统一 2D 的物理运动

Question

I have a player on XY position. After player's swipe position must be increased or decreased on 1 for x-area. Tryed to do this by addForce and velocity , but these ways are increasing x-area pos to infinity. So - How i can stop increasing after then position will be (1,0) after (0,0)? or mb anything else method can help me?

    void FixedUpdate()
    {
        if (Input.touchCount > 0)
        {
            theTouch = Input.GetTouch(0);

            if (theTouch.phase == TouchPhase.Began)
            {
                theTouchStartPos = theTouch.position;   
            }

            if (theTouch.phase == TouchPhase.Ended)
            {
                theTouchEndPos = theTouch.position;     

                if (theTouchEndPos.x <= theTouchStartPos.x)
                {
                    Debug.Log("swipe left");
                    Move(Vector2.left);
                }

                if (theTouchEndPos.x >= theTouchStartPos.x)
                {
                    Debug.Log("swipe right");
                    Move(Vector2.right);
                }
            }
        }
    }

    void Move(Vector2 direction)
    {
        //playerRb.velocity = new Vector2(direction.x, direction.y) * speed;
        float step = speed * Time.deltaTime;
        playerPos = Vector2.Lerp(playerPos, playerPos + direction, step);
    }

Answer 1

Do not take single frame inputs such as TouchPhase.Began or GetKeyDown in FixedUpdate() . It may work on your machine but it will not register the input every time on a faster machine.

"FixedUpdate is often called more frequently than Update. It can be called multiple times per frame, if the frame rate is low and it may not be called between frames at all if the frame rate is high ." - https://docs.unity3d.com/Manual/ExecutionOrder.html

What this means is that FixedUpdate is not guaranteed to run on the frame when you pressed down, so inputs that only last for one frame such as TouchPhase.Began or GetKeyDown may not register if the framerate is high enough.

It's surprising to me how noone else noticed this.

Answer 2

So it sounds like what you are trying to achieve is: Each swipe moves your player one "step" to left or right on the X-axis but smoothly.

In your current approach you call the Lerp only in one single frame when the touch ends so the object will barely move at all.

I would rather set the target x-axis position and let the playeroventowards that with the given speed in every physics update call.

As SpicyCatGames pointed out correctly, you should always separate the handling of User Input ( Update ) from the physics ( FixedUpdate ).

private float targetX;

private void Awake ()
{
    targetX = playerRb.position.x;
}

void FixedUpdate()
{
    // Move the player towards the target position but only on X
    // -> keep the Y velocity
    var position = playerRb.position;
    position.x = Mathf.MoveTowards(position.x, targetX, speed * Time.deltaTime);
    playerRb.MovePosition(position);
}

private void Update ()
{        
    if (Input.touchCount > 0)
    {
        theTouch = Input.GetTouch(0);

        if (theTouch.phase == TouchPhase.Began)
        {
            theTouchStartPos = theTouch.position;   
        }
        else if (theTouch.phase == TouchPhase.Ended)
        {
            theTouchEndPos = theTouch.position;     

            if (theTouchEndPos.x < theTouchStartPos.x)
            {
                Debug.Log("swipe left");
                Move(-1);
            }
            else if (theTouchEndPos.x > theTouchStartPos.x)
            {
                Debug.Log("swipe right");
                Move(1);
            }
        }
    }
}

void Move(float direction)
{
    targetX += direction;
}

Answer 3

This seems to get discussed a lot.

FixedUpdate is for physics "calculations" .

If you are animating something - that is to, say, you want to draw something each time the scene is drawn (perhaps changing the position) - you very much use Update, not FixedUpdate.

It's both pointless (and inaccurate) to change the position of on object on the FixedUpdate calls.

Say this represents time and the F are fixed update events and the D (draw) are update events

F  F  F    F F  F F  F  F   F  F  F  F F  F F  F 
 D       D           D  D       D         D

You (of course, obviously) only need to set the position of the object when you are about to draw it.

And indeed, when you're about to draw it: you (obviously) want the position it should be in at that actual time.

(The script given here shows "F" and "D" running .. https://docs.unity3d.com/ScriptReference/MonoBehaviour.FixedUpdate.html )

As it explains in the documentation, FixedUpdate is for physics >>calculations<<. So, if you are extending the physics engine, simulating something, applying the precise rules of a bullet game, or whatever. (I've rarely had to use it.) Note that interestingly, if for some reason you were calculating a planet or something and doing that every FixedUpdate, you'd still have to actually set the position of the planet (or whatever it is) at the exact moment when you're about to draw it! (ie in Update.)

---

It's another good example of the famously touchy Unity doco. In dozens of places they show something being moved (ie, using Update, when the draw happens). On that FixedUpdate page it very confusingly just mentions physics "calculations" which is very vague. If they had added a couple more words ("for behind the scenes physics calculations where you are not actually getting the position to draw something that's about to be drawn") it would all be clearer!