TextDisplayer.cs

using System;

public interface TextDisplayer {
    void Show();
    void Hide();

    void RegisterOnRequestNextDialog( Action method );
    void SetText( string text );
}

TextTyperDisplayer.cs

using UnityEngine;
using UnityEngine.UI;
using System;

public class TextTyperDisplayer : MonoBehaviour, TextDisplayer {
    [SerializeField]
    private Text dialog;
    [SerializeField]
    private Button clickMask;
    [SerializeField]
    [Range( 0.01f, 0.5f )]
    private float typeTime = 0.02f;

    private string wholeDialog = string.Empty;
    private int currentCharacterIndex;
    private float timer;
    private Action onRequestNextDialog = delegate { };

    private void Awake() {
        clickMask.onClick.AddListener( OnClick );
    }

    private void Update() {
        if ( IsTyping() ) {
            timer += Time.deltaTime;
            if ( timer >= typeTime ) {
                ++currentCharacterIndex;
                dialog.text = wholeDialog.Substring( 0, currentCharacterIndex );

                timer = 0f;
            }
        }
    }

    private void OnClick() {
        if ( IsTyping() ) {
            currentCharacterIndex = wholeDialog.Length;
            dialog.text = wholeDialog.Substring( 0, currentCharacterIndex );
        }
        else {
            onRequestNextDialog.Invoke();
        }
    }

    private bool IsTyping() {
        return currentCharacterIndex < wholeDialog.Length;
    }

    #region TextDisplayer implementation
    public void Show() {
        gameObject.SetActive( true );
    }

    public void Hide() {
        gameObject.SetActive( false );
    }

    public void RegisterOnRequestNextDialog( Action method ) {
        onRequestNextDialog = method;
    }

    public void SetText( string text ) {
        wholeDialog = text;
        currentCharacterIndex = 0;
        timer = 0f;

        dialog.text = string.Empty;
    }
    #endregion
}