It is now recommended to use Microsoft's MVVM Community Toolkit as a preferred alternative to this package. Its source generators allow to write boilerplate-free view models without any runtime penalty.

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fluentmvvm

An easy-to-use, fast ViewModelBase providing an intuitive fluent API to express property setters naturally.

Contents

• Features
• Fluent API
• Performance
• License

Further reading

• API Documentation
• How it works: Creating backing fields at runtime
• How it works: Improved performance with string interning

Features

To make use of fluentmvvm's features, the first step is to download the NuGet package.

Then simply let your view models inherit from FluentViewModelBase.

public class PersonViewModel : FluentViewModelBase

Property Declaration

Instead of defining backing fields for each property, simply use Get<T>() for the property getter and Set(value) for the property setter. fluentmvvm will create the backing fields at runtime, so that one can write the following code without having to worry about the backing field for the property:

public string Name
{
    get => this.GetString();
    set => this.Set(value);
}

Set raises a PropertyChanged event only if the new value is different from the old value.

There are overloads such as GetBoolean, GetInt32, etc. for all primitive types as well as GetDecimal, GetDateTime, and GetString. The Get<T> method can be used for all other value and reference types (structs, enums, classes).

Remarks

Note that this mechanism only works for public instance properties with a set accessor.

Not creating backing fields at runtime

If you do not want backing fields to be created at runtime, you can pass false to the createBackingFields parameter of FluentViewModelBase. You can then still use the fluent API with the Set<T>(T value, ref T oldValue, ...) overload.

Dependencies between properties

For cases where a (computed) property's value depends on other properties' values, Affects can be used after Set.

It would raise a PropertyChanged event for the specified property if the new value was different from the old one.

public string FullName => $"{this.FirstName} {this.LastName}";

public string FirstName
{
    get => this.GetString();
    set => this.Set(value)
               .Affects(nameof(this.FullName));
}

public string LastName
{
    get => this.GetString();
    set => this.Set(value)
               .Affects(nameof(this.FullName));
}

Dependencies between properties and commands

For cases when a commands CanExecute is based on some property's value, Affects can be used with the command to raise the CanExecuteChanged event for.

public ICommand BuyThingsCommand { get; }

public int Balance
{
    get => this.GetInt32();
    set => this.Set(value)
             .Affects(this.BuyThingsCommand);
}

Remarks

Affects accepts a parameter of type ICommand. However, if the command does not contain a

public void RaiseCanExecuteChanged()

method, an exception is thrown. You can also derive from the interface IWpfCommand provided by fluentmvvm to make sure that the method mentioned above exists on your command implementations.

Fluent API

FluentViewModelBase provides a fluent API that can be used in property setters.

this.When(<condition>)        // never or once
    .Set(value)               // exactly once
    .Affects(<property name>) // never, once or many times
    .Affects(<command>)       // never, once or many times
    .WasUpdated();            // never or once

Overview

• If the condition passed to When evaluates to false, the calls to Set and Affects do not execute anything.

• Set checks whether the new value differs from the current value of the property. If it does differ, it sets the value to the property and raises a PropertyChanged event. If it does not, nothing happens, and all following Affects calls do not execute either.

• When expressing a dependency between one property and another, Affects can be used with the name of the property to raise a PropertyChanged event for. The event is raised only if the new value of the property differed from the old value.

• If your command implementations derive from IWpfCommand, dependencies between properties and commands can also be expressed using Affects. That way, when the new value of the property differs from the old value, a CanExecuteChanged event is raised for the command.

• WasUpdated returns a bool value indicating whether the property value was updated (and thus whether a PropertyChanged event was raised for the property).

Performance

The benchmark result below compares fluentmvvm to a naive implementation without using any Set methods (called "default" in the figure) and to a more sophisticated approach (called "expression" in the figure) that provides a Set<T>(Expression<Func<T>> propertyExpression, T oldValue, T newValue) method (for example, Galasoft.MvvmLight does).

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19041.685 (2004/?/20H1)
Intel Core i7-9750H CPU 2.60GHz, 1 CPU, 12 logical and 6 physical cores
.NET Core SDK=5.0.101
  [Host]     : .NET Core 3.1.10 (CoreCLR 4.700.20.51601, CoreFX 4.700.20.51901), X64 RyuJIT
RunStrategy=Throughput

License

(c) Thomas Flinkow 2019 - 2021 · GitHub · email

Distributed under the MIT license. See LICENSE for more information.