[Silverlight动画]转向行为 - 追捕行为

对于追捕行为，它非常类似寻找行为。其实追捕的最后一个动作就是调用寻找。追捕的本质是预测目标所要到达的位置并事先赶到。

        public void pursue(Vehicle target) {
            double lookAheadTime = position.dist(target.position) / _maxSpeed;
            Vector2D predictedTarget = target.position.add(target.velocity.multiply(lookAheadTime));
            seek(predictedTarget);
        }

首先通过两者间距离除以最大速率计算出预测时间。这就得到了追上目标所需要的时间间隔（假设目标不再移动）。通过目标的速度乘以时间间隔得到预测移动距离，再加到当前位置上就是预测位置。最后，把这个预测位置作为寻找点。

测试一下。这次要创建三辆机车。一个是只顾移动的机车，其作为目标，另外两个转向机车，一个用寻找行为，一个用追捕行为。如果一切正常，追捕者靠着优越的算法会胜出。 

<UserControl
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:Steer" xmlns:ed="http://schemas.microsoft.com/expression/2010/drawing" x:Class="Steer.PursueTest"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">
    
    <Grid x:Name="LayoutRoot" Background="White">

    	<local:SteeredVehicle x:Name="mySeeker" HorizontalAlignment="Left" Height="40" VerticalAlignment="Top" Width="40" RenderTransformOrigin="0.5,0.5" d:IsHidden="True">
    		<ed:RegularPolygon Fill="#FFFF0C00" Height="40" InnerRadius="1" PointCount="3" Stretch="Fill" Stroke="Black" UseLayoutRounding="False" Width="40" RenderTransformOrigin="0.5,0.5" StrokeThickness="0">
    			<ed:RegularPolygon.RenderTransform>
    				<CompositeTransform Rotation="90"/>
    			</ed:RegularPolygon.RenderTransform>
    		</ed:RegularPolygon>
    	</local:SteeredVehicle>
    	<local:SteeredVehicle x:Name="myPursuer" HorizontalAlignment="Left" Height="40" VerticalAlignment="Top" Width="40" RenderTransformOrigin="0.5,0.5" d:IsHidden="True">
    		<ed:RegularPolygon Fill="Blue" Height="40" InnerRadius="1" PointCount="3" Stretch="Fill" Stroke="Black" UseLayoutRounding="False" Width="40" RenderTransformOrigin="0.5,0.5" StrokeThickness="0">
    			<ed:RegularPolygon.RenderTransform>
    				<CompositeTransform Rotation="90"/>
    			</ed:RegularPolygon.RenderTransform>
    		</ed:RegularPolygon>
    	</local:SteeredVehicle>
    	<local:SteeredVehicle x:Name="myTarget" HorizontalAlignment="Left" Height="40" VerticalAlignment="Top" Width="40" RenderTransformOrigin="0.5,0.5">
    		<ed:RegularPolygon Fill="#FFFD00FF" Height="40" InnerRadius="1" PointCount="5" Stretch="Fill" Stroke="Black" StrokeThickness="0" UseLayoutRounding="False" Width="40"/>
    	</local:SteeredVehicle>

    </Grid>
</UserControl>

    public partial class PursueTest : UserControl
    {
        public PursueTest()
        {
            InitializeComponent();
            Loaded += new RoutedEventHandler(PursueTest_Loaded);
        }

        void PursueTest_Loaded(object sender, RoutedEventArgs e)
        {
            mySeeker.x = 400;
            myPursuer.x = 400;
            myTarget.position = new Vector2D(200, 100);
            myTarget.velocity.length = 15;
            myTarget.velocity.angle = Math.PI / 4;

            CompositionTarget.Rendering += new EventHandler(CompositionTarget_Rendering);
        }

        void CompositionTarget_Rendering(object sender, EventArgs e)
        {
            mySeeker.seek(myTarget.position);
            mySeeker.update();

            myPursuer.pursue(myTarget);
            myPursuer.update();

            myTarget.update();
        }
    }