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Moving Forms and Creating Resize Gripper Boxes
 Download the Hit Test Sample project and DLL (25kb)
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Before you Begin
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This project requires the SSubTmr.DLL component. Make sure you have loaded and registered this before trying the project.
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This article provides code demonstrating how to modify the behaviour of a VB form so that parts of it behave like
title bars and resize borders. You can use this code to:
- Create windows without a title bar that you can click on anywhere to move around
- Build fake title bars
- Add a Resize Gripper box to the lower right-hand corner.
Whilst the principle behind this code isn't unique, this example code is a lot more useful than
the versions on certain other sites. To run this technique you need to subclass your form
for the WM_NCHITTEST message. And if you try to put a breakpoint into a form which is actively
subclassing this message, VB's IDE collapses rather like the author after two or three pints
of beer and can only be stopped by the three fingered salute (unlike the author).
This sample demonstrates how to isolate all the nasty code into a DLL so you debug your project
without fear of an untimely and embarassing demise. Of course, if you use this technique,
when you come to ship you can just add a single class into the project, remove the DLL reference
and skip the dependency.
The WM_NCHITTEST Message
When you move the mouse around the form, by default Windows automatically determines when you are at a border, or in
the title bar, and changes the mouse pointer as required and allows resizing or moving.
This default behaviour can be overridden by changing the response to the WM_NCHITTEST message which
is sent to the window whenever the mouse moves over it.
Here are the return values you can provide:
Public Enum EHitTestAreas
    HTERROR = (-2)
    HTTRANSPARENT = (-1)
    HTNOWHERE = 0
    HTCLIENT = 1
    HTCAPTION = 2
    HTSYSMENU = 3
    HTGROWBOX = 4
    HTMENU = 5
    HTHSCROLL = 6
    HTVSCROLL = 7
    HTMINBUTTON = 8
    HTMAXBUTTON = 9
    HTLEFT = 10
    HTRIGHT = 11
    HTTOP = 12
    HTTOPLEFT = 13
    HTBOTTOM = 15
    HTBOTTOMLEFT = 16
    HTBOTTOMRIGHT = 17
    HTBORDER = 18
End Enum
The most useful values are:
- HTCAPTION, which makes an area of your form behave exactly like
a title bar (so when you click on it you can drag the form around, or when you double click
the form is maximised) and
- HTBOTTOMRIGHT, which makes an area work as if it was part of the bottom right hand border.
This style is used in the resize gripper boxes in the bottom right hand corner of most
Windows status bars.
The HitTest DLL/Class
To take all the subclassing processing out of executable code, this class stores the
window handle(s) to subclass for the WM_NCHITTEST message and also each of the areas to
perform hit-testing on, along with the required response.
Method reference for the HitTest DLL:
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Method
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Description
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AttachHitTester
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Adds a VB control or form to the list of windows to perform hit testing on.
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DestroyHitTester
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Clears up all hit testers. Called automatically when the object is destroyed.
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AddArea
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Adds an area to be hit tested. The area can be referred to either by a key
or by the index returned by this function.
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PointInObject
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A helper function. Returns whether the given point (expressed in pixels, relative
to the form) is within the rectangle of a form or control.
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SetTestArea
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Sets the rectangle to be hit tested and the response for an area
added earlier with the AddArea method.
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SetTestAreaFromObject
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The same as SetTestArea, but takes a VB control or form to determine
the hit testing rectangle from.
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This version only supports rectangular areas but in theory it could be expanded to any
arbitrary region using the API region creation calls (see
Changing Window Shapes for more details) and PtInRegion.
Creating a Gripper Box
Using the Hit Tester class its easy to create a fake gripper box. The first point thing to
set up the area of the gripper box with the class, and set the response to HTBOTTOMRIGHT. Then
you want to draw the gripper box itself. The quickest way to do this is to use labels and
take advantage of the Marlett font which is supplied with all Win32 systems to draw the gripper.
Marlett Characters
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