UI Button OnClick Sensitivity for High DPI Devices

Questions & Answers
1

I’ve got a Unity project running on a Samsung Galaxy S5, and users are having issues pressing buttons.

They need to repeatedly click on any button using the OnClick handler. I believe this is because of very small movements in their finger when they touch the device reading as scrolling rather than a simple touch. Adding an OnPointerDown event trigger of course avoids the issue…

Unfortunately, some of my buttons are on items that take up large chunks of a ScrollRect. This means that I can’t easily switch to using EventTriggers to simply capture OnPointerDown - that would unnecessarily trigger the button when the person is actually scroll-dragging.

Is there any way I can change the ‘scroll’ sensitivity within the Button generally? I’d like to adjust it to suit Screen.dpi, since most devices with a higher resolution screen have an accompanying increase in touch resolution.

2

Looking at Unity’s UI implementation (on tag 5.3) I’ve found that (1) is called by (2) which uses pixelDragThreshold attribute defined by the current EventSystem instance in the scene…

A possible (and maybe the simplest) solution would be to use a script which will change that threshould based on the screen PPI (Screen.dpi) of each device.

1. private static bool ShouldStartDrag(...) @ PointerInputModule.cs : 228
2. protected virtual void ProcessDrag(...) @ PointerInputModule.cs : 248

The following script does the drag threshold adaptation, put this in your event system game object:

using UnityEngine;
using UnityEngine.EventSystems;

public class DragCorrector : MonoBehaviour
{
    public int baseTH = 6;
    public int basePPI = 210;
    public int dragTH = 0;

    void Start()
    {
        dragTH = baseTH * (int)Screen.dpi / basePPI;

        EventSystem es = GetComponent<EventSystem>();

        if (es) es.pixelDragThreshold = dragTH;
    }
}

Here’s a very simple script to check the device’s PPI… you can run this on different phones to see if Unity is calculating it right:

using UnityEngine;
using UnityEngine.UI;

public class PPIViewer : MonoBehaviour
{
    public Text label;

    void Start()
    {
        if (label != null)
        {
            label.text = "PPI: " + Screen.dpi.ToString();
        }
    }
}
5

This code is more efficient for me:
link text

public class DragThresholdUtil : MonoBehaviour {
  void Start () {
    int defaultValue = EventSystem.current.pixelDragThreshold;		
    EventSystem.current.pixelDragThreshold = 
            Mathf.Max(
                 defaultValue , 
                 (int) (defaultValue * Screen.dpi / 160f));
  }
}
3

Nice little fix. Would you be OK with me adding this to the UI extensions project? Should prove valuable.

4

Try to increase the Drag Treshold in the Event System to about 16 :slight_smile: .

6

I came up to following blog post when I was solving this issue:

Unity UI Drag Threshold

It introduces script that update Drag Threshold according to DPI.