Frog.cs

public class State
{
    public Action Enter { get; set; } = () => { };
    public Action Update { get; set; } = () => { };
    public Action Exit { get; set; } = () => { };

    public void Switch(ref State curState, State newState)
    {
        Exit();
        curState = newState;
        newState.Enter();
    }
}

public partial class Frog : Monster
{
    State idle = new();
    State preJump = new();
    State jump = new();
    State curState;

    public override void _Ready()
    {
        base._Ready();

        // idle
        idle.Enter = () =>
        {
            sprite.Play("idle");
            Switch(delay: 2, preJump);
        };

        // pre jump
        preJump.Enter = () =>
        {
            sprite.Play("jump_pre");
            Switch(delay: 0.75, jump);
        };
        preJump.Update = () =>
        {
            double strength = 0.4;
            sprite.Offset = new Vector2((float)GD.RandRange(-strength, strength), 0);
        };
        preJump.Exit = () =>
        {
            sprite.Offset = Vector2.Zero;
        };

        // jump
        jump.Enter = () =>
        {
            sprite.Play("jump_start");
        };

        curState = idle;
        curState.Enter();
    }

    void Switch(double delay, State newState) => 
        Global.CreateTimer(delay, () => curState.Switch(ref curState, newState));

    public override void _PhysicsProcess(double delta)
    {
        base._PhysicsProcess(delta);
        curState.Update();
    }
}