探索TimelinePlayableAPI，让Timeline为所欲为

今天我们再来看看Timeline中的Track，并且动手实现一个老年迪斯科场景。

Track

Timeline可以添加很多Track来对不同的对象做控制。

• Track   在Clip上控制动画，可以在有Animator组件但是没有AnimatorController的情况下直接播放动画。
• Audio Track   在Clip上控制音频播放。
• Activtion Track   在Clip上控制物体是否在场景中Active，GameObject.SetActive()的作用
• Control Track   在Clip上可以控制和时间相关的元素，可以控制粒子系统，克隆物体，控制另一个timeline等。厉害的是我们放在Control Track上的例子特效，可以随意地拖动Timeline来预览效果，对特效兄弟来说是应该很强大很爽的地方。
• Playable Track   控制继承自BasicPlayableBehaviour的clip
    （在2017.4.0f1版本里原先的BasicPlayableBehaviour因为性能原因已经被弃用，建议采用PlayableAsset和PlayableBehaviour）

如何有了这篇文章

虽然上面提供的几个track已经有很强大的功能了，但是实际操作中我们会发现这些远远不够，官方的Mannul上也没有更多的介绍，有效的资料是少之又少。那么官方演示的demo里的对话系统是怎么通过Timeline实现的呢？
于是我开始寻找，在Youtube上找到了Unite Europe2017上的演讲视频，Extending Timeline with your own playables(https://www.youtube.com/watch?v=uBPRfcox5hE&t=2331s)。

研究了一番，里面的例子就是LightControl将自己的理解分享给大家。让大家都能在Timeline里为所欲为。

深度理解Timeline

Timeline是建立在Playables API(https://docs. ​​unity3d​​.com/Manual/Playables.html)上的PlayableGraph系统，在这套系统中Timeline相当于是一个函数的作用，我们从Assset传入InputData，Timeline处理后输出OutputData到对应的Component。

这套系统中有4个关键部分，Data，Clip，Mixer，Track
我希望我的这张图可以帮助大家快速对应和理解这4个部分。

Timeline也是一个Templete模板，可以方便快速地被复用。

• The Data

用来存放数据，需要被序列化。继承自PlayableBehaviour

using UnityEngine;
using UnityEngine.Playables;

[System.Serializable]
public class LightData : PlayableBehaviour
{
    public float range;
    public Color color;
    public float intensity;
    [HideInInspector]
    public Transform lookTarget;

    public override void OnPlayableCreate(Playable playable)
    {
        var duration = playable.GetDuration();
        if (Mathf.Approximately((float)duration, 0))
        {
            throw new UnityException("A Clip Cannot have a duration of zero");
        }
    }
}

• The Clip

  也就是asset，继承自PlayableAsset。因为是asset，所以不能直接与场景中的物体关联，需要关联场景中的物体时要用ExposedReference来声明，并且通过Resolve方法赋值。

using UnityEngine;
using UnityEngine.Playables;

[System.Serializable]
public class LightClip : PlayableAsset
{
    public LightData templete = new LightData();
    public ExposedReference<Transform> lookTarget;
    // Factory method that generates a playable based on this asset
    public override Playable CreatePlayable(PlayableGraph graph, GameObject go)
    {
        var playable = ScriptPlayable<LightData>.Create(graph, templete);
        LightData clone = playable.GetBehaviour();
        clone.lookTarget = lookTarget.Resolve(graph.GetResolver());
        return playable;
    }
}

• The Mixer

  Mixer控制当前Track中所有的clip的行为，根据每个clip不同的输入权重，计算需要的结果。 Mixer在Timeline中算是最重要的部分了。

using UnityEngine;
using UnityEngine.Playables;

// A behaviour that is attached to a playable
public class LightMixer : PlayableBehaviour
{
    private float m_defaultRange = 1;
    private float m_defaultIntensity = 1;
    private Color m_defaultColor = Color.white;
    private Quaternion m_defaultRotation = Quaternion.identity;
    private Light m_trackBinding;

    private bool m_isFirstFrameProcess = true;

    // Called when the owning graph starts playing
    public override void OnGraphStart(Playable playable)
    {
    }

    // Called when the owning graph stops playing
    public override void OnGraphStop(Playable playable)
    {
        m_isFirstFrameProcess = false;
        if (m_trackBinding == null)
            return;
        m_trackBinding.range = m_defaultRange;
        m_trackBinding.color = m_defaultColor;
        m_trackBinding.intensity = m_defaultIntensity;
        m_trackBinding.transform.rotation = m_defaultRotation;
    }

    // Called when the state of the playable is set to Play
    public override void OnBehaviourPlay(Playable playable, FrameData info)
    {
    }

    // Called when the state of the playable is set to Paused
    public override void OnBehaviourPause(Playable playable, FrameData info)
    {

    }

    // Called each frame while the state is set to Play
    public override void PrepareFrame(Playable playable, FrameData info)
    {
    }

    public override void ProcessFrame(Playable playable, FrameData info, object playerData)
    {
        m_trackBinding = playerData as Light;
        if (m_trackBinding == null)
            return;

        if (!m_isFirstFrameProcess)
        {
            m_isFirstFrameProcess = true;
            m_defaultRange = m_trackBinding.range;
            m_defaultColor = m_trackBinding.color;
            m_defaultIntensity = m_trackBinding.intensity;
            m_defaultRotation = m_trackBinding.transform.rotation;
        }

        int inputCount = playable.GetInputCount();
        float blendRange = 0;
        Color blendColor = Color.clear;
        float blendIntensity = 0;
        Quaternion blendRotation = Quaternion.identity;

        float totalWeight = 0;
        float greatestWeight = 0;
        int currentInputs = 0;

        for (int i = 0; i < inputCount; i++)
        {
            ScriptPlayable<LightData> playableInput = (ScriptPlayable<LightData>)playable.GetInput(i);
            LightData input = playableInput.GetBehaviour();

            float inputWeight = playable.GetInputWeight(i);
            blendRange += input.range * inputWeight;
            blendColor += input.color * inputWeight;
            blendIntensity += input.intensity * inputWeight;
            if (input.lookTarget != null && inputWeight > 0)
                blendRotation *= Quaternion.LookRotation((input.lookTarget.position - m_trackBinding.transform.position) * inputWeight);

            totalWeight += inputWeight;

            if (inputWeight > greatestWeight)
                greatestWeight = inputWeight;

            if (!Mathf.Approximately(inputWeight, 0f))
                currentInputs++;
        }

        m_trackBinding.range = blendRange + m_defaultRange * (1 - totalWeight);
        m_trackBinding.color = blendColor + m_defaultColor * (1 - totalWeight);
        m_trackBinding.intensity = blendIntensity + m_defaultIntensity * (1 - totalWeight);
        m_trackBinding.transform.rotation = blendRotation;
    }
}

• The Track

  把Mixer计算的结果输出，可以指定输出到绑定的物体上。继承自TrackAsset。有好几个特性可以绑定，

• TrackColor : 定义在编辑器中Track的标识颜色
• BindingObejctType : 当前Track上的Mixer输出后的数据要传递的对象类型
• TrackClipType : 当前Track上的Clip的类型

using UnityEngine;
using UnityEngine.Playables;
using UnityEngine.Timeline;

[TrackColor(1f, 0f, 0f)]
[TrackClipType(typeof(LightClip))]
[TrackBindingType(typeof(Light))]
public class LightControlTrack : TrackAsset
{
    public override Playable CreateTrackMixer(PlayableGraph graph, GameObject go, int inputCount)
    {
        return ScriptPlayable<LightMixer>.Create(graph, inputCount);
    }
}

实践

通过上面的探究，我们要做老年disco效果，那就来定制一个自己的track，专门用来控制Light，我们叫LightControl

• 创建一个新的场景添加两个角色，一红一蓝
• 新建一个ParticleSytem，烘托一下Disco的气氛
• 创建一个新的Timeline，DiscoTimeline
• 在DiscoTimeline上新建2个AnimationTrack，一个控制蓝角色的动画，一个控制红角色的动画
• 在DiscoTimeline新建一个ControlTrack，控制刚才创建的ParticleSystem
• 在场景中新建一个SpotLight和一个PointLight
• 在DiscoTimeline中新建一个TrackGroup，命名为PlayableLight
• 在DiscoTimeline中新建我们的LightControl Track，将SpotLight与这个Track绑定，命名为SpotLight
• 在DiscoTimeline中新建我们的LightControl Track，将PointLight与这个Track绑定，命名为PointLight
• 将我们自定义的Clip添加2段到SpotLightTrack上，在一个Clip上的LookAtTarget上绑定蓝角色，另一个Clip上的LookAtTarget绑定到红角色
• 将我们自定义的Clip添加2段到PointLightTrack上，设置变换的灯光颜色和强度。 最后的Timeline就是这样