Reading touch input on Android drops the game from 60fps to 12-20fps.

Unity Engine
,
1

EDIT: Can’t be sure this is the problem any more.
Performance just seems to be generally unpredictable on Android. I can build, run, and get 20fps. Close the app, open it again, 20fps. Disconnect from PC, restart phone, open app, 20fps. Rebuild the EXACT SAME PROJECT WITH NO CHANGES and get 60fps every time I open the rebuilt app.

(Unity 2020.1.2f1)

This is the code I have on one object in the scene:

    void Update()
    {
        for (int i = 0; i < Input.touchCount; ++i)
        {
            Touch t = Input.GetTouch(i);
            Vector2 tpos = t.position;
            tpos.x /= Screen.width;
            tpos.y /= Screen.height;
            if (t.position.x < Screen.width * 0.5f)
            {
                if (t.phase == TouchPhase.Began)
                {
                    refx = tpos.x;
                }
                value = Mathf.Clamp((tpos.x - refx) * 7f, -1.0f, 1.0f);
            }
        }
    }

If I disable that object, the framerate is a stable 60fps. If I enable it, it drops to 12-20fps.

By the way: ‘value’ is just being applied to the steering of a car (derived from the Unity tutorial on building a car)

If I disable the steering object and feed a constant number to the steering instead of ‘value’, everything is fine and smooth.

EDIT: Stranger and stranger. I tried this:

       for (int j = 0; j < Input.touchCount; ++j) { }

        int i = 0;
        if (Input.touchCount > 0)
        {
       
            Touch t = Input.GetTouch(i);
            Vector2 tpos = t.position;
            tpos.x /= Screen.width;
            tpos.y /= Screen.height;
            if (t.position.x < Screen.width * 0.5f)
            {
                if (t.phase == TouchPhase.Began)
                {
                    refx = tpos.x;
                }
                value = Mathf.Clamp((tpos.x - refx) * 7f, -1.0f, 1.0f);
            }
       
        }

This also runs at 12-20fps, BUT if I delete the ‘for’ line at the top (which does nothing!) the game goes back up to 60fps.

EDIT: It seems that polling Input.touchCount in a for loop is HUGELY expensive. If I cache the result and do a for-loop over that, there’s no slowdown. It can’t be the number of times it’s polled that’s the issue, because the slowdown occurs even when the number of touches is zero (which would mean it’s only polled once).

2

Hmm that’s really interesting considering, according to the documentation, Input.touchCount is “Guaranteed not to change throughout the frame.”

But… I have to wonder, how many touches are actually happening in your game? Even with a for loop, if the value was 1 you would only be reading it 3 times.

3

Yeah, it wasn’t the number of pollings done, it was doing it inside a for() construct that caused the lag. Even when there were no touches it went super-slow.

4

Hmm that doesn’t make sense. A for loop should be any different from any other way of accessing the value. When touches is zero it should only be reading it once. Have you tried the profiler?