ref : http://kikikiroku.session.jp/unity-android-finger-gesture-down-up-drag-flick/

PlayerJoystick.cs

using UnityEngine;
using System.Collections;
using UnityEngine.EventSystems;
using UnityStandardAssets.CrossPlatformInput;
using UnityEngine.UI;

public class PlayerJoystick : Joystick, InputGesture {

	Image joystickImage;
	EventSystem es;
 
	public struct Range {
		public int x;
		public int y;
	}

	public Range movementRange;

	protected override void UpdateVirtualAxes(Vector3 value)
	{
		var delta = m_StartPos - value;
		delta.y = -delta.y;
		delta.x /= movementRange.x;
		delta.y /= movementRange.y;
		if (m_UseX)
		{
			m_HorizontalVirtualAxis.Update(-delta.x);
		}

		if (m_UseY)
		{
			m_VerticalVirtualAxis.Update(delta.y);
		}
	}
	public override void Start() {
		base.Start();
		SetupMovementRange();
	}

	public override void OnEnable() {
		base.OnEnable();
		InputGestureManager.Instance.RegisterGesture (this);
		ShowImage(false);
	}
	public override void OnDisable() {
		base.OnDisable();
		InputGestureManager.Instance.UnregisterGesture (this);
	}
	public override void OnPointerDown (PointerEventData data) {
		base.OnPointerDown(data);
		m_StartPos = transform.position = data.position;
		ShowImage(true);
		UpdateVirtualAxes(m_StartPos);
	} 
	public override void OnPointerUp (PointerEventData data) {
		base.OnPointerUp(data);
		ShowImage(false);
	}
	public override void OnDrag (PointerEventData data) {
		Vector3 newPos = Vector3.zero;
		if (m_UseX)
		{
			int delta = (int)(data.position.x - m_StartPos.x);
			delta = Mathf.Clamp(delta, - movementRange.x, movementRange.x);
			newPos.x = delta;
		}

		if (m_UseY)
		{
			int delta = (int)(data.position.y - m_StartPos.y);
			delta = Mathf.Clamp(delta, - movementRange.y, movementRange.y);
			newPos.y = delta;
		}
		transform.position = new Vector3(m_StartPos.x + newPos.x, m_StartPos.y + newPos.y, m_StartPos.z + newPos.z);
		UpdateVirtualAxes(transform.position);
	}
	
	public int Order {
		get { return 9999; }
	}

	public bool IsGestureProcess (GestureInfo info) {
		return true;
	}

	public void OnGestureDown (GestureInfo info) {
		OnPointerDown(GestureToPointer(info));

	}
	public void OnGestureUp (GestureInfo info) {
		OnPointerUp(GestureToPointer(info));
	}

	public void OnGestureDrag (GestureInfo info) {
		OnDrag(GestureToPointer(info));
	}

	public void OnGestureFlick (GestureInfo info) {
	}

	public void DoMove (Vector3 world_delta) {
	}

	public void ShowImage (bool isShow) {
		joystickImage = GetComponent<Image>();
		joystickImage.enabled = isShow;
	}
	private void SetupMovementRange () {
		movementRange.x = Screen.width / 5;
		movementRange.y = Screen.height / 10;
	}

	private PointerEventData GestureToPointer (GestureInfo info) {
		if (es == null) 
			es = GameObject.FindObjectOfType<UnityEngine.EventSystems.EventSystem>();
		PointerEventData data = new PointerEventData(es);
		data.position = info.ScreenPosition;
		return data;
	}
}