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   <TITLE>Assign event handlers at runtime</TITLE>

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

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<H3>

Assign event handlers at runtime?<BR>

</H3>

<P>

<B>Step 1:</B> Identify out what type of handler you will be writing. <TT>OnClick</TT> event

handlers are <TT>TNotifyEvent</TT> types. The <TT>OnIdle</TT> event of <TT>TApplication</TT> is

of the type <TT>TIdleEvent</TT>, and it passes two arguments to its handler. The VCL help file

can help you identify what type of handler you should create for a particular event. For

<TT>OnIdle</TT>, the help file lists:

</P>

<pre>

<b>__property</b> TIdleEvent OnIdle<b>;</b>

</pre>

<BR>

<P>

<B>Step 2:</B> Determine what type of variable the function should return (almost always

<TT>void</TT>), and what variables should be passed to the function. Unfortunately,

the help file leaves this part out. For events that could have been created at design time

(such as <TT>OnClick</TT>), you could create a dummy handler, and then cut and paste the function

declarations. For handlers that cannot be created at design time, you must refer to the include

files. </P>

<P>

The <TT>TApplication</TT> handler types are listed in FORMS.HPP. You might want to

open the file now and take a look. Find the <TT>typedef</TT> for <TT>TIdleEvent</TT>. It should

look something like:

<BR>

<pre>

<b>typedef</b> <b>void</b> <b>__fastcall</b> <b>(</b><b>__closure</b> <b>*</b>TIdleEvent<b>)</b><b>(</b>System<b>:</b><b>:</b>TObject<b>*</b> Sender<b>,</b>

                                                <b>bool</b> <b>&</b>Done<b>)</b><b>;</b>

</pre>

<P>

This means that an <TT>OnIdle</TT> function should look like this:

</P>

<pre>

<b>void</b> <b>__fastcall</b> TForm1<b>:</b><b>:</b>AppIdle<b>(</b>System<b>:</b><b>:</b>TObject <b>*</b>Sender<b>,</b> <b>bool</b> <b>&</b>Done<b>)</b>

</pre>

<BR>

<P>

<B>Step 3:</B> Add the function declaration to the class that is handling the event. For

<TT>OnIdle</TT> it would look like this:</P>

<pre>

<b>class</b> TForm1 <b>:</b> <b>public</b> TForm

<b>{</b>

    <b>__published</b><b>:</b>

    <b>private</b><b>:</b>

        <b>void</b> <b>__fastcall</b> AppIdle<b>(</b>System<b>:</b><b>:</b>TObject <b>*</b>Sender<b>,</b> <b>bool</b> <b>&</b>Done<b>)</b><b>;</b>

</pre>

<P>

<BR>

<B>Step 4:</B> Code the function body.

</P>

<pre>

<b>void</b> <b>__fastcall</b> TForm1<b>:</b><b>:</b>AppIdle<b>(</b>System<b>:</b><b>:</b>TObject <b>*</b>Sender<b>,</b> <b>bool</b> <b>&</b>Done<b>)</b>

<b>{</b>

    <font color="navy">// function body here</font>

<b>}</b>

</pre>

<BR>

<P>

<B>Step 5:</B> And now, the step you have all been waiting for. To associate your handler

function to a component's event, you must assign the function name to the handler. The most

convenient place to do this is inside the constructor of the class that contains the handler

function. </P>

<pre>

<b>__fastcall</b> TForm1<b>:</b><b>:</b>TForm1<b>(</b>TComponent<b>*</b> Owner<b>)</b>

    <b>:</b> TForm<b>(</b>Owner<b>)</b>

<b>{</b>

    Application<b>-></b>OnIdle <b>=</b> AppIdle<b>;</b>

<b>}</b>

</pre>

<P>

Notice that there is nothing fancy here. Just assign the function name to the handler. Do not

assign the address of the function to the handler.

</P>       



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