Problem about event triggers in iPhone

I had test functions about pointer down,pointer up and pointer click in event trigger component.

my script:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.EventSystems;
 
 public class aaaaa1 : MonoBehaviour
 {
     public int a = 0;
     public int b = 0;
     public int c = 0;
 
     public void pointdown ()
     {
         a++;
     }
 
     public void pointup ()
     {
         c++;
     }
 
     public void pointclick ()
     {
         b++;
     }
 
     void OnGUI ()
     {
         GUILayout.Label ("");
         GUILayout.Label ("");
         GUILayout.Label ("a:" + a.ToString ());
         GUILayout.Label ("b:" + b.ToString ());
         GUILayout.Label ("c:" + c.ToString ());
     }
 }

   

my setting:

It works well in editor, but then on iPhone, something seems very strange. When i pressed, the point down run twice, and even i drop in it point down run also. I wondered how could be this. And WHO met this question, had solved yet? Thanks a lot!