How to identify the Object type? - delphi

Question

Lets say I have a Treeview, and it contains items with Object pointers. How can I determine from the selected item what the Object is, so I can access it?
Here is a basic example of some classes and code to give an idea:
Note: TChildObject1 and TChildObject2 inherit from TMyObject.
type
TMyObject = class
private
FName: string;
public
property Name: string read FName write FName;
constructor Create(aName: string);
end;
type
TChildObject1 = class(TMyObject)
private
FSomeString: string;
public
property SomeString: string read FSomeString write FSomeString;
constructor Create(aName: string);
destructor Destroy; override;
end;
type
TChildObject2 = class(TMyObject)
private
FSomeInteger: integer;
public
property SomeInteger: integer read FSomeInteger write FSomeInteger;
constructor Create(aName: string);
destructor Destroy; override;
end;
Lets say they were created and added to a TTreeview like so:
procedure NewChild1(aTreeView: TTreeView; aName: string);
var
Obj: TChildObject1;
begin
Obj := TChildObject1.Create(aName);
try
aTreeView.Items.AddObject(nil, Obj.Name, Obj);
finally
Obj.Free;
end;
end;
procedure NewChild2(aTreeView: TTreeView; aName: string);
var
Obj: TChildObject2;
begin
Obj := TChildObject2.Create(aName);
try
aTreeView.Items.AddObject(nil, Obj.Name, Obj);
finally
Obj.Free;
end;
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
// add the items to the tree
NewChild1(TreeView1, 'Child Object 1');
NewChild2(TreeView1, 'Child Object 2');
end;
Now, when I select a Node in the Treeview, how can I determine which Object class the pointer leads to? I tried this, which is not working:
Note: This does not error, but it does not return the correct value (ie, does not pick up the correct object)
procedure TForm1.TreeView1Click(Sender: TObject);
var
Obj: TMyObject;
begin
if TreeView1.Selected <> nil then
begin
Obj := TMyObject(TreeView1.Selected.Data);
if Obj is TChildObject1 then
begin
Edit1.Text := 'this node is a child1 object';
end else
if Obj is TChildObject2 then
begin
Edit1.Text := 'and this node is child 2 object';
end;
end;
end;
I could do it something like below, but I don't think is the right way, it means a lot of checking, declaring, assigning etc.
procedure TForm1.TreeView1Click(Sender: TObject);
var
ChildObj1: TChildObject1;
ChildObj2: TChildObject2;
begin
if TreeView1.Selected <> nil then
begin
if TreeView1.Selected.Text = 'Child Object 1' then
begin
ChildObj1 := TreeView1.Selected.Data;
Edit1.Text := ChildObj1.SomeString;
end else
if TreeView1.Selected.Text = 'Child Object 2' then
begin
ChildObj2 := TreeView1.Selected.Data;
Edit1.Text := IntToStr(ChildObj2.SomeInteger);
end;
end;
end;
Tips and advice appreciated.

Answer 1

The main problem is which you are freeing the memory of the object that you are adding to the treeview. So the data of the nodes points to a invalid location.
To assign the objects to a node use a code like this
Obj := TChildObject1.Create(aName);
aTreeView.Items.AddObject(nil, Obj.Name, Obj);
and when you need dispose the data you can call the Free method for each node.
for i:= 0 to TreeView1.Items.Count - 1 do
begin
Obj:= TMyObject(TreeView1.Items.Item[i].Data);
if Assigned(Obj) then
Obj.Free;
end;