Unity的Timeline如何在clip激活的第一帧执行自定义代码

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

public class LightControlMixerBehaviour : PlayableBehaviour
{
    private float[] weights;
    private bool initWeight;
    public override void PrepareFrame(Playable playable, FrameData info)
    {
        base.PrepareFrame(playable, info);
        int inputCount = playable.GetInputCount ();

        if (!initWeight && inputCount > 0)
        {
            weights = new float[inputCount];
            initWeight = true;
        }
        for (int i = 0; i < inputCount; i++)
        {
            float inputWeight = playable.GetInputWeight(i);
            ScriptPlayable<LightControlBehaviour> inputPlayable = (ScriptPlayable<LightControlBehaviour>)playable.GetInput(i);
            LightControlBehaviour input = inputPlayable.GetBehaviour ();

            {
                input.OnStart();
                Debug.Log(inputWeight);
            }

            weights[i] = inputWeight;
        }
    }
}