joystick changing my character facing direction and moving within screen only

i'm new to unity3d, i readed a guide here

and i using a joystick controller that can easily found it using google name MPJoystick

i try to change the mouse click move to move using joystick, now my character can move, but the character facing direction won't, i have no idea why.

1 april 2014 9:50PM: after some try, finally i can change the facing direction, next i need to control the character inside the screen and not go over, anyone know how?i tried using the Matf.Clamp solutions that i found but not working, no idea why

here is my code

controller.cs

 using UnityEngine;
 using System.Collections;
 
 public class Controller : MonoBehaviour {
     
     public float moveSpeed;
     public float screenWidth;
     public float screenHeight;
     
     public MPJoystick moveJoystick;  
 
     private Vector3 moveDirection;
     public Vector3 faceDirection;
 
     public float turnSpeed;
 
     public Vector3 playerSize;
     public float distance;
     public float leftBorder;
     public float rightBorder;
     public float bottomBorder;
     public float topBorder;
 
     public void Awake() {
         
     }
 
     void Start () {
         screenWidth = Screen.width;
         screenHeight = Screen.height;
         playerSize = renderer.bounds.size;
         distance = (transform.position - Camera.main.transform.position).z;
         leftBorder = Camera.main.ViewportToWorldPoint (new Vector3 (0, 0, distance)).x + (playerSize.x/2);
         rightBorder = Camera.main.ViewportToWorldPoint (new Vector3 (1, 0, distance)).x - (playerSize.x/2);
         
         bottomBorder = Camera.main.ViewportToWorldPoint (new Vector3 (0, 0, distance)).y + (playerSize.y/2);
         topBorder = Camera.main.ViewportToWorldPoint (new Vector3 (0, 1, distance)).y - (playerSize.y/2);
 
         faceDirection = Vector3.right;
     }
 
     void Update () {
         Vector3 currentPosition = transform.position;
         float touchKey_x =  moveJoystick.position.x;
         float touchKey_y =  moveJoystick.position.y;
 
         if (touchKey_x == 0 && touchKey_y == 0) {
             return;
         } else {
 
             /* Change facing direction */
             faceDirection.y = moveDirection.y + touchKey_y;
             faceDirection.x = moveDirection.x + touchKey_x;
             float targetAngle = Mathf.Atan2(faceDirection.y, faceDirection.x) * Mathf.Rad2Deg;
             transform.rotation = 
                 Quaternion.Slerp( transform.rotation, 
                                  Quaternion.Euler( 0, 0, targetAngle ), 
                                  turnSpeed * Time.deltaTime );
 
             /* i think the next 3 line is change the facing direction? am i right? answer: i think i'm wrong */
             moveDirection.x = moveDirection.x + touchKey_x;
             moveDirection.y = moveDirection.y + touchKey_y;
             moveDirection.Normalize ();
 
             /* i guess here is the moving code? am i right? */
             Vector3 target = moveDirection * moveSpeed + currentPosition;
             /*target.x = Mathf.Clamp (target.x, leftBorder, rightBorder);
             target.y = Mathf.Clamp (target.y, topBorder, rightBorder);*/
             transform.position = Vector3.Lerp (currentPosition, target, Time.deltaTime);
         }
     }
 }

and how can i avoid the character go over the screen? if i try without the target.x = Math.Clamp blablabla and target.y = Math.Clamp blablabla, the character can move well but the problem is the character can go over the screen,

but when i try using Math.Clamp, the character move become so weird, sometime i move top but go bottom, i move bottom but go top, sometime stuck at center but sometime can go over...