="Black"
                                      Visibility="Collapsed"
                                      IsActive="False"
                                      Width="20"
                                      Height="20"
                                      HorizontalAlignment="Center"
                                      VerticalAlignment="Center" />
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
   

   </p></details>

4. Add a DataLoaderView in your xaml.

   
   <dl:DataLoaderView Source="{Binding MyAsyncValue}"
                      Style="{StaticResource DefaultDataLoaderViewStyle}">
     <DataTemplate>
       <StackPanel>
         <TextBlock Text="{Binding Data}" />
       </StackPanel>
     </DataTemplate>
   </dl:DataLoaderView>
   

5. Start your application! Note that by default using a DataLoaderView automatically triggers a initial load from the IDataLoader.

Features

Load async data

The load function of the IDataLoader must simply return a Task of the data type (here MyData).

async Task<MyData> LoadData(CancellationToken ct, IDataLoaderRequest request)
{
  // Put your async operation here.
  return default(MyData);
}

This is where you would typically execute an API call or get data from any other async source.

This method receives a CancellationToken as the operation might be cancelled. Cancellation can happen for a few reasons.

• When the IDataLoader is disposed.
• If another request was made and the concurrent mode is CancelPrevious.

Get the DataLoader's state

You can check the state of a data loader using the IDataLoader.State property. You can observe its changes using the IDataLoader.StateChanged event.

The state of a DataLoader can be summarized as a combination of the following properties.

Example

Imagine a scenario where you land on a page where the data has never been loaded before from an API.

1. The system would first be on an initial state (not loading, no data, no error).
2. The system would then start loading the data from the API.
3. The system would then expose the data returned from the API.

This could be represented by the following state flow.

stateDiagram-v2
  direction LR
  s1: IsLoading(false)<br>Data(null)<br>Error(null)
  s2: <b>IsLoading(true)</b><br>Data(null)<br>Error(null)
  s3: IsLoading(false)<br><b>Data(myData)</b><br>Error(null)

  s1 --> s2 : Start loading
  s2 --> s3 : Receive data

Let's go deeper and imagine that we lose network and do a pull to refresh.

4. You lose connection and refresh the page.
5. The system would then start loading the data from the API.
6. The system notifies you that there was an error, but you still have your previous data.

From the last sequence, we would continue with the following states.

stateDiagram-v2
  direction LR
  s3: IsLoading(false)<br>Data(myData)<br>Error(null)
  s4: <b>IsLoading(true)</b><br>Data(myData)<br>Error(null)
  s5: IsLoading(false)<br>Data(myData)<br><b>Error(WebException)</b>

  s3 --> s4 : Start refresh
  s4 --> s5 : Receive error<br>because not network

Other useful state properties are available:

Define IsEmpty

Apps often have a specific visual when content is empty. You can configure your DataLoader to evaluate the IDataLoaderState.IsEmpty property based on a predicate of your choice. Simply use IDataLoaderBuilder.WithEmptySelector.

MyAsyncList = DataLoaderBuilder<List<int>>
  .Empty
  .WithName("MyAsyncList")
  .WithLoadMethod(LoadMyAsyncList)
  .WithEmptySelector(state => state.Data is null || state.Data.Count == 0)
  .Build();

DataLoaderView

The DataLoaderView is the UI equivalent of a IDataLoader. It represents the different states of the IDataLoader using the following visual states. This is the list of all supported visual states and their respective visual state group.

Here is the table of all possible visual state combinations.

💡 You don't need to use all those states. It's possible to create a decent style without using the CombinedStates at all. The CombinedStates are available to offer support for advanced scenarios.

DataLoaderView also has a few template properties to more easily define its ControlTemplate. You can use any combination of the following in you styles. Remember that the source of truth comes from the visual states.

💡 Again, you don't need to use all those properties. It's very possible that you don't use ErrorNotificationTemplate at all. Perhaps your style only uses ContentTemplate.

Protection against UI flickering

To avoid seeing flickery transitions between the different visual states, the DataLoaderView supports different throttling options.

• StateMinimumDuration: This represents the minimum duration of a visual state. You should keep this high enough to prevent visual states from changing too fast. This is the property that prevents flickers.
• StateChangingThrottleDelay: This represents the time before updating to a new state during which another update can happen. You should keep this low enough to prevent slowing down your user experience. This is the property that prevents seeing loading states that are not necessary.

<dl:DataLoaderView Source="{Binding MyDataLoader}"
                   StateMinimumDuration="0:0:1.5"
                   StateChangingThrottleDelay="0:0:0.100" />

Triggers

It's possible to add IDataLoaderTrigger objects to a DataLoader using IDataLoader.AddTrigger. This can be useful to automatically refresh the data. It also clearly indicates what are the dependencies of DataLoader.

There are a few implementations provided.

• ManualDataLoaderTrigger: Manually triggers a load request.
• ObservableDataLoaderTrigger: Triggers a load request when the observable pushes a new value.

There are some more implementations in the Chinook.DataLoader.DynamicMvvm package.

• DynamicCommandDataLoaderTrigger: Triggers a load request when the IDynamicCommand.IsExecuting property changes.
• DynamicPropertyDataLoaderTrigger: Triggers a load request when the value of a IDynamicProperty changes.

You can also create your own triggers by inheriting from DataLoaderTriggerBase. Simply call RaiseLoadRequested when a load request should be requested. You can also write a trigger from scratch by implementing IDataLoaderTrigger.

💡 DataLoaderView has a AutoLoad property which, as its name suggests, automatically requests an initial load from the DataLoader when it's displayed. This property defaults to true which means that you don't need to do anything to trigger the initial load when you use DataLoaderView.

Protection against concurrent loads

Now that we know what can trigger loads, we can see how it could be possible to request loads too fast and cause concurrent loads. The DataLoader doesn't allow concurrent loads. There are currently two behaviors to choose from when concurrent loads would happen.

• CancelPrevious: When there's a new request while a previous one is already loading, the previous request gets cancelled. This is the default behavior.
• DiscardNew: When there's a new request while a previous one is already loading, the new request gets discarded (it doesn't even start).

You can set the desired behavior using IDataLoaderBuilder.WithConcurrentMode.

Requests and contexts

To provide metadata on the loading operation, two important pieces are used.

• IDataLoaderRequest: Represents the request to load data.
• IDataLoaderContext: Contains user-defined key-value-pairs. It's exposed from the IDataLoaderRequest.

You can get or set additional information from the IDataLoader using the IDataLoaderRequest parameter.

Task<MyData> LoadData(CancellationToken ct, IDataLoaderRequest request)
{
  // This is the trigger that caused that loading operation.
  var sourceTrigger = request.SourceTrigger;

  // This is the context that was passed to the loading operation from the trigger.
  var context = request.Context;

  // You can read and write using the context.
  context["myMessage"] = "Hello!";

  // Your async operation here.
}

The request is also available as part of the IDataLoaderState and DataLoaderViewState. This means both request and context can be used in XAML data binding. This can be useful when implementing custom styles of DataLoaderView.

You can bind to the context from a style's ControlTemplate as follows.

<ControlTemplate TargetType="dl:DataLoaderView">
  <Grid>
    
    <TextBlock Text="{Binding State.Request.Context[myMessage], RelativeSource={RelativeSource Mode=TemplatedParent}"/>
  </Grid>
</ControlTemplate>

Triggers can also provide values through the context. Note however that the values provided by a trigger are only accessible from the load function when it originated from that trigger.

Integration with Chinook.DynamicMvvm

If you use Chinook.DynamicMvvm, know that there are built-in extensions methods that allow for fluent declaration of IDataLoader.

1. Add the Chinook.DataLoader.DynamicMvvm package to your project.
2. Register an IDataLoaderBuilderFactory in the IServiceProvider that your ViewModels use. Here is a simple code sample that does that using Microsoft.Extensions.DependencyInjection and Microsoft.Extensions.Hosting.

   var serviceProvider = new HostBuilder()
     .ConfigureServices(serviceCollection => serviceCollection
       .AddSingleton<IDataLoaderBuilderFactory, IDataLoaderBuilderFactory>()
     )
     .Build()
     .Services;
   

3. Use IViewModel.GetDataLoader to declare your DataLoader.
4. Optionally use IViewModel.GetCommandFromDataLoaderRefresh if you want a command that refreshes the DataLoader. (This can be useful for things like pull-to-refresh.)

public class MyViewModel : ViewModelBase
{
  public IDynamicCommand RefreshData => this.GetCommandFromDataLoaderRefresh(Data);

  public IDataLoader Data => this.GetDataLoader(GetData, b => b
    // You can optionally configure your data loader here
  );

  private async Task<MyData> GetData(CancellationToken ct)
  {
    // Get data from your API...
  }
}

💡 You can install the Visual Studio Extension Chinook Snippets and use code snippets to quickly generate DataLoader declarations when using Chinook.DynamicMvvm. All snippets related to IDataLoader start with ckdl (for Chinook DataLoader).

Automatically dispose previous data

Using IDataLoaderBuilder.DisposePreviousData, you can configure an IDataLoader that will automatically dispose its data when new data is loaded. This works when Data directly implements IDisposable or when it's an IEnumerable containing items implementing IDisposable.

public IDataLoader<DadJokesItemViewModel[]> Jokes => this.GetDataLoader(LoadJokes, b => b
    // Dispose the previous ItemViewModels when Jokes produces new values.
    .DisposePreviousData()
);

Automatically re-evaluate IsEmpty

Some types change on their own. A good example is ObservableCollection. DataLoader supports connecting to its Data in order to automatically update its state. Using IDataLoaderBuilder.UpdateOnCollectionChanged, you can configure an IDataLoader that will automatically re-evaluate IsEmpty when the collection changes. This works with any collection that implements INotifyCollectionChanged.

public IDataLoader<ObservableCollection<ToDoItem>> ToDoList => this.GetDataLoader(LoadToDoList, b => b
    // Re-evaluate is Empty when the collection changes.
    .UpdateOnCollectionChanged()
);

For types that don't implement INotifyCollectionChanged, you can use IDataLoaderBuilder.SubscribeToData to implement an equivalent behavior.

⚙ Note that UpdateOnCollectionChanged is actually implemented using SubscribeToData. The SubscribeToData building block allows this kind of behavior for any type that is observable, meaning an object that exposes an event, implements or exposes IObservable<T>, or has any other subscription mechanism.

Automatically dispose subscriptions

You might need intermediate objects that needs disposing when loading data. A good example is the subscription when creating an observable collection using DynamicData.

public IDataLoader<ReadOnlyObservableCollection<int>> MyList => this.GetDataLoader(LoadMyList);

private async Task<ReadOnlyObservableCollection<int>> LoadMyList(CancellationToken ct, IDataLoaderRequest request)
{
  DynamicData.IObservableList<int> itemsSource = await GetItems(ct);
  itemsSource
    .Connect()
    .Bind(out var list)
    .Subscribe()
    // Dispose this subscription when the next one is created.
    .DisposeWithNextLoad(request);

  return list;
}

Using the IDisposable.DisposeWithNextLoad extensions method, you can ensure that only 1 disposable stays active at a time. Alternatively, you can use the IDataLoaderRequest.AddToSequenceDisposableGroup extension method which does the same thing.

Note that even when there are no subsequent loads, the disposable still gets disposed when the IDataLoader itself gets disposed.

Load on a background thread

Using IDataLoaderBuilder.OnBackgroundThread, you can configure an IDataLoader that will invoke its load function from Task.Run. This can be helpful to avoid loading data on the UI thread.

public IDataLoader<string> Stuff => this.GetDataLoader(LoadStuff, b => b
    // Invoke the load from a new task.
    .OnBackgroundThread()
);

Define default behaviors

Using DataLoaderBuilderFactory, you can create a IDataLoaderBuilderFactory that provides default behaviors in all the IDataLoaderBuilder instances it creates. Here you can see how to create an IDataLoaderBuilderFactory using Microsoft.Extensions.DependencyInjection.

private static IServiceCollection AddDataLoaderFactory(this IServiceCollection services)
{
  return services.AddSingleton<IDataLoaderBuilderFactory>(s =>
  {
    return new DataLoaderBuilderFactory(b => b
      .OnBackgroundThread()
      .WithEmptySelector(GetIsEmpty)
    );

    bool GetIsEmpty(IDataLoaderState state)
    {
      return state.Data == null || (state.Data is IEnumerable enumerable &&    !enumerable.Cast<object>().Any());
    }
  });
}

💡 When using Chinook.DynamicMvvm, the IViewModel.GetDataLoader extension methods automatically use IDataLoaderBuilderFactory registered in the IServiceProvider to generate the IDataLoader instances.

Support for custom behaviors

The architecture of IDataLoader and IDataLoaderBuilder was made in a way that more custom behaviors can be implemented. Using IDataLoaderBuilder.DelegatingStrategies, you can add processing both before and after the actual load function in the same fashion as you would using a stack of System.Net.Http.HttpMessageHandler. Combining this with triggers, IDataLoaderRequest, and IDataLoaderContext, brings a lot of flexibility because the strategy has access to the request and context objects.

⚙ Even the recipes we've seen so far (like SubscribeToData or DisposeWithNextLoad) are built on top of that extensible foundation.

License

This project is licensed under the Apache 2.0 license - see the LICENSE file for details.

Contributing

Please read CONTRIBUTING.md for details on the process for contributing to this project.

Be mindful of our Code of Conduct.