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   <TITLE>Create an MDI child in the maximized state</TITLE>

   <META NAME="Author" CONTENT="Harold Howe">

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<H3>

Create an MDI child in the maximized state

</H3>

<P>

If you try to set the <TT>WindowState</TT> of an MDI child form to <TT>wsMaximized</TT>, you

may notice that the form doesn't initially draw itself in the maximized state. The form first

appears in a non-maximized state before resizing to the maximized state. To the user, this

behavior may look odd.

</P>

<UL>

<LI><A HREF="#explanation">Background Information</A>

<LI><A HREF="#solution"   >Solution</A>

</UL>

<BR>

<A NAME="explanation">

<H4>Background Information:</H4>

<P>

The problem is caused by VCL code that runs as a result of the <TT>AfterConstruction</TT>

method of <TT>TCustomForm</TT>. <TT>AfterConstruction</TT> contains an <TT>if</TT> statement

that sets the <TT>Visible</TT> property of the form to <TT>true</TT>.

</P>

<pre>

<b>void</b> <b>__fastcall</b> TCustomForm<b>:</b><b>:</b>AfterConstruction<b>(</b><b>)</b>

<b>{</b>

    <b>...</b>

    <b>...</b>

    <b>if</b> <b>(</b>FFormState<b>.</b>Contains<b>(</b>fsVisible<b>)</b><b>)</b>

        Visible <b>=</b> <b>true</b><b>;</b>

<b>}</b>

</pre>

<P>

<TT>Visible</TT> is set based on the contents of the form's private <TT>FFormState</TT> set

variable. For MDI child forms, the VCL ensures that <TT>FFormState</TT> will always contain

<TT>fsVisible</TT>, irregardless of how you maniplulate the <TT>Visible</TT> property of the

form in code (the IDE will not let you set <TT>Visible</TT> to false for an MDI child).

</P>

<P>

The assignment to <TT>Visible</TT> eventually posts a user message to the form called

<TT>CM_VISIBLECHANGING</TT>. This message is handled by <TT>TWinControl</TT>, which

responds by calling the <TT>UpdateControlState</TT> function, and <TT>UpdateControlState</TT>

in turn calls <TT>UpdateShowing</TT>. <TT>TWinControl::UpdateShowing</TT> performs another user message called

<TT>CM_SHOWINGCHANGED</TT>. <TT>TCustomForm</TT> catches this user message, and at this

point, things begin to make sense.

</P>

<pre>

<b>void</b> <b>__fastcall</b> TCustomForm<b>:</b><b>:</b>CMShowingChanged<b>(</b>TMessage <b>&</b>Message<b>)</b>

<b>{</b>

    <b>...</b>

    <b>...</b>

        <b>if</b> <b>(</b>FormStyle <b>==</b> fsMDIChild<b>)</b>

        <b>{</b>

            <font color="navy">// {Fake a size message to get MDI to behave }</font>

            <b>if</b> <b>(</b>FWindowState <b>==</b> wsMaximized<b>)</b>

            <b>{</b>

                SendMessage<b>(</b>Application<b>.</b>MainForm<b>.</b>ClientHandle<b>,</b>

                            WM_MDIRESTORE<b>,</b> Handle<b>,</b> <font color="blue">0</font><b>)</b><b>;</b>

                ShowWindow<b>(</b>Handle<b>,</b> SW_SHOWMAXIMIZED<b>)</b><b>;</b>

            <b>}</b>

        <b>...</b>

<b>}</b>

</pre>

<P>

The <TT>SendMessage</TT> statement turns out to be the culprit. Sending <TT>WM_MDIRESTORE</TT> to

the MDI client paints the MDI child in a non-maximized state on the screen. When the MDI

client receives the message, it sends other messages back to the MDI child form. These

messages include non-client painting messages and the <TT>WM_MDIACTIVATE</TT> message. The

painting messages combine to produce one annoying result: the MDI child is completely drawn

in the non-maximized state.

</P>

<A NAME="solution">

<H4>Solution:</H4>

<P>

Altering what the VCL does inside <TT>CMShowingChanged</TT> would be difficult, but we can

tell Windows not to repaint the MDI client area while <TT>TCustomForm</TT> is sending the

troublesome <TT>WM_MDIRESTORE</TT> message. We can force the MDI client window to suppress

all drawing until we give the OK. The API function <TT>LockWindowUpdate</TT> allows us to prevent the

MDI client from being painted until after the form's constructor has returned. Here is a

code example that suppresses painting of the MDI client until after the goofy

<TT>SendMessage</TT>'s have taken their effect. As a result, the intermediate painting of

the MDI window in the non-maximized state never makes it to the screen.

</P>

<pre>

<font color="navy">// set WindowState to wsMaximized at design time.</font>

LockWindowUpdate<b>(</b>Application<b>-></b>MainForm<b>-></b>ClientHandle<b>)</b><b>;</b>

TMDIChild <b>*</b>child <b>=</b> <b>new</b> TMDIChild<b>(</b>Application<b>)</b><b>;</b>

child<b>-></b>Caption <b>=</b> <font color="blue">"Maximized MDI"</font><b>;</b>

LockWindowUpdate<b>(</b>NULL<b>)</b><b>;</b>

</pre>

<BR>

<P>

<B>Note:</B> Only one window can be locked at a time. The <TT>HWND</TT> argument to

<TT>LockWindowUpdate</TT> determines which window we want to suppress. Passing <TT>NULL</TT>

to <TT>LockWindowUpdate</TT> deactivates the locking scheme.

</P>

<P>

<B>Note:</B> Notice that we lock the MDI client window and not the mainform of the program.

MDI child windows are children of the MDI client window, not the mainform. The

<TT>ClientHandle</TT> property of <TT>TForm</TT> returns the <TT>HWND</TT> of the MDI

client window. The <TT>ClientHandle</TT> property is only valid for forms that have

<TT>FormStyle</TT> set to <TT>fsMDIForm</TT>.

</P>

<P>

<B>Note:</B> If you create your MDI child windows from within the MDI parent, you can omit

the <TT>Application-&gt;Mainform</TT> prefix to <TT>ClientHandle</TT>. In other words, you

can call <TT>LockWindowUpdate</TT> like this:

</P>

<pre>

LockWindowUpdate<b>(</b>ClientHandle<b>)</b><b>;</b>

</pre>

<P>

<B>Note:</B> When the MDI form is initially drawn in the non-maximized state, the window is

not sized based on the <TT>Width</TT> and <TT>Height</TT> properties that you specified at

design time. The operating system sizes the window itself. If you step through

<TT>TCustomForm::CreateHandle</TT>, you will see that the size and position members of the

<TT>TMDICreateStruct</TT> structure are initialized to <TT>CW_USEDEFAULT</TT>. This tells

Windows to size the form however it sees fit. In fact, the <TT>Width</TT>, <TT>Height</TT>,

<TT>Left</TT>, and <TT>Top</TT> properties of the MDI child form are re-calculated

after the <TT>WM_MDICREATE</TT> message has been sent. The default values are leftovers from the

<TT>CreateParams</TT> function. You should be able to override <TT>CreateParams</TT> if you

need to alter the initial width, height, or position of the form.

</P>

<P>

<B>Note:</B> A couple of other functions play a role in why an MDI child is initially drawn

in the non-maximized state. The functions execute in response to the <TT>WM_MDIRESTORE</TT>

message that is sent from <TT>TCustomForm::CMShowingChanged</TT>. These functions include

<TT>WMMDIActivate</TT>, <TT>SetActive</TT>, and <TT>MergeMenu</TT> (all methods of

<TT>TCustomForm</TT>. The <TT>WM_MDIRESTORE</TT> message triggers a </TT>WM_MDIACTIVATE</TT>

message. The <TT>WMMDIActivate</TT> handler function calls <TT>SetActive</TT>.

<TT>SetActive</TT> calls <TT>MergeMenu</TT>, and <TT>MergeMenu</TT> does this:

</P>

<pre>

<font color="navy">// assume MergeState is true</font>

<b>int</b> Size<b>;</b>

<b>if</b><b>(</b>MergeState <b>&&</b> <b>(</b>FormStyle <b>==</b> fsMDIChild<b>)</b> <b>&&</b> <b>(</b>WindowState <b>=</b> wsMaximized<b>)</b>

<b>{</b>

    <font color="navy">// { Force MDI to put back the system menu of a maximized child }</font>

    Size <b>=</b> ClientWidth <b>+</b> <b>(</b><b>int</b><b>(</b>ClientHeight<b>)</b> <b><<</b> <font color="blue">16</font><b>)</b><b>;</b>

    SendMessage<b>(</b>Handle<b>,</b> WM_SIZE<b>,</b> SIZE_RESTORED<b>,</b> Size<b>)</b><b>;</b>

    SendMessage<b>(</b>Handle<b>,</b> WM_SIZE<b>,</b> SIZE_MAXIMIZED<b>,</b> Size<b>)</b><b>;</b>

<b>}</b>

</pre>

<P>

When this code runs, the MDI child is still in the non-maximized state. The first

<TT>WM_SIZE</TT> message causes the MDI child to finish painting itself in the non-maximized

state. All of this code executes from within the very first <TT>SendMessage</TT> call in

<TT>CMShowingChanged</TT>.

</P>

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