[WPF自定义控件库] 模仿UWP的ProgressRing

1. 为什么需要ProgressRing

虽然我认为这个控件库的控件需要模仿Aero2的外观，但总有例外，其中一个就是ProgressRing。ProgressRing是来自UWP的控件，部分代码参考了这里。ProgressRing的使用方式运行效果如下：

<kino:ProgressRing IsActive="True"
                       Height="40"
                       Width="40"
                       Margin="8"
                       MinHeight="9"
                       MinWidth="9" />

在Windows 10中ProgressRing十分常见，而且十分好用。它还支持自适应尺寸，在紧凑的地方使用ProgressRing会给UI增色不少，而且不会显得格格不入：

那为什么不使用ProgressBar？其中一个原因是ProgressBar功能太多，而我很多时候只需要一个简单的显示正在等待的元素，另一个原因是条状的ProgressBar在紧凑的地方不好看，所以才需要结构相对简单的ProgressRing。

2. 基本结构

[TemplateVisualState(GroupName = VisualStates.GroupActive, Name = VisualStates.StateActive)]
[TemplateVisualState(GroupName = VisualStates.GroupActive, Name = VisualStates.StateInactive)]
public partial class ProgressRing : Control
{
    // Using a DependencyProperty as the backing store for IsActive.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty IsActiveProperty =
        DependencyProperty.Register("IsActive", typeof(bool), typeof(ProgressRing), new PropertyMetadata(false, new PropertyChangedCallback(IsActiveChanged)));

    private bool hasAppliedTemplate = false;

    public ProgressRing()
    {
        DefaultStyleKey = typeof(ProgressRing);
    }

    public bool IsActive
    {
        get { return (bool)GetValue(IsActiveProperty); }
        set { SetValue(IsActiveProperty, value); }
    }

    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        hasAppliedTemplate = true;
        UpdateState(IsActive);
    }

    private static void IsActiveChanged(DependencyObject d, DependencyPropertyChangedEventArgs args)
    {
        var pr = (ProgressRing)d;
        var isActive = (bool)args.NewValue;
        pr.UpdateState(isActive);
    }

    private void UpdateState(bool isActive)
    {
        if (hasAppliedTemplate)
        {
            string state = isActive ? VisualStates.StateActive : VisualStates.StateInactive;
            VisualStateManager.GoToState(this, state, true);
        }
    }
}

ProgressRing的基本代码如上所示，它只包含IsActive这个属性，并使用这个属性控制它在Active和Inactive两种状态之间切换。参考Silverlight Toolkit，我也把常用的各种VisualState的状态名称作为常量写到一个统一的VisualStates类里：

#region GroupActive

/// <summary>
/// Active state.
/// </summary>
public const string StateActive = "Active";

/// <summary>
/// Inactive state.
/// </summary>
public const string StateInactive = "Inactive";

/// <summary>
/// Active state group.
/// </summary>
public const string GroupActive = "ActiveStates";
#endregion GroupActive

3. 旋转

XAML部分几乎全部照抄UWP的ProgressRing，所以实际运行效果和UWP的ProgressRing很像，区别很小。

通常来说，ProgressRing的Active状态持续时间不会太长，而且ProgressRing的尺寸也不会太大，所以ProgressRing的Active状态可以说不计成本。Active状态下有5个Ellipse 不停旋转，或者说做绕着中心点做圆周运动，而为了不需要任何计算圆周中心点的代码，ProgressRing给每个Ellipse外面都套上一个Canvas，让这整个Canvas旋转。XAML大概这样：

<Storyboard RepeatBehavior="Forever" x:Key="Sb">
    <DoubleAnimationUsingKeyFrames Storyboard.TargetName="E1R" BeginTime="0" Storyboard.TargetProperty="Angle">
        <SplineDoubleKeyFrame KeyTime="0" Value="-110" KeySpline="0.13,0.21,0.1,0.7" />
        <SplineDoubleKeyFrame KeyTime="0:0:0.433" Value="10" KeySpline="0.02,0.33,0.38,0.77" />
        <SplineDoubleKeyFrame KeyTime="0:0:1.2" Value="93" />
        <SplineDoubleKeyFrame KeyTime="0:0:1.617" Value="205" KeySpline="0.57,0.17,0.95,0.75" />
        <SplineDoubleKeyFrame KeyTime="0:0:2.017" Value="357" KeySpline="0,0.19,0.07,0.72" />
        <SplineDoubleKeyFrame KeyTime="0:0:2.783" Value="439" />
        <SplineDoubleKeyFrame KeyTime="0:0:3.217" Value="585" KeySpline="0,0,0.95,0.37" />
    </DoubleAnimationUsingKeyFrames>
</Storyboard>


<Canvas RenderTransformOrigin=".5,.5" Height="100" Width="100">
    <Canvas.RenderTransform>
        <RotateTransform x:Name="E1R" />
    </Canvas.RenderTransform>
    <Ellipse x:Name="E1"
    Width="20"
    Height="20"
    Fill="MediumPurple" />
</Canvas>

然后运行效果这样：

4. 自适应大小

为了让ProgressRing中各个Ellipse都可以自适应大小，ProgressRing提供了一个TemplateSettings属性，类型为TemplateSettingValues，它里面包含以下记个依赖属性：

public double MaxSideLength
{
    get { return (double)GetValue(MaxSideLengthProperty); }
    set { SetValue(MaxSideLengthProperty, value); }
}

public double EllipseDiameter
{
    get { return (double)GetValue(EllipseDiameterProperty); }
    set { SetValue(EllipseDiameterProperty, value); }
}

public Thickness EllipseOffset
{
    get { return (Thickness)GetValue(EllipseOffsetProperty); }
    set { SetValue(EllipseOffsetProperty, value); }
}

XAML中的元素大小及布局绑定到这些属性：

<Grid x:Name="Ring"
      Background="{TemplateBinding Background}"
      MaxWidth="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=TemplateSettings.MaxSideLength}"
      MaxHeight="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=TemplateSettings.MaxSideLength}"
      Visibility="Collapsed"
      RenderTransformOrigin=".5,.5"
      FlowDirection="LeftToRight">
    <Canvas RenderTransformOrigin=".5,.5">
        <Canvas.RenderTransform>
            <RotateTransform x:Name="E1R" />
        </Canvas.RenderTransform>
        <Ellipse x:Name="E1"
        Style="{StaticResource ProgressRingEllipseStyle}"
        Width="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=TemplateSettings.EllipseDiameter}"
        Height="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=TemplateSettings.EllipseDiameter}"
        Margin="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=TemplateSettings.EllipseOffset}"
        Fill="{TemplateBinding Foreground}" />
    </Canvas>

每当ProgressRing调用MeasureOverrride都重新计算这些值：

protected override System.Windows.Size MeasureOverride(System.Windows.Size availableSize)
{
    var width = 20d;
    var height = 20d;
    if (System.ComponentModel.DesignerProperties.GetIsInDesignMode(this) == false)
    {
        width = double.IsNaN(Width) == false ? Width : availableSize.Width;
        height = double.IsNaN(Height) == false ? Height : availableSize.Height;
    }

    TemplateSettings = new TemplateSettingValues(Math.Min(width, height));
    return base.MeasureOverride(availableSize);
}

public TemplateSettingValues(double width)
{
    if (width <= 40)
    {
        EllipseDiameter = (width / 10) + 1;
    }
    else
    {
        EllipseDiameter = width / 10;
    }
    MaxSideLength = width - EllipseDiameter;
    EllipseOffset = new System.Windows.Thickness(0, EllipseDiameter * 2.5, 0, 0);
}

这样就实现了外观的自适应大小功能。需要注意的是，过去很多人喜欢将这种重新计算大小的操作放到LayoutUpdated事件中进行，但LayoutUpdated是整个布局的最后一步，这时候如果改变了控件的大小有可能重新触发Measure和Arrange及LayoutUpdated，这很可能引起“布局循环”的异常。正确的做法是将计算尺寸及改变尺寸的操作都放到最初的MeasureOverride中。

TemplateSettings在UWP中很长见到，它的其它用法可以参考这篇文章：了解模板化控件：UI指南