A Simple MVVM Example[Forward]
In my opinion, if you are using WPF or Silverlight you should be using the MVVM design pattern. It is perfectly suited to the technology and allows you to keep your code clean and easy to maintain.
The problem is, there are a lot of online resources for MVVM, each with their own way of implementing the design pattern and it can be overwhelming. I would like to present MVVM in the simplest way possible using just the basics.
So lets start at the beginning.
MVVM
MVVM is short for Model-View-ViewModel.
Models are simple class objects that hold data. They should only contain properties and property validation. They are not responsible for getting data, saving data, click events, complex calculations, business rules, or any of that stuff.
Views are the UI used to display data. In most cases, they can be DataTemplates which is simply a template that tells the application how to display a class. It is OK to put code behind your view IF that code is related to the View only, such as setting focus or running animations.
ViewModels are where the magic happens. This is where the majority of your code-behind goes: data access, click events, complex calculations, business rules validation, etc. They are typically built to reflect a View. For example, if a View contains a ListBox of objects, a Selected object, and a Save button, the ViewModel will have an ObservableCollection ObectList, Model SelectedObject, and ICommand SaveCommand.
MVVM Example
I’ve put together a small sample showing these 3 layers and how they relate to each other. You’ll notice that other than property/method names, none of the objects need to know anything about the others. Once the interfaces have been designed, each layer can be built completely independent of the others.
Sample Model
For this example I’ve used a Product Model. You’ll notice that the only thing this class contains is properties and change notification code.
Usually I would also implement IDataErrorInfo here for property validation, however I have left this out for now.
public class ProductModel : ObservableObject
{
#region Fields private int _productId;
private string _productName;
private decimal _unitPrice; #endregion // Fields #region Properties public int ProductId
{
get { return _productId; }
set
{
if (value != _productId)
{
_productId = value;
OnPropertyChanged("ProductId");
}
}
} public string ProductName
{
get { return _productName; }
set
{
if (value != _productName)
{
_productName = value;
OnPropertyChanged("ProductName");
}
}
} public decimal UnitPrice
{
get { return _unitPrice; }
set
{
if (value != _unitPrice)
{
_unitPrice = value;
OnPropertyChanged("UnitPrice");
}
}
} #endregion // Properties
}
The class inherits from ObservableObject, which is a custom class I use to avoid having to rewrite the property change notification code repeatedly. I would actually recommend looking into Microsoft PRISM’s NotificationObject or MVVM Light’s ViewModelBase which does the same thing once you are comfortable with MVVM, but for now I wanted to keep 3rd party libraries out of this and to show the code.
public abstract class ObservableObject : INotifyPropertyChanged
{
#region INotifyPropertyChanged Members /// <summary>
/// Raised when a property on this object has a new value.
/// </summary>
public event PropertyChangedEventHandler PropertyChanged; /// <summary>
/// Raises this object's PropertyChanged event.
/// </summary>
/// <param name="propertyName">The property that has a new value.</param>
protected virtual void OnPropertyChanged(string propertyName)
{
this.VerifyPropertyName(propertyName); if (this.PropertyChanged != null)
{
var e = new PropertyChangedEventArgs(propertyName);
this.PropertyChanged(this, e);
}
} #endregion // INotifyPropertyChanged Members #region Debugging Aides /// <summary>
/// Warns the developer if this object does not have
/// a public property with the specified name. This
/// method does not exist in a Release build.
/// </summary>
[Conditional("DEBUG")]
[DebuggerStepThrough]
public virtual void VerifyPropertyName(string propertyName)
{
// Verify that the property name matches a real,
// public, instance property on this object.
if (TypeDescriptor.GetProperties(this)[propertyName] == null)
{
string msg = "Invalid property name: " + propertyName; if (this.ThrowOnInvalidPropertyName)
throw new Exception(msg);
else
Debug.Fail(msg);
}
} /// <summary>
/// Returns whether an exception is thrown, or if a Debug.Fail() is used
/// when an invalid property name is passed to the VerifyPropertyName method.
/// The default value is false, but subclasses used by unit tests might
/// override this property's getter to return true.
/// </summary>
protected virtual bool ThrowOnInvalidPropertyName { get; private set; } #endregion // Debugging Aides
}
In addition to the INotifyPropertyChanged methods, there is also a debug method to validate the PropertyName. This is because the PropertyChange notification gets passed in as a String, and I have caught myself forgetting to change this string when I change the name of a Property.
Note: The PropertyChanged notification exists to alert the View that a value has changed so it knows to update. I have seen suggestions to drop it from the Model and to expose the Model’s properties to the View from the ViewModel instead of the Model, however I find in most cases this complicates things and requires extra coding. Exposing the Model to the View via the ViewModel is much simpler, although either method is valid.
Sample ViewModel
I am doing the ViewModel next because I need it before I can create the View. This should contain everything the User would need to interact with the page. Right now it contains 4 properties: a ProductModel, a GetProduct command, a SaveProduct command, an a ProductId used for looking up a product.
public class ProductViewModel : ObservableObject
{
#region Fields private int _productId;
private ProductModel _currentProduct;
private ICommand _getProductCommand;
private ICommand _saveProductCommand; #endregion #region Public Properties/Commands public ProductModel CurrentProduct
{
get { return _currentProduct; }
set
{
if (value != _currentProduct)
{
_currentProduct = value;
OnPropertyChanged("CurrentProduct");
}
}
} public ICommand SaveProductCommand
{
get
{
if (_saveProductCommand == null)
{
_saveProductCommand = new RelayCommand(
param => SaveProduct(),
param => (CurrentProduct != null)
);
}
return _saveProductCommand;
}
} public ICommand GetProductCommand
{
get
{
if (_getProductCommand == null)
{
_getProductCommand = new RelayCommand(
param => GetProduct(),
param => ProductId > 0
);
}
return _getProductCommand;
}
} public int ProductId
{
get { return _productId; }
set
{
if (value != _productId)
{
_productId = value;
OnPropertyChanged("ProductId");
}
}
} #endregion #region Private Helpers private void GetProduct()
{
// You should get the product from the database
// but for now we'll just return a new object
ProductModel p = new ProductModel();
p.ProductId = ProductId;
p.ProductName = "Test Product";
p.UnitPrice = 10.00;
CurrentProduct = p;
} private void SaveProduct()
{
// You would implement your Product save here
} #endregion
}
There is another new class here: the RelayCommand. This is essential for MVVM to work. It is a command that is meant to be executed by other classes to run code in this class by invoking delegates. Once again, I’d recommend checking out the MVVM Light Toolkit’s version of this command when you are more comfortable with MVVM, but I wanted to keep this simple so have included this code here.
/// <summary>
/// A command whose sole purpose is to relay its functionality to other
/// objects by invoking delegates. The default return value for the
/// CanExecute method is 'true'.
/// </summary>
public class RelayCommand : ICommand
{
#region Fields readonly Action<object> _execute;
readonly Predicate<object> _canExecute; #endregion // Fields #region Constructors /// <summary>
/// Creates a new command that can always execute.
/// </summary>
/// <param name="execute">The execution logic.</param>
public RelayCommand(Action<object> execute)
: this(execute, null)
{
} /// <summary>
/// Creates a new command.
/// </summary>
/// <param name="execute">The execution logic.</param>
/// <param name="canExecute">The execution status logic.</param>
public RelayCommand(Action<object> execute, Predicate<object> canExecute)
{
if (execute == null)
throw new ArgumentNullException("execute"); _execute = execute;
_canExecute = canExecute;
} #endregion // Constructors #region ICommand Members [DebuggerStepThrough]
public bool CanExecute(object parameters)
{
return _canExecute == null ? true : _canExecute(parameters);
} public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
} public void Execute(object parameters)
{
_execute(parameters);
} #endregion // ICommand Members
}
Sample View
And now the Views. These are DataTemplates which define how a class should be displayed to the User. There are many ways to add these templates to your application, but the simplest way is to just add them to the startup window’s Resources.
<Window.Resources>
<DataTemplate DataType="{x:Type local:ProductModel}">
<Border BorderBrush="Black" BorderThickness="1" Padding="20">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions> <TextBlock Grid.Column="0" Grid.Row="0" Text="ID" VerticalAlignment="Center" />
<TextBox Grid.Row="0" Grid.Column="1" Text="{Binding ProductId}" /> <TextBlock Grid.Column="0" Grid.Row="1" Text="Name" VerticalAlignment="Center" />
<TextBox Grid.Row="1" Grid.Column="1" Text="{Binding ProductName}" /> <TextBlock Grid.Column="0" Grid.Row="2" Text="Unit Price" VerticalAlignment="Center" />
<TextBox Grid.Row="2" Grid.Column="1" Text="{Binding UnitPrice}" /> </Grid>
</Border>
</DataTemplate> <DataTemplate DataType="{x:Type local:ProductViewModel}">
<DockPanel Margin="20">
<DockPanel DockPanel.Dock="Top">
<TextBlock Margin="10,2" DockPanel.Dock="Left" Text="Enter Product Id" VerticalAlignment="Center" /> <TextBox Margin="10,2" Width="50" VerticalAlignment="Center" Text="{Binding Path=ProductId, UpdateSourceTrigger=PropertyChanged}" /> <Button Content="Save Product" DockPanel.Dock="Right" Margin="10,2" VerticalAlignment="Center"
Command="{Binding Path=SaveProductCommand}" Width="100" /> <Button Content="Get Product" DockPanel.Dock="Right" Margin="10,2" VerticalAlignment="Center"
Command="{Binding Path=GetProductCommand}" IsDefault="True" Width="100" />
</DockPanel> <ContentControl Margin="20,10" Content="{Binding Path=CurrentProduct}" />
</DockPanel>
</DataTemplate>
</Window.Resources>
The View defines two DataTemplates: one for the ProductModel, and one for the ProductViewModel. You’ll need to add a namespace reference to the Window definition pointing to your Views/ViewModels so you can define the DataTypes. Each DataTemplate only binds to properties belonging to the class it is made for.
In the ViewModel template, there is a ContentControl that is bound to ProductViewModel.CurrentProduct. When this control tries to display the CurrentProduct, it will use the ProductModel DataTemplate.
Starting the Sample
And finally, to start the application add the following on startup:
MainWindow app = new MainWindow();
ProductViewModel viewModel = new ProductViewModel();
app.DataContext = viewModel;
app.Show();
This is found in the code behind the startup file – usually App.xaml.cs.
This creates your Window (the one with the DataTemplates defined in Window.Resources), creates a ViewModel, and it sets the Window’s DataContext to the ViewModel.
And there you have it. A basic look at MVVM.
UPDATE
Sample code can be found here.
Notes
There are many other ways to do the things shown here, but I wanted to give you a good starting point before you start diving into the confusing world of MVVM.
The important thing to remember about using MVVM is your Forms, Pages, Buttons, TextBoxes, etc (the Views) are NOT your application. Your ViewModels are. The Views are merely a user-friendly way to interact with your ViewModels.
So if you want to change pages, you should not be changing pages in the View, but instead you should be setting something like the AppViewModel.CurrentPage = YourPageViewModel. If you want to run a Save method, you don’t put that behind a button’s Click event, but rather bind the Button.Command to a ViewModel’s ICommand property.
I started with Josh Smith’s article on MVVM, which was a good read but for a beginner like me, some of these concepts flew right over my head.
I’ve never done a blog or tutorial before, but I noticed there is a lot of confusion about what MVVM is and how to use it. Since I struggled through the maze of material online to figure out what MVVM is and how its used, I thought I’d try and write a simpler explanation. I hope this clarifies things a bit and doesn’t make it worse :)
>> Next – Navigation with MVVM
A Simple MVVM Example[Forward]的更多相关文章
- MVVM开发模式简单实例MVVM Demo
本文主要是翻译Rachel Lim的一篇有关MVVM模式介绍的博文 A Simple MVVM Example 并具体给出了一个简单的Demo(原文是以WPF开发的,对于我自己添加或修改的一部分会用红 ...
- WPF MVVM使用prism4.1搭建
WPF MVVM使用prism4.1搭建 MVVM即Model-View-ViewModel,MVVM模式与MVP(Model-View-Presenter)模式相似,主要目的是分离视图(View)和 ...
- WPF MVVM模式的一些理解
/*本文转自 http://www.cnblogs.com/sirkevin/archive/2012/11/28/2793471.html */ 使用WPF+Mvvm开发一年多,期间由于对Mvvm模 ...
- 使用Simple MvvmToolkit开发Android和iOS程序
详情见:Android and iOS Development with Simple MVVM Toolkit? Yes you can! :http://blog.tonysneed.com/20 ...
- 实用的MVVM:ImageView
最近在学习WPF,遇到一本入门好书,推荐给大家<Windows Presentation Foundation 4.5 Cookbook> 做项目首先要从MVVM开始,现在把他的Simpl ...
- Android Mvvm模式的理解
1. Mvvm是什么,Mvvm是怎么来的?Mvvm模式广泛应用在WPF项目开发中,使用此模式可以把UI和业务逻辑分离开,使UI设计人员和业务逻辑人员能够分工明确. Mvvm模式是根据MVP模式来的,可 ...
- WCF学习系列二---【WCF Interview Questions – Part 2 翻译系列】
http://www.topwcftutorials.net/2012/09/wcf-faqs-part2.html WCF Interview Questions – Part 2 This WCF ...
- A Complete List of .NET Open Source Developer Projects
http://scottge.net/2015/07/08/a-complete-list-of-net-open-source-developer-projects/?utm_source=tuic ...
- MVC Controller Dependency Injection for Beginners【翻译】
在codeproject看到一篇文章,群里的一个朋友要帮忙我翻译一下顺便贴出来,这篇文章适合新手,也算是对MEF的一个简单用法的介绍. Introduction In a simple stateme ...
随机推荐
- innobackupex 重启MySQL
当重新修改了MySQL的数据目录时: 重启报错: Starting MySQL.The server quit without updating PID file (/[FAILED]l/mysql/ ...
- 【实战】初识ListView及提高效率
简介: ListView是手机上最常用的控件之一,几乎所有的程序都会用到,手机屏幕空间有限,当需要显示大量数据的时候,就需要借助ListView来实现,允许用户通过手指上下滑动的方式将屏幕外的数据滚动 ...
- React Native填坑之旅--HTTP请求篇
如果不能从头到尾的建立一个RN应用,那么RN将失色不少.本以为HTTP请求部分需要使用Native的实现,Android和iOS各回各家,各调各库了.Google了一下之后居然RN可以使用fetch库 ...
- (light OJ 1005) Rooks dp
http://www.lightoj.com/volume_showproblem.php?problem=1005 PDF (English) Statistics Forum Tim ...
- python 处理中文文件时的编码问题,尤其是utf-8和gbk
python代码文件的编码 py文件默认是ASCII编码,中文在显示时会做一个ASCII到系统默认编码的转换,这时就会出错:SyntaxError: Non-ASCII character.需要在代码 ...
- [Xamarin] 關於Internal Storage ,存取App內部使用資料 (转帖)
最近在開發App,會使用到必須要處理一些App所使用的資料,上網路查一下Android 得作法,包含我自己也實作了一下,可能是因為對Java || Android 不是很孰悉,常常錯在 java.la ...
- Dynamic CRM 2013学习笔记(二十八)用JS动态设置字段的change事件、必填、禁用以及可见
我们知道通过界面设置字段的change事件,是否是必填,是否可见非常容易.但有时我们需要动态地根据某些条件来设置,这时有需要通过js来动态地控制了. 下面分别介绍如何用js来动态设置. 一.动态设 ...
- 用sass画蜗牛
一.sass的好处 用css画图也算是简单的实战吧,虽然用到的东西还比较少..用过之后,发现sass主要有以下优势: 可维护性.最重要的一点,可维护性的很大一部分来自变量 嗯,最简单的例子,画图总要有 ...
- C#,Java,C -循环冗余检验:CRC-16-CCITT查表法
C#代码 using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace ...
- C#与数据库访问技术总结(十三)之DataReader对象
DataReader对象与数据获取 DataReader对象以“基于连接”的方式来访问数据库. 也就是说,在访问数据库.执行SQL操作时,DataReader要求一直连在数据库上. 这将会给数据库的连 ...