Blend_技巧篇_淡入淡出

1、实现原理

1.1 利用UIElement.OpacityMask属性，用于改变对象区域的不透明度的画笔。可以使元素的特定区域透明或部分透明，从而实现比较新颖的效果。

1.2 OpacityMask属性接受任何画刷，可利用LinearGradientBrush线性渐变画刷，通过对渐变画刷中各颜色点加以动画处理即可。

2、渐变淡入实现

渐变淡入效果，可通过事件触发器触发Loaded事件实现，所以可以仅用前端XAML语言实现。

2.1 设置对象的OpacityMask属性

<Grid.OpacityMask>
    <LinearGradientBrush StartPoint="0.5,1" EndPoint="0.5,0">
        <GradientStop Color="#00000000" Offset="0"/>
        <GradientStop Color="#00000000" Offset="1"/>
        <GradientStop Color="#00000000" Offset="1"/>
    </LinearGradientBrush>
</Grid.OpacityMask>

2.2 设置对象的事件触发器

<Grid.Triggers>
    <EventTrigger RoutedEvent="Loaded">
        <EventTrigger.Actions>
            <BeginStoryboard>
                <Storyboard>
                    <DoubleAnimation From="1" To="0" Duration="0:0:1.5" Storyboard.TargetProperty="OpacityMask.(GradientBrush.GradientStops)[1].Offset"/>
                    <DoubleAnimation From="1" To="0" Duration="0:0:1" BeginTime="0:0:0.5" Storyboard.TargetProperty="OpacityMask.(GradientBrush.GradientStops)[2].Offset"/>
                    <ColorAnimation To="#FF000000" Duration="0" Storyboard.TargetProperty="OpacityMask.(GradientBrush.GradientStops)[2].Color"/>
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger.Actions>
    </EventTrigger>
</Grid.Triggers>

3、渐变淡出实现

渐变淡出效果，由于事件触发器事件需要路由事件触发，所以需要使用后端代码实现。

3.1 准备渐变淡出的动画和画刷资源

<Window.Resources>
    <Storyboard x:Key="ClosedStoryboard" Storyboard.TargetName="LoginGrid">
        <DoubleAnimation From="1" To="0" Duration="0:0:2" Storyboard.TargetProperty="OpacityMask.(GradientBrush.GradientStops)[1].Offset"/>
        <DoubleAnimation From="1" To="0" Duration="0:0:1.5" BeginTime="0:0:0.5"Storyboard.TargetProperty="OpacityMask.(GradientBrush.GradientStops)[2].Offset"/>
        <ColorAnimation To="#00000000" Duration="0" Storyboard.TargetProperty="OpacityMask.(GradientBrush.GradientStops)[2].Color"/>
    </Storyboard>
    <LinearGradientBrush x:Key="ClosedBrush" StartPoint="0.5,0" EndPoint="0.5,1">
        <GradientStop Color="#FF000000" Offset="0"/>
        <GradientStop Color="#FF000000" Offset="1"/>
        <GradientStop Color="#FF000000" Offset="1"/>
    </LinearGradientBrush>
</Window.Resources>

3.2 后端通过绑定按钮的Click事件实现

private void btnCancel_Click(object sender, RoutedEventArgs e)
{
    this.IsEnabled = false;

    LoginGrid.OpacityMask = this.Resources["ClosedBrush"] as LinearGradientBrush;
    Storyboard std = this.Resources["ClosedStoryboard"] as Storyboard;
    std.Completed += delegate { this.Close(); };

    std.Begin();
}