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How to build a TTreeView from a file Turn on/off line numbers in source code. Switch to Orginial background IDE or DSP color Comment or reply to this aritlce/tip for discussion. Bookmark this article to my favorite article(s). Print this article
Delphi 2.x
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			Author: Tomas Rutkauskas

I would like to populate a TTreeView from a simple file with the following structure

Key: Integer (unique)
Name: String (description)
Parent: Integer (key of parent in treeview)

I assume that the key and parent fields are all I need to build the treeview 
(parent = 0 would be a root node)


I would break this down into two steps:

1) Read the file into memory
2) Populate the treeview using a recursive function

1) One method of doing this would be by building a TCollection/ TCollectionItem 
pair of classes. The TCollectionItems just need three fields:

1   TInputItem = class(TCollectionItem)
2   private
3     fKey: integer;
4     fName: string;
5     fParent: integer;
6   public
7     property Key: integer read fKey write fKey;
8     property Name: string read fName write fName;
9     property Parent: integer read fParent write fParent;
10  end;

Note: using properties is not strictly necessary, but is good style as it allows 
easier subsequent amendment.

Now we could use a standard TCollection to hold our TInputItems but it is neater to 
have a descendent of this too:

11  TInputCollection = class(TCollection)
12  public
13    function AddItem(const AName: string; AKey, AParent: integer): TInputItem;
14    property InputItem[index: integer]: TInputItem read GetInputItem; default;
15  end;

Creating a default property like InputItem above makes coding very tidy. It allows 
us to do the following:

16  var
17    InputCollection: InputCollection;
18    ix: integer;
20  InputCollection := TInputCollection.Create(TInputItem);
21  InputCollection.AddItem('First', 1, 0);
22  InputCollection.AddItem('Second', 2, 0);
23  InputCollection.AddItem('FirstChild', 3, 1);
25  for ix := 0 to InputCollection.Count - 1 do
26    if InputCollection[ix].Parent = 0 then
27      {DoSomething};

The last line, because of the index property being declared default, is the same as:

28  if InputCollection.InputItem[ix].Parent = 0 then
29    {DoSomething;}
31  //Without the property at all, you would code:
33  if TInputItem(InputCollection.Items[ix]).Parent = 0 thenDoSomething;
34  {DoSomething;}

In order to support the above, the implementation of the two methods:
36  function TInputCollection.AddItem(const AName: string; AKey, AParent: integer):
37    TInputItem;
38  begin
39    Result := Add as TInputItem;
40    Result.Key := AKey;
41    Result.Name := AName;
42    Result.Parent := AParent;
43  end;
45  function TInputCollection.GetInputItem(index: integer): TInputItem;
46  begin
47    Result := Items[ix] as TInputItem;
48  end;
50  //We can now design an overall structure of a PopulateTree procedure:
52  procedure PopulateTree(tv: TTreeView);
53  var
54    ic: TInputCollection;
55  begin
56    ic := TInputCollection.Create(TInputItem);
57    try
58      LoadTreeItems(ic);
59      PopulateTreeItems(tv, nil, ic, 0);
60    finally
61      ic.Free;
62    end;
63  end;
65  //LoadTreeItems can be tested via code similar to:
67  procedure LoadTreeItems(ic: TInputCollection);
68  begin
69    ic.AddItem('First', 1, 0);
70    ic.AddItem('Second', 2, 0);
71    ic.AddItem('FirstChild', 3, 1);
72  end;

before replacing with your own loop through your input file. PopulateTreeItems is 
passed the treeview, the parent node and the parent id and it is a recursive 

2) Having done all the above, this part is now very easy. PopulateTreeItems 
iterates through the collection looking for items that match the passed parent id. 
For each item that matches, it adds a treenode and then calls PopulateTreeItems 
passing itself as the parent:
74  procedure PopulateTreeItems(tv: TTreeView; pnode: TTreeNode; ic: TInputCollection;
75    parent: integer);
76  var
77    node: TTreeNode;
78    ix: integer;
79  begin
80    for ix := 0 to ic.Count - 1 do
81    begin
82      if ic[ix].Parent = parent then
83      begin
84        node := tv.Items.Add(pnode, ic[ix].Name);
85        PopulateTreeItems(tv, node, ic, ic[ix].Key); {recursive call}
86      end;
87    end;
88  end;

I apologise in advance if there are problems with the above code. It is completely untested. In practice, I don't do things quite like that, but populate treenodes on demand via the OnExpand event handler.

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