The ViewModel pattern

The ViewModel Pattern
Simone ChiarettaArchitect, Council of the EU
http://codeclimber.net.nz
Twitter: @simonech
June 23rd, 2010

Who the hell am I?
Simone Chiaretta
Microsoft MVP ASP.NET
ASP Insider
Blogger – http://codeclimber.net.nz
ItalianALT.NET UG Founder
OpenSource developer
Climber
All Around Nice Guy
Disclaimer:"The viewsexpressed are purelythose of the speaker and may not in anycircumstancesberegarded as stating an official position of the Council"

WhatisViewModel

Workflow of a MVC Application
Controller asks for the data to the Model
The request gets to the Controller
Model
2
1
3
Controller
Browser
Model returns the data back to the Controller
Controller formats data and sends them to the View
View
4
5
View builds the page that is sent back to the Browser
3

Workflow of a MVC Application
Controller asks for the DomainModel to the BLL
The request gets to the Controller
BLL
2
1
3
Controller
Browser
BLL returns the DomainModel to the Controller
Controller formats DomainModelinto ViewModeland sends it to the View
View
4
5
View builds the page that is sent back to the Browser
4

DomainModel != ViewModel
DomainModel
Data + Behaviours
Hierarchical, complex types
ViewModel
Only Data
Flat, only strings

WhyDomainModelisnotgood?
Viewsshouldnotknowhowto traverse the DM
Viewsusuallyneedlessproperties
UsingORMsyoumight start a SQL querybymistake

Howto do it?
Copy the propertiesneededfrom DM to VM
Possiblyflatten data

DomainModel != ViewModel
How to avoid getting bored writing tedious mapping code?

IntroducingAutoMapper

Automapper
DevelopedbyJimmyBogard
Latestrelease 1.1
Active Mailing list
Downloadablefrom: http://automapper.codeplex.com/

Features
Flattening
Projection
FluentAPIconfiguration
Mapping of List and Collections
Mapping of NestedObjects
CustomTypeConverter
CustomValueResolver
CustomFormatters
NullSubstitution

Basic Usage
DefineMapping
Mapper.CreateMap<Post, ShowPostModel>();
UseMapping
Mapper.Map<ListModel, ListViewModel>(viewModel)

Projection
Mapper.CreateMap<EditPageViewModel, Post>()
.ForMember(
p => p.Id,
opt => opt.MapFrom(src => src.Post.Id))

CustomValueResolver
Mapper.CreateMap<EditPageViewModel, Post>()
.ForMember(
p => p.Category,
opt => opt.ResolveUsing<CustomResolver>()
)
public class CustomResolver : ValueResolver<EditPageViewModel, Category>
{
protected override Category ResolveCore(
EditPageViewModel source)
{
return new InMemoryPostModel()
.GetCategory(source.Post.CategoryId);
}
}

NullSubstitution
Mapper.CreateMap<Post, ShowPostModel>()
.ForMember(
p => p.CategoryName,
opt => opt.NullSubstitute("No Category")
);

Contacts – Simone Chiaretta
MSN: simone_ch@hotmail.com
Blog:
English: http://codeclimber.net.nz/
Italian: http://blogs.ugidotnet.org/piyo/
Twitter: @simonech
16

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Disclaimer:"The viewsexpressed are purelythose of the speaker and may not in anycircumstancesberegarded as stating an official position of the Council"

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The ViewModel pattern

by Simone Chiaretta

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