How can I dynamically inject code into event handlers in Delphi?

Question

For debugging / performance tests I would like to dynamically add logging code to all event handlers of components of a given type at run time.

For example, for all

Answer 1

You can use the following scheme to rewire the datasets:

type
  TDataSetEventWrapper = class
  private
    FDataSet: TDataSet;
    FOrgAfterOpen: TDataSetNotifyEvent;
    FOrgBeforeOpen: TDataSetNotifyEvent;
    procedure MyAfterOpen(DataSet: TDataSet);
    procedure MyBeforeOpen(DataSet: TDataSet);
  protected
    property DataSet: TDataSet read FDataSet;
  public
    constructor Create(ADataSet: TDataSet);
    destructor Destroy; override;
  end;

constructor TDataSetEventWrapper.Create(ADataSet: TDataSet);
begin
  Assert(ADataSet <> nil);
  inherited Create;
  FDataSet := ADataSet;
  FOrgAfterOpen := FDataSet.AfterOpen;
  FOrgBeforeOpen := FDataSet.BeforeOpen;
  FDataSet.AfterOpen := MyAfterOpen;
  FDataSet.BeforeOpen := MyBeforeOpen;
end;

destructor TDataSetEventWrapper.Destroy;
begin
  FDataSet.AfterOpen := FOrgAfterOpen;
  FDataSet.BeforeOpen := FOrgBeforeOpen;
  inherited;
end;

procedure TDataSetEventWrapper.MyBeforeOpen(DataSet: TDataSet);
begin
  if Assigned(FOrgBeforeOpen) then
    FOrgBeforeOpen(DataSet);
end;

procedure TDataSetEventWrapper.MyAfterOpen(DataSet: TDataSet);
begin
  if Assigned(FOrgAfterOpen) then
    FOrgAfterOpen(DataSet);
end;

Inside MyAfterOpen and MyBeforeOpen you can bring in your code before, after or around the call to the original event handler.

Collect the wrapper objects in a TObjectList with OwnsObjects := true and everything will revert to the original when you clear or free the objectlist.

Caution: For this code to work the events have to be wired already when you create the wrappers and manually reassigning those events is forbidden.