如何实现显示“在此处输入"的文本框? [英] How do I implement a TextBox that displays "Type here"?

问题描述

在用户将文本输入到 TextBox 之前显示Type here to ..."是当今众所周知的可用性功能.如何在 C# 中实现这一功能?

Displaying "Type here to ..." until the user enters text into a TextBox is a well-known usability feature nowadays. How would one implement this feature in C#?

我的想法是重写 OnTextChanged，但是处理文本从和到Type here"的变化的逻辑有点棘手......

My idea is to override OnTextChanged, but the logic to handle the changes of text from and to "Type here" is a bit tricky...

在初始化时显示在此处输入"并在第一次输入时将其删除很容易，但我想在每次输入的文本变空时显示消息.

Displaying "Type here" on initialization and removing it on first input is easy, but I want to display the message every time the entered text becomes empty.

推荐答案

你要找的是一个带有watermark"的TextBox.

What you're looking for is a TextBox with a "watermark".

这里有一个 C# 的示例实现，全部归功于 Wael Alghool.

There's a sample implementation for C# here, all credits to Wael Alghool.

他的代码的相关部分是:

The relevant part of his code is:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Drawing;

namespace wmgCMS
{
    class WaterMarkTextBox : TextBox
    {
        private Font oldFont = null;
        private Boolean waterMarkTextEnabled = false;

        #region Attributes 
            private Color _waterMarkColor = Color.Gray;
            public Color WaterMarkColor
            {
                get { return _waterMarkColor; }
                set { _waterMarkColor = value; Invalidate();/*thanks to Bernhard Elbl
                                                              for Invalidate()*/ }
            }

            private string _waterMarkText = "Water Mark";
            public string WaterMarkText
            {
                get { return _waterMarkText; }
                set { _waterMarkText = value; Invalidate(); }
            }
        #endregion

        //Default constructor
        public WaterMarkTextBox()
        {
            JoinEvents(true);
        }

        //Override OnCreateControl ... thanks to  "lpgray .. codeproject guy"
        protected override void OnCreateControl() 
        { 
            base.OnCreateControl();
            WaterMark_Toggel(null, null); 
        }

        //Override OnPaint
        protected override void OnPaint(PaintEventArgs args)
        {
            // Use the same font that was defined in base class
            System.Drawing.Font drawFont = new System.Drawing.Font(Font.FontFamily,
                Font.Size, Font.Style, Font.Unit);
            //Create new brush with gray color or 
            SolidBrush drawBrush = new SolidBrush(WaterMarkColor);//use Water mark color
            //Draw Text or WaterMark
            args.Graphics.DrawString((waterMarkTextEnabled ? WaterMarkText : Text),
                drawFont, drawBrush, new PointF(0.0F, 0.0F));
            base.OnPaint(args);
        }

        private void JoinEvents(Boolean join)
        {
            if (join)
            {
                this.TextChanged += new System.EventHandler(this.WaterMark_Toggel);
                this.LostFocus += new System.EventHandler(this.WaterMark_Toggel);
                this.FontChanged += new System.EventHandler(this.WaterMark_FontChanged);
                //No one of the above events will start immeddiatlly 
                //TextBox control still in constructing, so,
                //Font object (for example) couldn't be catched from within
                //WaterMark_Toggle
                //So, call WaterMark_Toggel through OnCreateControl after TextBox
                //is totally created
                //No doupt, it will be only one time call

                //Old solution uses Timer.Tick event to check Create property
            }
        }

        private void WaterMark_Toggel(object sender, EventArgs args )
        {
            if (this.Text.Length <= 0)
                EnableWaterMark();
            else
                DisbaleWaterMark();
        }

        private void EnableWaterMark()
        {
            //Save current font until returning the UserPaint style to false (NOTE:
            //It is a try and error advice)
            oldFont = new System.Drawing.Font(Font.FontFamily, Font.Size, Font.Style,
               Font.Unit);
            //Enable OnPaint event handler
            this.SetStyle(ControlStyles.UserPaint, true);
            this.waterMarkTextEnabled = true;
            //Triger OnPaint immediatly
            Refresh();
        }

        private void DisbaleWaterMark()
        {
            //Disbale OnPaint event handler
            this.waterMarkTextEnabled = false;
            this.SetStyle(ControlStyles.UserPaint, false);
            //Return back oldFont if existed
            if(oldFont != null)
                this.Font = new System.Drawing.Font(oldFont.FontFamily, oldFont.Size,
                    oldFont.Style, oldFont.Unit);
        }

        private void WaterMark_FontChanged(object sender, EventArgs args)
        {
            if (waterMarkTextEnabled)
            {
                oldFont = new System.Drawing.Font(Font.FontFamily,Font.Size,Font.Style,
                    Font.Unit);
                Refresh();
            }
        }
    }
}

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