How to make a custom ComboBox (OwnerDrawFixed) loo

问题:

I am making a custom ComboBox, inherited from Winforms' standard ComboBox. For my custom ComboBox, I set DrawMode to OwnerDrawFixed and DropDownStyle to DropDownList. Then I write my own OnDrawItem method. But I ended up like this:

How do I make my Custom ComboBox to look like the Standard one?

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Update 1: ButtonRenderer

After searching all around, I found the ButtonRenderer class. It provides a DrawButton static/shared method which -- as the name implies -- draws the proper 3D button. I'm experimenting with it now.

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Update 2: What overwrites my control?

I tried using the Graphics properties of various objects I can think of, but I always fail. Finally, I tried the Graphics of the form, and apparently something is overwriting my button.

Here's the code:

Protected Overrides Sub OnDrawItem(ByVal e As System.Windows.Forms.DrawItemEventArgs)
  Dim TextToDraw As String = _DefaultText
  __Brush_Window.Color = Color.FromKnownColor(KnownColor.Window)
  __Brush_Disabled.Color = Color.FromKnownColor(KnownColor.GrayText)
  __Brush_Enabled.Color = Color.FromKnownColor(KnownColor.WindowText)
  If e.Index >= 0 Then
    TextToDraw = _DataSource.ItemText(e.Index)
  End If
  If TextToDraw.StartsWith("---") Then TextToDraw = StrDup(3, ChrW(&H2500)) ' U+2500 is "Box Drawing Light Horizontal"
  If (e.State And DrawItemState.ComboBoxEdit) > 0 Then
    'ButtonRenderer.DrawButton(e.Graphics, e.Bounds, VisualStyles.PushButtonState.Default)
  Else
    e.DrawBackground()
  End If
  With e
    If _IsEnabled(.Index) Then
      .Graphics.DrawString(TextToDraw, Me.Font, __Brush_Enabled, .Bounds.X, .Bounds.Y)
    Else
      '.Graphics.FillRectangle(__Brush_Window, .Bounds)
      .Graphics.DrawString(TextToDraw, Me.Font, __Brush_Disabled, .Bounds.X, .Bounds.Y)
    End If
  End With
  TextToDraw = Nothing
  ButtonRenderer.DrawButton(Me.Parent.CreateGraphics, Me.ClientRectangle, VisualStyles.PushButtonState.Default)

  'MyBase.OnDrawItem(e)
End Sub

And here's the result:

Replacing Me.Parent.CreateGraphics with e.Graphics got me this:

And doing the above + replacing Me.ClientRectangle with e.Bounds got me this:

Can anyone point me whose Graphics I must use for the ButtonRenderer.DrawButton method?

PS: The bluish border is due to my using PushButtonState.Default instead of PushButtonState.Normal

---

I Found An Answer! (see below)

回答1:

I forgot where I found the answer... I'll edit this answer when I remember.

But apparently, I need to set the Systems.Windows.Forms.ControlStyles flags. Especially the ControlStyles.UserPaint flag.

So, my New() now looks like this:

Private _ButtonArea as New Rectangle

Public Sub New()
  ' This call is required by the designer.
  InitializeComponent()
  ' Add any initialization after the InitializeComponent() call.
  MyBase.SetStyle(ControlStyles.Opaque Or ControlStyles.UserPaint, True)
  MyBase.DrawMode = Windows.Forms.DrawMode.OwnerDrawFixed
  MyBase.DropDownStyle = ComboBoxStyle.DropDownList
  ' Cache the button's modified ClientRectangle (see Note)
  With _ButtonArea
    .X = Me.ClientRectangle.X - 1
    .Y = Me.ClientRectangle.Y - 1
    .Width = Me.ClientRectangle.Width + 2
    .Height = Me.ClientRectangle.Height + 2
  End With
End Sub

And now I can hook into the OnPaint event:

Protected Overrides Sub OnPaint(ByVal e As System.Windows.Forms.PaintEventArgs)
  If Me.DroppedDown Then
    ButtonRenderer.DrawButton(Me.CreateGraphics, _ButtonArea, VisualStyles.PushButtonState.Pressed)
  Else
    ButtonRenderer.DrawButton(Me.CreateGraphics, _ButtonArea, VisualStyles.PushButtonState.Normal)
  End If
  MyBase.OnPaint(e)
End Sub

Note: Yes, the _ButtonArea rectangle must be enlarged by 1 pixel to all directions (up, down, left, right), or else there will be a 1-pixel 'perimeter' around the ButtonRenderer that shows garbage. Made me crazy for awhile until I read that I must enlarge the Control's rect for ButtonRenderer.

回答2:

I had this problem myself and the reply by pepoluan got me started. I still think a few things are missing in order to get a ComboBox with looks and behavior similar to the standard ComboBox with DropDownStyle=DropDownList though.

DropDownArrow
We also need to draw the DropDownArrow. I played around with the ComboBoxRenderer, but it draws a dark border around the area of the drop down arrow so that didn't work.

My final solution was to simply draw a similar arrow and render it onto the button in the OnPaint method.

Hot Item Behavior
We also need to ensure our ComboBox has a hot item behavior similar to the standard ComboBox. I don't know of any simple and reliable method to know when a mouse is no longer above the control. Therefore I suggest using a Timer that checks at each tick whether the mouse is still over the control.

Edit Just added a KeyUp event handler to make sure the control would update correctly when a selection was made using the keyboard. Also made a minor correction of where the text was rendered, to ensure it is more similar to the vanilla combobox' text positioning.

Below is the full code of my customized ComboBox. It allows you to display images on each item and is always rendered as in the DropDownList style, but hopefully it should be easy to accommodate the code to your own solution.

using System;
using System.ComponentModel;
using System.Drawing;
using System.Drawing.Design;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.Windows.Forms;
using System.Windows.Forms.VisualStyles;

namespace CustomControls
{
    /// <summary>
    /// This is a special ComboBox that each item may conatins an image.
    /// </summary>
    public class ImageComboBox : ComboBox
    {
        private static readonly Size arrowSize = new Size(18, 20);


        private bool itemIsHot;

        /* Since properties such as SelectedIndex and SelectedItems may change when the mouser is hovering over items in the drop down list
         * we need a property that will store the item that has been selected by comitted selection so we know what to draw as the selected item.*/
        private object comittedSelection;

        private readonly ImgHolder dropDownArrow = ImgHolder.Create(ImageComboBox.DropDownArrow());

        private Timer hotItemTimer;

        public Font SelectedItemFont { get; set; }

        public Padding ImageMargin { get; set; }

        //
        // Summary:
        //     Gets or sets the path of the property to use as the image for the items
        //     in the System.Windows.Forms.ListControl.
        //
        // Returns:
        //     A System.String representing a single property name of the System.Windows.Forms.ListControl.DataSource
        //     property value, or a hierarchy of period-delimited property names that resolves
        //     to a property name of the final data-bound object. The default is an empty string
        //     ("").
        //
        // Exceptions:
        //   T:System.ArgumentException:
        //     The specified property path cannot be resolved through the object specified by
        //     the System.Windows.Forms.ListControl.DataSource property.
        [DefaultValue("")]
        [Editor("System.Windows.Forms.Design.DataMemberFieldEditor, System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor))]
        public string ImageMember { get; set; }


        public ImageComboBox()
        {
            base.SetStyle(ControlStyles.Opaque | ControlStyles.UserPaint, true);

            //All the elements in the control are drawn manually.
            base.DrawMode = DrawMode.OwnerDrawFixed;

            //Specifies that the list is displayed by clicking the down arrow and that the text portion is not editable. 
            //This means that the user cannot enter a new value. 
            //Only values already in the list can be selected.
            this.DropDownStyle = ComboBoxStyle.DropDownList;

            //using DrawItem event we need to draw item
            this.DrawItem += this.ComboBoxDrawItemEvent;

            this.hotItemTimer = new Timer();
            this.hotItemTimer.Interval = 250;
            this.hotItemTimer.Tick += this.HotItemTimer_Tick;

            this.MouseEnter += this.ImageComboBox_MouseEnter;

            this.KeyUp += this.ImageComboBox_KeyUp;

            this.SelectedItemFont = this.Font;
            this.ImageMargin = new Padding(4, 4, 5, 4);

            this.SelectionChangeCommitted += this.ImageComboBox_SelectionChangeCommitted;
            this.SelectedIndexChanged += this.ImageComboBox_SelectedIndexChanged;
        }


        private static Image DropDownArrow()
        {
            var arrow = new Bitmap(8, 4, PixelFormat.Format32bppArgb);

            using (Graphics g = Graphics.FromImage(arrow))
            {
                g.CompositingQuality = CompositingQuality.HighQuality;

                g.FillPolygon(Brushes.Black, ImageComboBox.CreateArrowHeadPoints());
            }

            return arrow;
        }

        private static PointF[] CreateArrowHeadPoints()
        {
            return new PointF[4] { new PointF(0, 0), new PointF(7F, 0), new PointF(3.5F, 3.5F), new PointF(0, 0) };
        }

        private static void DrawComboBoxItem(Graphics g, string text, Image image, Rectangle itemArea, int itemHeight, int itemWidth, Padding imageMargin
            , Brush brush, Font font)
        {
            if (image != null)
            {
                // recalculate margins so image is always approximately vertically centered
                int extraImageMargin = itemHeight - image.Height;

                int imageMarginTop = Math.Max(imageMargin.Top, extraImageMargin / 2);
                int imageMarginBotttom = Math.Max(imageMargin.Bottom, extraImageMargin / 2);

                g.DrawImage(image, itemArea.X + imageMargin.Left, itemArea.Y + imageMarginTop, itemHeight, itemHeight - (imageMarginBotttom
                    + imageMarginTop));
            }

            const double TEXT_MARGIN_TOP_PROPORTION = 1.1;
            const double TEXT_MARGIN_BOTTOM_PROPORTION = 2 - TEXT_MARGIN_TOP_PROPORTION;

            int textMarginTop = (int)Math.Round((TEXT_MARGIN_TOP_PROPORTION * itemHeight - g.MeasureString(text, font).Height) / 2.0, 0);
            int textMarginBottom = (int)Math.Round((TEXT_MARGIN_BOTTOM_PROPORTION * itemHeight - g.MeasureString(text, font).Height) / 2.0, 0);

            //we need to draw the item as string because we made drawmode to ownervariable
            g.DrawString(text, font, brush, new RectangleF(itemArea.X + itemHeight + imageMargin.Left + imageMargin.Right, itemArea.Y + textMarginTop
                , itemWidth, itemHeight - textMarginBottom));
        }


        private string GetDistplayText(object item)
        {
            if (this.DisplayMember == string.Empty) { return item.ToString(); }
            else
            {
                var display = item.GetType().GetProperty(this.DisplayMember).GetValue(item).ToString();

                return display ?? item.ToString();
            }

        }

        private Image GetImage(object item)
        {
            if (this.ImageMember == string.Empty) { return null; }
            else { return item.GetType().GetProperty(this.ImageMember).GetValue(item) as Image; }

        }

        private void ImageComboBox_SelectionChangeCommitted(object sender, EventArgs e)
        {
            this.comittedSelection = this.Items[this.SelectedIndex];
        }

        private void HotItemTimer_Tick(object sender, EventArgs e)
        {
            if (!this.RectangleToScreen(this.ClientRectangle).Contains(Cursor.Position)) { this.TurnOffHotItem(); }
        }

        private void ImageComboBox_KeyUp(object sender, KeyEventArgs e)
        {
            this.Invalidate();
        }

        private void ImageComboBox_MouseEnter(object sender, EventArgs e)
        {
            this.TurnOnHotItem();
        }

        private void ImageComboBox_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (!this.DroppedDown)
            {
                if (this.SelectedIndex > -1) { this.comittedSelection = this.Items[this.SelectedIndex]; }
                else { this.comittedSelection = null; }

            }
        }

        private void TurnOnHotItem()
        {
            this.itemIsHot = true;
            this.hotItemTimer.Enabled = true;
        }

        private void TurnOffHotItem()
        {
            this.itemIsHot = false;
            this.hotItemTimer.Enabled = false;
            this.Invalidate(this.ClientRectangle);
        }

        /// <summary>
        /// Draws overridden items.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void ComboBoxDrawItemEvent(object sender, DrawItemEventArgs e)
        {
            //Draw backgroud of the item
            e.DrawBackground();
            if (e.Index != -1)
            {
                Brush brush;

                if (e.State.HasFlag(DrawItemState.Focus) || e.State.HasFlag(DrawItemState.Selected)) { brush = Brushes.White; }
                else { brush = Brushes.Black; }

                object item = this.Items[e.Index];

                ImageComboBox.DrawComboBoxItem(e.Graphics, this.GetDistplayText(item), this.GetImage(item), e.Bounds, this.ItemHeight, this.DropDownWidth
                    , new Padding(0, 1, 5, 1), brush, this.Font);
            }

        }

        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);

        // define the area of the control where we will write the text
        var topTextRectangle = new Rectangle(e.ClipRectangle.X - 1, e.ClipRectangle.Y - 1, e.ClipRectangle.Width + 2, e.ClipRectangle.Height + 2);

        using (var controlImage = new Bitmap(e.ClipRectangle.Width, e.ClipRectangle.Height, PixelFormat.Format32bppArgb))
        {
            using (Graphics ctrlG = Graphics.FromImage(controlImage))
            {
                /* Render the control. We use ButtonRenderer and not ComboBoxRenderer because we want the control to appear with the DropDownList style. */
                if (this.DroppedDown) { ButtonRenderer.DrawButton(ctrlG, topTextRectangle, PushButtonState.Pressed); }
                else if (this.itemIsHot) { ButtonRenderer.DrawButton(ctrlG, topTextRectangle, PushButtonState.Hot); }
                else { ButtonRenderer.DrawButton(ctrlG, topTextRectangle, PushButtonState.Normal); }


                // Draw item, if any has been selected
                if (this.comittedSelection != null)
                {
                    ImageComboBox.DrawComboBoxItem(ctrlG, this.GetDistplayText(this.comittedSelection), this.GetImage(this.comittedSelection)
                        , topTextRectangle, this.Height, this.Width - ImageComboBox.arrowSize.Width, this.ImageMargin, Brushes.Black, this.SelectedItemFont);
                }


                /* Now we need to draw the arrow. If we use ComboBoxRenderer for this job, it will display a distinct border around the dropDownArrow and we don't want that. As an alternative we define the area where the arrow should be drawn, and then procede to draw it. */
                var dropDownButtonArea = new RectangleF(topTextRectangle.X + topTextRectangle.Width - (ImageComboBox.arrowSize.Width
                            + this.dropDownArrow.Image.Width) / 2.0F, topTextRectangle.Y + topTextRectangle.Height - (topTextRectangle.Height
                            + this.dropDownArrow.Image.Height) / 2.0F, this.dropDownArrow.Image.Width, this.dropDownArrow.Image.Height);

                ctrlG.DrawImage(this.dropDownArrow.Image, dropDownButtonArea);

            }

            if (this.Enabled) { e.Graphics.DrawImage(controlImage, 0, 0); }
            else { ControlPaint.DrawImageDisabled(e.Graphics, controlImage, 0, 0, Color.Transparent); }

        }
        }
    }

internal struct ImgHolder
    {
        internal Image Image
        {
            get
            {
                return this._image ?? new Bitmap(1, 1); ;
            }
        }
        private Image _image;

        internal ImgHolder(Bitmap data)
        {
            _image = data;
        }
        internal ImgHolder(Image data)
        {
            _image = data;
        }

        internal static ImgHolder Create(Image data)
        {
            return new ImgHolder(data);
        }
        internal static ImgHolder Create(Bitmap data)
        {
            return new ImgHolder(data);
        }
    }

}