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Search (While You Type) in a ComboBox
Today's users almost expect this to happen in all search boxes. After all, if their favourite search engine juggles with tens of thousand keywords
while they type, and suggests matching phrases on the fly,why shouldn't they expect the same from your application?
Naturally, the unnamed search engine applies very advanced technology, using massive pre-indexed tables with complex relationships, and search
statistics. This isn't the topic of this article. The search used here is deliberately not optimized and is only fitting for very small tables (let's say
below 10'000 records).
Instead, the present article focuses on the user interface. A combo box is used to show the results of the current search, and is updated on the fly,
while the focus remains in the control. This allows to narrow down the search until the desired information is located. Note that this is different
from the built-in auto-complete feature, which works only if the user always types the first letters of the information displayed in the combo.
The specific while you type aspect might not be suitable for every application. However, if that option is turned off, what remains is a more
classical search box, but using the combo's drop-down section as search result window. This technique is very well received by users, even if they
have to press Tab in order to see the result of the search. So, even if that aspect isn't what you are looking for, you might still be interested in the
search mechanism and in the attached demo database.
It must be said that the complete solution is rather technical. Several advanced techniques are used, which are not always explained in full. This
article was written for Access developers who need to create a search box with a professional look and feel, and not for novice users creating their
first criteria form.
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only sometimes, the user will have to check each time which it is.
This leads to the `not in list event. In almost all cases, the editing will end with a search string that is not as such in the list. However, if a single
hotel has been identified, it makes sense to automatically select that record. As a matter of fact, the first hotel in the list could be selected whenever
the user tabs out of the control.
A Timer Form
Perhaps this would warrant a distinct article. Each form has a timer event, triggered when the timer interval drops to zero. If a timer is needed, it is
therefore possible to create a tiny form for the sole purpose of counting down some interval, and to trap its timer event. This is of course a bit of an
overkill opening an entire form just for a timer but since Visual Basic isn't itself multi-threaded, this is the only option. The form really onlyneeds a module (for easy instantiation) and an active timer event.
Another form's class module using the timer form could look like this:Option Explicit
Dim WithEvents Timer As Form
Private Sub cmdTimer_Click()
Set Timer = New Form_zsfrmTimer
Timer.TimerInterval = 1000
End Sub
Private Sub Timer_Timer()
MsgBox "Time out!"
Set Timer = NothingEnd Sub
The command button creates a new (hidden) instance of the timer form, called zsfrmTimer, and sets its timer interval to one thousand milliseconds.
A second later, provided the button wasn't clicked again, the timer event is triggered.
When the first keystroke occurs in the combo box, a timer form is instantiated. At each keystroke, the timer interval is reset. If the timer runs out,
the event is used to filter the combo box.
The mechanism is quite simple, and can be used whenever a timer is needed. On a given form, the form's own timer is of course the best choice,
but the aim here was to leave the main form's timer untouched, so that the same implementation of the search combo could be used on all forms of
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is created, which will be instantiated when needed. The call from each form is simply this:Option Explicit
Dim ComboHotel As New claComboHotel
Private Sub Form_Load()
ComboHotel.Init cboHotel
End Sub
This means that, in order to add such a combo to a new form, it's sufficient to copy it from another form and to add two lines to the form's module.
This makes it very easy to create a consistent interface throughout the application.
The ComboBox
Before it can be copied from form to form, it needs to be created. In this case, it will have five columns:
a hidden ID column (the number of the hotel),
the first non-hidden column combining the name with the city,
the name of the establishment alone,
the name of the city,
the two-letter state abbreviation.
However, it doesn't need to have a row source: that is provided by the class module.Const cstrSelect _
= " SELECT ID," _
& " Chr(9)+Establishment & ', '+City AS Display," _
& " Establishment," _
& " City," _
& " State" _
& " FROM Hotels"Const cstrOrderBy _
= " ORDER BY City, Establishment, ID"
Private Sub ResetRowSource(Optional Criteria)
If IsMissing(Criteria) Then Criteria = mvarCriteria
mcboAny.RowSource = cstrSelect & " WHERE " + Criteria & cstrOrderBy
mfDirty = True
End Sub
The only surprise here is the Chr(9) in front of the first visible column. This is the internal code for the Tab character, which cannot be entered in
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any field in Access. Although this character is not visible, it completely shuts down the auto-complete feature. When the user types royal, it will
not match the Royal Manotel, Genve, because of the missing leading tab character.
Notice also that the standard sort order isn't alphabetic. In this instance, it's more useful to sort on the city than on the hotel name alone. After
entering the keyword jeune (for youth hostels), the city becomes the key column.
Initialization of the Combo
The call to theInitmethod from the form allows the class module to trap all needed events.Public Sub Init( _
Combo As ComboBox, _
Optional Criteria = Null)
Debug.Assert Combo.OnChange = "[Event Procedure]"
Debug.Assert Combo.OnEnter = "[Event Procedure]"
Debug.Assert Combo.OnExit = "[Event Procedure]"
Debug.Assert Combo.OnNotInList = "[Event Procedure]"
Set mcboAny = Combo
mvarCriteria = Criteria
ResetRowSource
End Sub
The Debug lines are useful at design time; they will stop execution if the combo box doesn't trigger the four events needed. As can be seen in the
form module above, the events do not need to be handled by the form (unless this is needed for some other purpose), but they must be active.
An optional criteria can be used at initialization, for example if only some hotels should be selectable on that particular form (only active hotels,only hotels for which a contract exists, etc). This criteria is a top-level filter for that particular instance.
Combo Events
The basic events are entering, exiting, and changing the content.Private Sub mcboAny_Change()
If mfrmClock Is Nothing Then Set mfrmClock = New Form_zsfrmTimer
mfrmClock.TimerInterval = 300
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End Sub
Private Sub mcboAny_Enter()
If mcboAny.ListCount Then Else
End Sub
Private Sub mcboAny_Exit(Cancel As Integer)
Set mfrmClock = Nothing
If mfDirty Then ResetRowSource: mfDirty = False
mvarLast = Null
End Sub
As explained above, the `change event doesn't requery the combo box. Instead, it resets a timer to 0.3 seconds. When that time elapses without
user input, the requery is triggered. Note that the timer form is created only once and recycled for the entire editing session. The `exit event is aclean-up routine, releasing the timer form and resetting the combo box to it's unfiltered state.
The `enter event contains a simple yet efficient hack: it accesses the combo's list count. As a side effect, this forces Access to actually populate the
list. This makes the scroll bar much more friendly and immediately usable to scroll through all records (instead of only the first batch of retrieved
records). This makes sense in this demo, but not necessarily if the list is large.
Since auto-complete has been shut down, the `not in list event should handle the exit from the combo in a graceful manner. If the user ends the
session with a mouse or arrow key selection of a hotel, then a hotel has been selected. If not, the event performs the selection.Private Sub mcboAny_NotInList(NewData As String, Response As Integer)
If mfrmClock.TimerInterval Then mfrmClock_Timer
With mcboAny
If .ListCount = 0 Then
.RowSource = "SELECT Null, Null, '*** no match ***'"
.Dropdown .Undo
Response = acDataErrContinue
mfDirty = True
Else
.RowSource = "SELECT " & .ItemData(0) & ", '" & NewData & "'"
Response = acDataErrAdded
mfDirty = True
End If
End With
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mvarLast = "*"
End Sub
The core of the function simply selects the first item from the list. If no items were found, the combo is used as a message box, and shows no
match. The code can easily be modified to handle the case where more than one item is in the list. Instead of just taking the first one, the function
could force the user to make a selection among them, with or without displaying an error message. This is elaborated in the demo database.
Line 3 is a sanity check for really fast typists. If this event is triggered before the combo box was even refreshed, the list would not correspond to
the current search criteria. In that case, and that case only, the normal time-out of 0.3 seconds is skipped and the combo is updated without delay. If
a user has discovered that the letters tik uniquely select a given hotel, he or she will not wait for the list refresh and tab out immediately...
The Search Itself
This is the most complex, yet probably the least useful code sample, as it depends totally on the present data and on the arbitrary search syntax
created for it. However, a few essential programming techniques are worth explaining.Private Sub mfrmClock_Timer()
Static sfBusy As Boolean
Dim strCols() As String
Dim strWords() As String
Dim varW As Variant
Dim varWhere As Variant
On Error GoTo Done:
Do While sfBusy: DoEvents: Loop
sfBusy = True
mfrmClock.TimerInterval = 0
If mcboAny.ListIndex >= 0 Then GoTo Done
mfDirty = True
varWhere = "(" + mvarCriteria + ")"
strCols = Split(mcboAny.Text, ",")
If UBound(strCols) >= 0 Then
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strWords = Split(strCols(0), " ")
For Each varW In strWords
If Len(varW) Then _
varWhere = varWhere + " And " _
& "Establishment Like '*" + Swiss(varW) + "*'"
Next varW
End If
If UBound(strCols) >= 1 Then
varW = Trim(strCols(1))
If IsStateCode(varW) Then
varWhere = varWhere + " And " _
& "State='" & varW & "'"ElseIf Len(varW) Then
varWhere = varWhere + " And " _
& "City Like '" + Swiss(varW) + "*'"
End If
End If
If mfrmClock.TimerInterval Then GoTo Done
If Nz(varWhere) Nz(mvarLast) Then
ResetRowSource varWhere
If mcboAny.ListCount Then Else
DoEvents
mcboAny.Dropdown
mvarLast = varWhere
End If
Done:
sfBusy = False
End Sub
The core of the function splits the search criteria at the comma, and further splits the first part at every space. Lines 20-28 build the criteria against
the hotel name, and lines 30-39 against either the state field or against the city field. The boolean function IsStateCode() performs a simple lookup
of any two-character search string in the appropriate table.
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Line 11 is called a semaphore. This technique is essential for any code that could potentially be reentrant, meaning in this case that it could get
called again before it has finished running. For VBA, this is perhaps a very slight risk, but it's bad practice to disregard it entirely. Similarly, after
creating the criteria for the requery, line XX checks whether a new timer has been set (meaning an additional character was entered). If that is the
case, the current filter is already obsolete and should be discarded. Finally, line 52 clears the semaphore and the next timer event can be processed.
The in Htel
A final refinement is needed, namely the treatment of accented characters. Since the sample data is a list of hotel names, the fact that some, but not
all, are written with an cannot be ignored. Creating accent insensitive searches in Accessisn't a simple topic, but since the application is limited
to Swiss hotels, there are only five languages to consider, with only a handful of diacriticals.Function Swiss(ByVal pstrText As String) As String
pstrText = Replace(pstrText, "a", "[a]") pstrText = Replace(pstrText, "e", "[e]")
pstrText = Replace(pstrText, "i", "[i]")
pstrText = Replace(pstrText, "o", "[o]")
pstrText = Replace(pstrText, "u", "[u]")
pstrText = Replace(pstrText, "c", "[c]")
Swiss = pstrText
End Function
The function simply replaces a few characters, including o, by a pattern for the Like operator listing all possible variants, e.g. [o]. This way,
the user can type hotel for Htel and zurich for Zrich without harm.
Summary
As stated in the introduction, on the fly searching is only comfortable if the search itself is nearly instantaneous, or at least under the 0.5 secondsthreshold. Searching through a few hundred records doesn't require any optimization, but for serious data a specific data structure might be needed
to achieve a sufficiently low response time.
Still, even with a decent efficiency, it is rarely possible to requery forcibly between everykeystroke. Formally, the search should occur in a separate
process, using the time between keystrokes to narrow down and refine the search. This is not possible within Access, so the solution suggested here
is to use a short timer.
1) Each keystroke resets a form timer, and doesn't requery automatically. A fast typist can enter the entire search string without any interruption.
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