xaml mvvm(2)之属性绑定

通过微软INotifyPropertyChanged接口，可以实现对UI实时更新，不管是数据源或者目标对象，可以实现相互通知。

下面我们根据INotifyPropertyChanged编写一个扩展类。该类是基于C#5.0特性，这里我们介绍一下System.Runtime.CompilerServices命名空间下的CallerMemberName特性，当RaisePropertyChanged的属性名称参数为空，而通过编译器可以智能加上，可以通过反编译工具知晓，这点改进这点很人性化。注：如果开发版本framework 4.0，则需要安装KB2468871补丁或者更新framework 4.0以上版本。

为这个扩展类添加命名空间如下。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq.Expressions;
using System.Reflection;
using System.Runtime.CompilerServices;

public class ObservableObject : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    protected PropertyChangedEventHandler PropertyChangedHandler
    {
        get
        {
            return this.PropertyChanged;
        }
    }
    [Conditional("DEBUG"), DebuggerStepThrough]
    public void VerifyPropertyName(string propertyName)
    {
        Type type = base.GetType();
        if (!string.IsNullOrEmpty(propertyName) && type.GetTypeInfo().GetDeclaredProperty(propertyName) == null)
        {
            throw new ArgumentException("Property not found", propertyName);
        }
    }
    protected virtual void RaisePropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChangedEventHandler propertyChanged = this.PropertyChanged;
        if (propertyChanged != null)
        {
            propertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
    protected virtual void RaisePropertyChanged<T>(Expression<Func<T>> propertyExpression)
    {
        PropertyChangedEventHandler propertyChanged = this.PropertyChanged;
        if (propertyChanged != null)
        {
            string propertyName = ObservableObject.GetPropertyName<T>(propertyExpression);
            propertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
    protected static string GetPropertyName<T>(Expression<Func<T>> propertyExpression)
    {
        if (propertyExpression == null)
        {
            throw new ArgumentNullException("propertyExpression");
        }
        MemberExpression memberExpression = propertyExpression.Body as MemberExpression;
        if (memberExpression == null)
        {
            throw new ArgumentException("Invalid argument", "propertyExpression");
        }
        PropertyInfo propertyInfo = memberExpression.Member as PropertyInfo;
        if (propertyInfo == null)
        {
            throw new ArgumentException("Argument is not a property", "propertyExpression");
        }
        return propertyInfo.Name;
    }
    protected bool Set<T>(Expression<Func<T>> propertyExpression, ref T field, T newValue)
    {
        if (EqualityComparer<T>.Default.Equals(field, newValue))
        {
            return false;
        }
        field = newValue;
        this.RaisePropertyChanged<T>(propertyExpression);
        return true;
    }
    protected bool Set<T>(string propertyName, ref T field, T newValue)
    {
        if (EqualityComparer<T>.Default.Equals(field, newValue))
        {
            return false;
        }
        field = newValue;
        this.RaisePropertyChanged(propertyName);
        return true;
    }
    protected bool Set<T>(ref T field, T newValue, [CallerMemberName] string propertyName = null)
    {
        return this.Set<T>(propertyName, ref field, newValue);
    }
}

下面我们来继承这个类编写一个Animal类对象。在这里我们用到RaisePropertyChanged三种不同的实现方式，达到一样的绑定效果。

public class Animal : ObservableObject
{
    private string m_Cat;
    public string Cat
    {
        get { return m_Cat; }
        set { m_Cat = value; RaisePropertyChanged("Cat"); }
    }

    private string m_Dog;
    public string Dog
    {
        get { return m_Dog; }
        set { m_Dog = value; RaisePropertyChanged(); }
    }

    private string m_Tiger;
    public string Tiger
    {
        get { return m_Tiger; }
        set { m_Tiger = value; RaisePropertyChanged(() => this.Tiger); }
    }
}

下面我们来建立model视图类。在该类中用的事件绑定和model对象实现，我们会在后续介绍。

public class MainPageViewModel : Core.ViewModelBase
{
    public MainPageViewModel()
    {
        MyAnimal = new Animal();
    }

    private Animal m_MyAnimal;
    public Animal MyAnimal
    {
        get { return m_MyAnimal; }
        set { m_MyAnimal = value; RaisePropertyChanged("MyAnimal"); }
    }

    public ICommand OKCommand
    {
        get
        {
            return new RelayCommand(() =>
            {
                MyAnimal.Dog = "eating";
                MyAnimal.Cat = "sleeping";
                MyAnimal.Tiger = "hungry";
            });
        }
    }
}

前台xaml。

<Grid DataContext="{Binding Path=MainPageViewModel}">
    <StackPanel>
        <StackPanel Orientation="Horizontal">
            <TextBlock FontSize="25" Text="cat is:" />
            <TextBlock FontSize="25" Text="{Binding MyAnimal.Cat}" />
        </StackPanel>
        <StackPanel Orientation="Horizontal">
            <TextBlock FontSize="25" Text="dog is:" />
            <TextBlock FontSize="25" Text="{Binding MyAnimal.Dog}" />
        </StackPanel>
        <StackPanel Orientation="Horizontal">
            <TextBlock FontSize="25" Text="Tiger is:" />
            <TextBlock FontSize="25" Text="{Binding MyAnimal.Tiger}" />
        </StackPanel>
        <Button Width="60" Content="OK" Command="{Binding OKCommand}" />
    </StackPanel>
</Grid>

运行效果。