Unity UGUI 打印机效果

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class Typewriter : MonoBehaviour {

	public string con;
	Text _conText;
	Text _Context{
		get{ 
			if (_conText == null) {
			
				_conText = GetComponent<Text> ();
			}
			return _conText;
		}
	}
	public float speed = 10f;
	public bool isOnStart;

	public System.Action ac;

    public bool isWriting;

	// Use this for initialization
	void Start () {
		if (isOnStart) {
			StartCoroutine (StartWrite ());
		}
	}
	public void StopAll()
    {
        StopAllCoroutines();
    }
	public void ToPrinter(string conText, System.Action ac = null)
	{
        this.con = conText;
		this.ac = ac;

		StartCoroutine (StartWrite());

	}

	IEnumerator StartWrite()
	{
        isWriting = true;
        int curPos = 0;
		int length = con.Length;
		while (curPos < length) {
			curPos += 1;
			_Context.text = con.Substring (0, curPos);
			yield return new WaitForSeconds (1/speed);
		}
        isWriting = false;
		if (ac != null) {
			ac ();
		}
	}
}