A focus on the FOCUS property [Revelation On-Line Wiki]

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====== A focus on the FOCUS property ======

=== Published 08 AUG 2019 at 04:35:57PM ===

There are two methods for setting the input focus in OpenInsight, and there is a subtle implementation difference that can impact your applications if you're not careful how you apply them.

The first method is to use an object's own **FOCUS** property and set it to TRUE$ like so:

<code>
Call Set_Property_Only( @Window : ".EDL_SURNAME", "FOCUS", TRUE$ )
</code>

This moves the focus to the specified object, but the event queue will be flushed both before and after the focus has been set, //thereby preventing any events raised as a result of the focus being moved from being processed//.  This method was originally designed for use with validation routines so the focus could be reset to an invalid control "safely".

The second method is to use the **SYSTEM** object's **FOCUS** property:

<code>
Call Set_Property_Only( "SYSTEM", "FOCUS", @Window : ".EDL_SURNAME" )
</code>

This moves the focus, but any events triggered as a result of moving the focus (like **LOSTFOCUS** and **GOTFOCUS** events) //will be processed//.

That all sounds straightforward enough, but using the first method can lead to unexpected results if you are relying on an event already in the queue that you subsequently need - not a common situation but one we encountered recently while converting an old form to v10.  In our case we had a menu failing to show when the focus was on a specific control, and it turned out that the **LOSTFOCUS** event for the control was setting it's **FOCUS** property to TRUE$.  This had the effect of killing a pending **MENUDROPDOWN** event (new in v10) that created the menu to display, hence no menu.

In this case the solution is to use the **SYSTEM BLOCKEVENTS** property to turn off events being triggered while the focus is moved and then restore event processing afterwards:

<code>
Call Set_Property_Only( "SYSTEM", "BLOCKEVENTS", TRUE$ )
Call Set_Property_Only( "SYSTEM", "FOCUS", @Window : ".EDL_SURNAME" )
Call Set_Property_Only( "SYSTEM", "BLOCKEVENTS", FALSE$ )
</code>

This prevented any **LOSTFOCUS** and **GOTFOCUS** events from being raised, but the **MENUDROPDOWN** event was still in the queue.

Using the **SYSTEM** **FOCUS** and **BLOCKEVENTS** properties in this way is a far "safer" alternative when moving the focus because you have full control over how events are handled, and I would always advocate using this method over using a control's own **FOCUS** property.

 


=== Comments ===


----

=== At 08 AUG 2019 04:40PM Donald Bakke wrote:  ===

<QUOTE>Good advice for OI v9 applications as well!

</QUOTE>

----

=== At 08 AUG 2019 04:42PM David wrote:  ===

<QUOTE>Great information ... thanks

</QUOTE>





== Original ID: revdevx.com/?p=2898 ==