(WPF, MVVM) Slider Binding.

對於Button的Command的綁定可以通過實現ICommand接口來進行，但是Slider並沒有Command屬性。

另外如果要實現MVVM模式的話，需要將一些Method和Slider的Event進行綁定，如何進行呢？

（對於UIElement的一些Event進行綁定一定有一些通用的方法，目前還沒有深入研究。）

 

首先，Slider Value的綁定是很簡單的, 綁定Slider的Value屬性即可。

（1）ViewModel

    public class SliderViewModel : ViewModelBase
    {
        private string selectedValue;

        public SliderViewModel()
        {

        }

        public string SelectedValue
        {
            get
            {
                return this.selectedValue;
            }
            set
            {
                if (this.selectedValue != value)
                {
                    this.selectedValue = value;
                    base.OnPropertyChanged("SelectedValue"); 
                }
            }
        }
    }

(2) View, 設定 DataContext 為ViewModel， 綁定SelectValue到 Slider的Value 和TextBlock的Text屬性上。

這樣當拖動Slider時，Slider的值會傳給SelectedValue, 然后SelectValue會傳給TexBlock上。

 x:Class="WpfApplication2.View.SliderView"
             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:vm="clr-namespace:WpfApplication2.ViewModel"
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <UserControl.DataContext>
        <vm:SliderViewModel />
    </UserControl.DataContext>
    <Grid>
        <Slider HorizontalAlignment="Left"
                Margin="28,102,0,0"
                VerticalAlignment="Top"
                Width="158" 
                Minimum="0"
                Maximum="100"
                Value="{Binding SelectedValue}"/>
        <TextBlock HorizontalAlignment="Left"
                   Margin="203,102,0,0"
                   TextWrapping="Wrap"
                   Text="{Binding SelectedValue}"
                   VerticalAlignment="Top" />

    </Grid>
</UserControl>

效果如下：

如果不想顯示double值，可以設定Slider的屬性。

                TickFrequency="1"
                IsSnapToTickEnabled="True"
                TickPlacement="None" />

其次， 當用鼠標或者鍵盤移動滑塊結束的時候，需要進行一些處理。如果不用MVVM的方式的話，可以在Slider的Event里面增加處理。

用MVVM的話，就稍微麻煩一些。

（1）下載或者復制C:\Program Files (x86)\Microsoft SDKs\Expression\Blend\.NETFramework\v4.0\Libraries\System.Windows.Interactivity.dll

        在工程中Reference 這個dll，命名空間里面增加：using System.Windows.Interactivity

 （2）新寫個SliderValueChangedBehavior 繼承Behavior<Slider>

    /// <summary>
    /// Helps find the user-selected value of a slider only when the keyboard/mouse gesture has ended.
    /// </summary>
    public class SliderValueChangedBehavior : Behavior<Slider>
    {
        /// <summary>
        /// Keys down.
        /// </summary>
        private int keysDown;

        /// <summary>
        /// Indicate whether to capture the value on latest key up.
        /// </summary>
        private bool applyKeyUpValue;

        #region Dependency property Value

        /// <summary>
        /// DataBindable value.
        /// </summary>
        public double Value
        {
            get { return (double)GetValue(ValueProperty); }
            set { SetValue(ValueProperty, value); }
        }

        public static readonly DependencyProperty ValueProperty = DependencyProperty.Register(
            "Value",
            typeof(double),
            typeof(SliderValueChangedBehavior),
            new PropertyMetadata(default(double), OnValuePropertyChanged));

        #endregion

        #region Dependency property Value

        /// <summary>
        /// DataBindable Command
        /// </summary>
        public ICommand Command
        {
            get { return (ICommand)GetValue(CommandProperty); }
            set { SetValue(CommandProperty, value); }
        }

        public static readonly DependencyProperty CommandProperty = DependencyProperty.Register(
            "Command",
            typeof(ICommand),
            typeof(SliderValueChangedBehavior),
            new PropertyMetadata(null));

        #endregion

        /// <summary>
        /// On behavior attached.
        /// </summary>
        protected override void OnAttached()
        {
            this.AssociatedObject.KeyUp += this.OnKeyUp;
            this.AssociatedObject.KeyDown += this.OnKeyDown;
            this.AssociatedObject.ValueChanged += this.OnValueChanged;

            base.OnAttached();
        }

        /// <summary>
        /// On behavior detaching.
        /// </summary>
        protected override void OnDetaching()
        {
            base.OnDetaching();

            this.AssociatedObject.KeyUp -= this.OnKeyUp;
            this.AssociatedObject.KeyDown -= this.OnKeyDown;
            this.AssociatedObject.ValueChanged -= this.OnValueChanged;
        }

        /// <summary>
        /// On Value dependency property change.
        /// </summary>
        private static void OnValuePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var me = (SliderValueChangedBehavior)d;
            if (me.AssociatedObject != null)
                me.Value = (double)e.NewValue;
        }

        /// <summary>
        /// Occurs when the slider's value change.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void OnValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
        {
            if (Mouse.Captured != null)
            {
                this.AssociatedObject.LostMouseCapture += this.OnLostMouseCapture;
            }
            else if (this.keysDown != 0)
            {
                this.applyKeyUpValue = true;
            }
            else
            {
                this.ApplyValue();
            }
        }

        private void OnLostMouseCapture(object sender, MouseEventArgs e)
        {
            this.AssociatedObject.LostMouseCapture -= this.OnLostMouseCapture;
            this.ApplyValue();
        }

        private void OnKeyUp(object sender, KeyEventArgs e)
        {
            if (this.keysDown-- != 0)
            {
                this.ApplyValue();
            }
        }

        private void OnKeyDown(object sender, KeyEventArgs e)
        {
            this.keysDown++;
        }

        /// <summary>
        /// Applies the current value in the Value dependency property and raises the command.
        /// </summary>
        private void ApplyValue()
        {
            this.Value = this.AssociatedObject.Value;

            if (this.Command != null)
                this.Command.Execute(this.Value);
        }
    }

 （3） 在View中為Slider的行為添加Command

            <i:Interaction.Behaviors>
                <helper:SliderValueChangedBehavior Command="{Binding ValueChangedCommand}"
                                                  />
            </i:Interaction.Behaviors>

（4）在ViewModel中實現當Slider值個改變的時候進行一些處理。

        public ICommand ValueChangedCommand
        {
            get
            {
                if (this.valueChangedCommmand == null)
                {
                    this.valueChangedCommmand = new RelayCommand(
                        param => this.PopValue(),
                        null);
                }

                return this.valueChangedCommmand; 
            }
        }

        private void PopValue()
        {
            MessageBox.Show(String.Format("Selected value is {0}", this.selectedValue)); 
        }

最后，進行調試，發現ValueChangedCommand當每次Silder值變更的時候都會被執行。

那么如何實現最后一次值變更時，才執行Command呢？