Understanding The MVVM Pattern (TechDays Belgium)

Understanding the MVVM pattern Laurent Bugnion Senior User Experience Integrator IdentityMine http://[blog|www].galasoft.ch

What is MVVM? • A weird acronym • Model – View – ViewModel • « MVVM in a SLOOB with SLUT » • Woven into WPF/SL • Some confusion: • MVVM is really just the pattern • The rest are helpers, practices, etc

The MVC pattern Model View Controller

The Passive View pattern Model View Controller

The Presentation Model pattern Model View DataBinding Presentation Model (ViewModel)

Blendability • Testability, Maintainability, Blendability • “The ability to be edited in Blend” • (and Visual Studio designer…) • Differentiate code in design and runtime • Create design time data

Hello MVVM (and Hi Blend)

Bridging the gap View DataBinding Commands Messages ViewModel Model

Bridging the gap View View Messages ViewModel ViewModel View Messages + callback ViewModel

Commands • “Point of entry” for a method” • Can be data bound • ICommand interface • Execute method • CanExecute method • CanExecuteChanged event handler

Commands and RelayCommands

RelayCommands RelayCommand MyCommand = new RelayCommand( () => ..., () => true | false); RelayCommand<T> MyGenericCommand = new RelayCommand<T>( p => ..., p => true | false);

Messaging • Should be simple by default • Possible to open filtered channels • To a target Type (or an Interface) • With a Token • With various Messenger instances • No constraints at all

Messaging

Messaging Messenger.Default.Register<T>( this, p => DoSomething(p)); Messenger.Default.Send<T>(p);

MVVM Light Toolkit • “Breaking the monotony” • http://www.galasoft.ch/mvvm/getstarted • http://mvvmlight.codeplex.com • V3 released at MIX10 • Keep it small • Keep it simple

MVVM Light Libraries V3 • ESSENTIALS • RelayCommand • Messenger • ViewModelBase • EXTRAS • EventToCommand • DispatcherHelper

MVVM Light Tools V3 • Project templates (VS + Blend) • Item templates (VS + Blend) • Code snippets

MVVM Light Toolkit, Unity, Blendability

Flaws of MVVM • Too much code needed • INotifyPropertyChanged • Commands • Some operations are tricky (set focus…) • Not applicable to ASP.NET, etc • Message to community is not clear

Debunking the myths • “No code in the code behind” • “If you put code in the View's code-behind, the MVVM police will take your family away” • “No need for converters” • “MVVM is only suitable for big projects” • “MVVM costs performance” • “MVVM is complex / makes my head hurt”

Debunking the myths • “I don’t need separation because I don’t have a designer” • “You cannot use MVVM with other patterns” • “MVVM will make you rich and attractive”

MVVM on the Windows Phone 7

http://blog.galasoft.ch http://www.galasoft.ch/mvvm/getstarted @LBugnion

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Understanding The MVVM Pattern (TechDays Belgium)

by Laurent Bugnion

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