WPF程序假死啊~~~循环中间内容不显示~~~

写了个延时输出的程序，不过界面假死....而且循环中间的字都没显示...只是在假死结束后直接显示循环最后一步的结果...求解释啊...为什么WPF中textBox没有Update的操作啊....唉...很急啊private void button1_Click(object sender, RoutedEventArgs e)
        {
            
            
                string mainstr = textBox1.Text;
                string substr1 = textBox2.Text;
                char substr = Convert.ToChar(substr1);
                bool flag = false;
                for (int i = 0; i < mainstr.Length; i++)
                {
                    if (flag)
                    {
                        break;
                    }
                    else
                    {
                        int pand = mainstr[i].CompareTo(substr);
                        bool panduan = Convert.ToBoolean(pand);
                        if (panduan)
                        {
                            textBox3.Text = "该字符" + mainstr[i] + "不是所需字符，继续比较下一个";
                            Thread.Sleep(1000);
                        }
                        else
                        {
                            textBox3.Text = "字符已经找到，其位置为" + (i + 1);
                            Thread.Sleep(1000);
                            flag = true;
                        }
                    }
            
            
                }
        }