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Providing an OnChange Event

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Now that users of the calendar can change the date within the calendar, it makes sense to allow applications to respond to those changes.

Add an OnChange event to TSampleCalendar.

  1. Declare the event, a field to store the event, and a dynamic method to call the event: type
  TSampleCalendar = class(TCustomGrid)
  private
    FOnChange: TNotifyEvent;
  protected
    procedure Change; dynamic;
  .
  .
  .
  published
    property OnChange: TNotifyEvent read FOnChange write FOnChange;
  .
  .
  . 
class PACKAGE TSampleCalendar : public TCustomGrid
{
private:
    TNotifyEvent FOnChange;
    .
    .
    .
protected:
    virtual void __fastcall Change();
__published:
    __property TNotifyEvent OnChange = {read=FOnChange, write=FOnChange};
    .
    .
    .
}
  1. Write the Change method:

    procedure TSampleCalendar.Change;
    begin
      if Assigned(FOnChange) then FOnChange(Self);
    end; 
    
    void __fastcall TSampleCalendar::Change()
    {
      if(FOnChange != NULL) FOnChange(this);
    }
    
  2. Add statements calling Change to the end of the SetCalendarDate and SetDateElement methods:

    procedure TSampleCalendar.SetCalendarDate(Value: TDateTime);
    begin
      FDate := Value;
      UpdateCalendar;
      Change;                                               { this is the only new statement }
    end;
    procedure TSampleCalendar.SetDateElement(Index: Integer; Value: Integer);
    begin
        .                                            { many statements setting element values }
        .
        .
        FDate := EncodeDate(AYear, AMonth, ADay);
        UpdateCalendar;
        Change;                                                                { this is new }
      end;
    end; 
    
    void __fastcall TSampleCalendar::SetCalendarDate(TDateTime Value)
    {
      FDate = Value;
      UpdateCalendar();
      Change();              // this is the only new statement
    }
    void __fastcall TSampleCalendar::SetDateElement(int Index, int Value)
    {
      .
      .
      .    // many statements setting element values
      FDate = TDateTime(AYear, AMonth, ADay);
      UpdateCalendar();
      Change();              // this is new
    }
    

Applications using the calendar component can now respond to changes in the date of the component by attaching handlers to the OnChange event.

See Also

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