在本章中，我们将介绍如何连接ViewModel.这是我们讨论View第一个结构的最后一章的延续.现在，第一种结构的下一种形式是元模式，它被称为 ViewModelLocator .它是一个伪模式，并且位于MVVM模式之上.
在MVVM中，每个View都需要连接起来ViewModelLocator是一种集中代码和解耦视图的简单方法.
这意味着它不必明确了解ViewModel类型以及如何构建它.
有许多不同的方法使用ViewModelLocator，但在这里我们使用与PRISM框架中最相似的那个.
ViewModelLocator提供标准，一致的，声明性和松散耦合的方式来查看第一个构造，自动化ViewModel连接到View的过程.下图表示ViewModelLocator的高级过程.
第1步 : 找出正在构建的View类型.
第2步 : 确定特定视图类型的ViewModel.
第3步 : 构建ViewModel.
第4步 : 将Views DataContext设置为ViewModel.
要理解基本概念，让我们通过继续上一章中的相同示例来查看ViewModelLocator的简单示例.如果查看StudentView.xaml文件，您将看到我们已静态连接ViewModel.
现在，如下面的程序所示，注释这些XAML代码也会删除代码代码隐藏.
<UserControl x:Class = "MVVMDemo.Views.StudentView"
xmlns = "http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x = "http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc = "http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d = "http://schemas.microsoft.com/expression/blend/2008"
xmlns:local = "clr-namespace:MVVMDemo.Views"
xmlns:viewModel = "clr-namespace:MVVMDemo.ViewModel"
mc:Ignorable = "d" d:DesignHeight = "300" d:DesignWidth = "300">
<!--<UserControl.DataContext>
<viewModel:StudentViewModel/>
</UserControl.DataContext>-->
<Grid>
<StackPanel HorizontalAlignment = "Left">
<ItemsControl ItemsSource = "{Binding Path = Students}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<StackPanel Orientation = "Horizontal">
<TextBox Text = "{Binding Path = FirstName, Mode = TwoWay}"
Width = "100" Margin = "3 5 3 5"/>
<TextBox Text = "{Binding Path = LastName, Mode = TwoWay}"
Width = "100" Margin = "0 5 3 5"/>
<TextBlock Text = "{Binding Path = FullName, Mode = OneWay}"
Margin = "0 5 3 5"/>
</StackPanel>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
</StackPanel>
</Grid>
</UserControl>现在让我们创建一个新的文件夹VML并添加一个新的公共类ViewModelLocator，它将包含一个附加属性(依赖属性)AutoHookedUpViewModel，如下面的代码所示.
public static bool GetAutoHookedUpViewModel(DependencyObject obj) {
return (bool)obj.GetValue(AutoHookedUpViewModelProperty);
}
public static void SetAutoHookedUpViewModel(DependencyObject obj, bool value) {
obj.SetValue(AutoHookedUpViewModelProperty, value);
}
// Using a DependencyProperty as the backing store for AutoHookedUpViewModel.
//This enables animation, styling, binding, etc...
public static readonly DependencyProperty AutoHookedUpViewModelProperty =
DependencyProperty.RegisterAttached("AutoHookedUpViewModel",
typeof(bool), typeof(ViewModelLocator), new PropertyMetadata(false,
AutoHookedUpViewModelChanged));现在你可以看到一个基本的附件属性定义.要向属性添加行为，我们需要为此属性添加更改的事件处理程序，其中包含连接ViewModel for View的自动过程.执行此操作的代码如下 :
private static void AutoHookedUpViewModelChanged(DependencyObject d,
DependencyPropertyChangedEventArgs e) {
if (DesignerProperties.GetIsInDesignMode(d)) return;
var viewType = d.GetType();
string str = viewType.FullName;
str = str.Replace(".Views.", ".ViewModel.");
var viewTypeName = str;
var viewModelTypeName = viewTypeName + "Model";
var viewModelType = Type.GetType(viewModelTypeName);
var viewModel = Activator.CreateInstance(viewModelType);
((FrameworkElement)d).DataContext = viewModel;
}以下是ViewModelLocator类的完整实现.
using System;
using System.ComponentModel;
using System.Windows;
namespace MVVMDemo.VML {
public static class ViewModelLocator {
public static bool GetAutoHookedUpViewModel(DependencyObject obj) {
return (bool)obj.GetValue(AutoHookedUpViewModelProperty);
}
public static void SetAutoHookedUpViewModel(DependencyObject obj, bool value) {
obj.SetValue(AutoHookedUpViewModelProperty, value);
}
// Using a DependencyProperty as the backing store for AutoHookedUpViewModel.
//This enables animation, styling, binding, etc...
public static readonly DependencyProperty AutoHookedUpViewModelProperty =
DependencyProperty.RegisterAttached("AutoHookedUpViewModel",
typeof(bool), typeof(ViewModelLocator), new
PropertyMetadata(false, AutoHookedUpViewModelChanged));
private static void AutoHookedUpViewModelChanged(DependencyObject d,
DependencyPropertyChangedEventArgs e) {
if (DesignerProperties.GetIsInDesignMode(d)) return;
var viewType = d.GetType();
string str = viewType.FullName;
str = str.Replace(".Views.", ".ViewModel.");
var viewTypeName = str;
var viewModelTypeName = viewTypeName + "Model";
var viewModelType = Type.GetType(viewModelTypeName);
var viewModel = Activator.CreateInstance(viewModelType);
((FrameworkElement)d).DataContext = viewModel;
}
}
}首先要做的是添加命名空间，以便我们可以访问ViewModelLocator输入我们项目的根目录.然后在作为视图类型的route元素上，添加AutoHookedUpViewModel属性并将其设置为true.
xmlns:vml = "clr-namespace:MVVMDemo.VML" vml:ViewModelLocator.AutoHookedUpViewModel = "True"
以下是StudentView.xaml文件的完整实现.
<UserControl x:Class = "MVVMDemo.Views.StudentView"
xmlns = "http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x = "http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc = "http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d = "http://schemas.microsoft.com/expression/blend/2008"
xmlns:local = "clr-namespace:MVVMDemo.Views"
xmlns:viewModel = "clr-namespace:MVVMDemo.ViewModel"
xmlns:vml = "clr-namespace:MVVMDemo.VML"
vml:ViewModelLocator.AutoHookedUpViewModel = "True"
mc:Ignorable = "d" d:DesignHeight = "300" d:DesignWidth = "300">
<!--<UserControl.DataContext>
<viewModel:StudentViewModel/>
</UserControl.DataContext>-->
<Grid>
<StackPanel HorizontalAlignment = "Left">
<ItemsControl ItemsSource = "{Binding Path = Students}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<StackPanel Orientation = "Horizontal">
<TextBox Text = "{Binding Path = FirstName, Mode = TwoWay}"
Width = "100" Margin = "3 5 3 5"/>
<TextBox Text = "{Binding Path = LastName, Mode = TwoWay}"
Width = "100" Margin = "0 5 3 5"/>
<TextBlock Text = "{Binding Path = FullName, Mode = OneWay}"
Margin = "0 5 3 5"/>
</StackPanel>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
</StackPanel>
</Grid>
</UserControl>编译并执行上述代码时，您将看到ViewModelLocator正在连接该特定View的ViewModel.
要注意的一个关键是视图不再耦合一种方式，了解ViewModel的类型或构造方式.这一切都被移到了ViewModelLocator内部的中心位置.
