Getting started with mvvm-light

Remarks#

MVVM-light is a toolkit written in C# which helps to accelerate the creation and development of MVVM applications in WPF, Silverlight, Windows Store, Windows Phone and Xamarin.

Webpage: https://www.mvvmlight.net/

There is a cross-platform MVVM sample from the author of the library which can be found at https://github.com/lbugnion/sample-crossplatform-flowers

RelayCommand

The RelayCommand implements the ICommand interface and can therefore be used to bind to Commands in XAML (as the Command property of the Button element)

The constructor takes two arguments; the first is an Action which will be executed if ICommand.Execute is called (e.g. the user clicks on the button), the second one is a Func<bool> which determines if the action can be executed (defaults to true, called canExecute in the following paragraph).

the basic structure is as follows:

public ICommand MyCommand => new RelayCommand(
    () =>
    {
        //execute action
        Message = "clicked Button";
    },
    () =>
    {
        //return true if button should be enabled or not
        return true;
    }
);

Some notable effects:

If canExecute returns false, the Button will be disabled for the user
Before the action is really executed, canExecute will be checked again
You can call MyCommand.RaiseCanExecuteChanged(); to force reevaluation of the canExecute Func

RelayCommand

The RelayCommand<T> is similar to the RelayCommand, but allows to directly pass an object to the command. It implements the ICommand interface and can therefore be used to bind to Commands in XAML (e.g. as the Command property of the Button element). You can then use the CommandParameter property to pass the object to the command.

XAML example:

<Button Command="{Binding MyCommand}" CommandParameter="{Binding MyModel}" />

The constructor takes two arguments; the first is an Action which will be executed if ICommand.Execute is called (e.g. the user clicks on the button), the second one is a Func<string,bool> which determines if the action can be executed (defaults to true, called canExecute in the following paragraph). the basic structure is as follows:

public RelayCommand<string> MyCommand => new RelayCommand<string>(
    obj =>
    {
        //execute action
        Message = obj;
    },
    obj =>
    {
        //return true if button should be enabled or not
        return obj != "allowed";
    }
);

Some notable effects:

If canExecute returns false, the Button will be disabled for the user
Before the action is really executed, canExecute will be checked again
You can call MyCommand.RaiseCanExecuteChanged(); to force reevaluation of the canExecute Func

ObservableObject

The ObservableObject class contains some helpful methods to help with the MVVM pattern.

The RaisePropertyChanged provides a compile safe method to raise property changed events.
It can be called with

RaisePropertyChanged(() => MyProperty);

The Set method can be used in the property setter to set the new value and raise the property changed event (only if change occurred). It returns true if change occurred and false otherwise.
example usage:

private string _myValue;
public string MyValue    
{
    get { return _myValue; }
    set { Set(ref _myValue, value); }
}

ViewModelBase

ViewModelBase extends ObservableObject and adds some methods useful for viewmodels.

The property IsInDesignMode or IsInDesignModeStatic allows to determine if the code is executed in the design mode (in Visual Studio Design View) or not. The two properties are exactly the same.