ExclamationBar.cs

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

[Script(ScriptRole.Client)]
public class ExclamationBar : MonoBehaviour
{
    public float FadeSmoothingFactor;

    static ExclamationBar _instance;
    CanvasGroup _canvasGroup;
    Text _textElement;
    bool _visible;
    float _disappearTime;

    void Awake()
    {
        _instance = this;
        _canvasGroup = GetComponent<CanvasGroup>();
        _textElement = GetComponentInChildren<Text>();
    }

    public static void Exclaim(string message, float duration = 3.0f)
    {
        _instance._textElement.text = message;
        _instance._visible = true;
        _instance._disappearTime = Time.time + duration;
    }

    void Update()
    {
        if (_visible)
        {
            _canvasGroup.alpha = Mathf.Lerp(_canvasGroup.alpha, 1, Time.deltaTime * FadeSmoothingFactor);

            if (Time.time > _disappearTime)
                _visible = false;
        }
        else
            _canvasGroup.alpha = Mathf.Lerp(_canvasGroup.alpha, 0, Time.deltaTime * FadeSmoothingFactor);
    }
}