Delphi Component Development - Propagate Events inside component - delphi

Question

I am trying to develop a new TEdit-Component.
TDBFilterEdit = class(TEdit)
The component is meant to Filter an associated DataSet based on the string that is entered in its Edit-Field.
this is what my component looks like:
type
TDBFilterEdit = class(TEdit)
private
{ Private-Deklarationen }
fFilter:String;
fDataSource:TDataSource;
fDataSet:TDataSet;
fText:string;
protected
{ Protected-Deklarationen }
procedure SetFilter(value:String);
procedure SetDS(value:TDataSource);
procedure FilterRecords(DataSet:TDataSet; var Accept:Boolean);
procedure Change(Sender:TObject);
procedure SetText(value:String);
public
{ Public-Deklarationen }
constructor Create(AOwner:TComponent);
published
{ Published-Deklarationen }
property Text:String read fText write SetText;
property Filter:String read fFilter write SetFilter;
property DataSource:TDataSource read fDataSource write SetDS;
end;
Now, I am pretty Novice when it comes to component-development. My first Idea was to Override the OnFilterRecord-method of the Dataset as soon as the DataSource gets assigned to my component and trigger it whenever the text of my Edit-component changes.
procedure TDBFilterEdit.SetDS(value:TDataSource);
var
myaccept:Boolean;
begin
fDataSource:=value;
fDataSet:=fDataSource.DataSet;
if fDataSet=nil then Exit;
fDataSet.OnFilterRecord:=FilterRecords;
if Filter<>'' then fDataSet.OnFilterRecord(fDataSet,myaccept);
end;
My Problem is, I don't know how to make the component aware that its Text-property got updated. I tried overriding the OnChange-Method with following code
procedure TDBFilterEdit.Change(Sender:TObject);
begin
Filter:=Text;
inherited Change();
end;
however, to no avail so far.

Answer 1

My Problem is, I don't know how to make the component aware that its Text-property got updated.
The Text property is inherited from TControl. When the property value changes, TControl issues a CM_TEXTCHANGED notification message to itself. Descendant classes can handle that message by either:
using a message handler:
type
TDBFilterEdit = class(TEdit)
...
procedure CMTextChanged(var Message: TMessage); message CM_TEXTCHANGED;
...
published
...
// DO NOT redeclare the Text property here!
// It is already published by TEdit...
end;
procedure TDBFilterEdit.CMTextChanged(var Message: TMessage);
begin
inherited;
// use new Text value as needed...
Filter := Text;
end;
overriding the virtual WndProc() method.
type
TDBFilterEdit = class(TEdit)
...
protected
...
procedure WndProc(var Message: TMessage); override;
...
end;
procedure TDBFilterEdit.WndProc(var Message: TMessage);
begin
inherited;
if Message.Msg = CM_TEXTCHANGED then
begin
// use new Text value as needed...
Filter := Text;
end;
end;
As for the rest of your component, it should look more like this:
type
TDBFilterEdit = class(TEdit)
private
{ Private-Deklarationen }
fDataSource: TDataSource;
fDataSet: TDataSet;
fFilter: String;
procedure FilterRecords(DataSet: TDataSet; var Accept: Boolean);
procedure SetDataSource(Value: TDataSource);
procedure SetDataSet(Value: TDataSet);
procedure SetFilter(const Value: String);
procedure StateChanged(Sender: TObject);
procedure UpdateDataSetFilter;
protected
{ Protected-Deklarationen }
procedure Notification(AComponent: TComponent; Operation: TOperation); override;
procedure WndProc(var Message: TMessage); override;
public
{ Public-Deklarationen }
destructor Destroy; override;
published
{ Published-Deklarationen }
property DataSource: TDataSource read fDataSource write SetDataSource;
property Filter: String read fFilter write SetFilter;
end;
...
destructor TDBFilterEdit.Destroy;
begin
SetDataSource(nil);
inherited;
end;
procedure TDBFilterEdit.FilterRecords(DataSet: TDataSet; var Accept: Boolean);
begin
// ...
end;
procedure TDBFilterEdit.Notification(AComponent: TComponent; Operation: TOperation);
begin
inherited Notification(AComponent, Operation);
if Operation = opRemove then
begin
if AComponent = fDataSource then
begin
SetDataSet(nil);
fDataSource := nil;
end
else if AComponent = fDataSet then
begin
fDataSet := nil;
end;
end;
end;
procedure TDBFilterEdit.SetFilter(const Value: String);
begin
if fFilter <> Value then
begin
fFilter := Value;
UpdateDataSetFilter;
end;
end;
procedure TDBFilterEdit.SetDataSource(Value: TDataSource);
begin
if fDataSource <> Value then
begin
SetDataSet(nil);
if fDataSource <> nil then
begin
fDataSource.RemoveFreeNotification(Self);
fDataSource.OnStateChange := nil;
end;
fDataSource := Value;
if fDataSource <> nil then
begin
fDataSource.FreeNotification(Self);
fDataSource.OnStateChange := StateChanged;
SetDataSet(fDataSource.DataSet);
end;
end;
end;
procedure TDBFilterEdit.SetDataSet(Value: TDataSet);
begin
if fDataSet <> Value then
begin
if fDataSet <> nil then
begin
fDataSet.RemoveFreeNotification(Self);
fDataSet.OnFilterRecord := nil;
end;
fDataSet := Value;
if fDataSet <> nil then
begin
fDataSet.FreeNotification(Self);
fDataSet.OnFilterRecord := FilterRecords;
UpdateDataSetFilter;
end;
end;
end;
procedure TDBFilterEdit.StateChanged(Sender: TObject);
begin
if fDataSource.DataSet <> fDataSet then
SetDataSet(fDataSource.DataSet);
end;
procedure TDBFilterEdit.UpdateDataSetFilter;
begin
if fDataSet <> nil then
begin
fDataSet.Filter := fFilter;
fDataSet.Filtered := fFilter <> '';
end;
end;
procedure TDBFilterEdit.WndProc(var Message: TMessage);
begin
inherited;
if Message.Msg = CM_TEXTCHANGED then
Filter := Text;
end;
UPDATE: sorry, my bad. The CM_TEXTCHANGED message is only sent when the Text property is updated programmably in code. To detect when the user changed the text, you need to handle the Win32 EN_CHANGE notification instead:
using a message handler:
type
TDBFilterEdit = class(TEdit)
...
procedure CMTextChanged(var Message: TMessage); message CM_TEXTCHANGED;
procedure CNCommand(var Message: TWMCommand); message CN_COMMAND;
...
published
...
// DO NOT redeclare the Text property here!
// It is already published by TEdit...
end;
procedure TDBFilterEdit.CMTextChanged(var Message: TMessage);
begin
inherited;
// use new Text value as needed...
Filter := Text;
end;
procedure TDBFilterEdit.CNCommand(var Message: TWMCommand);
begin
inherited;
if Message.NotifyCode = EN_CHANGE then
begin
// use new Text value as needed...
Filter := Text;
end;
end;
overriding the virtual WndProc() method.
type
TDBFilterEdit = class(TEdit)
...
protected
...
procedure WndProc(var Message: TMessage); override;
...
end;
procedure TDBFilterEdit.WndProc(var Message: TMessage);
begin
inherited;
case Message.Msg of
CM_TEXTCHANGED: begin
// use new Text value as needed...
Filter := Text;
end;
CN_COMMAND: begin
if TWMCommand(Message).NotifyCode = EN_CHANGE then
begin
// use new Text value as needed...
Filter := Text;
end;
end;
end;
end;
In fact, TCustomEdit already handles EN_CHANGE for you, and will call its virtual Change() method (to fire its OnChange event), which you can override:
type
TDBFilterEdit = class(TEdit)
...
protected
...
procedure Change; override;
...
end;
procedure TDBFilterEdit.Change;
begin
inherited;
// use new Text value as needed...
Filter := Text;
end;