delphi - TVirtualStringTree 中的 GetText 被触发更多次

Question

请看下面的代码:

unit Unit2;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, VirtualTrees, Vcl.StdCtrls;

type
  TTreeData = record
    Fields: array of String;
  end;
  PTreeData = ^TTreeData;

const
  szVirtualTree = SizeOf(TTreeData);

type
  TForm2 = class(TForm)
    tree: TVirtualStringTree;
    Button1: TButton;
    procedure treeGetText(Sender: TBaseVirtualTree; Node: PVirtualNode;
      Column: TColumnIndex; TextType: TVSTTextType; var CellText: string);
    procedure treeGetNodeDataSize(Sender: TBaseVirtualTree;
      var NodeDataSize: Integer);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}

procedure TForm2.Button1Click(Sender: TObject);
var
  LTreeData: PTreeData;
  Node: PVirtualNode;
  cols, rows, col, row: Integer;
begin
  cols:= 2;
  rows:= 2;

  Tree.Header.Columns.Clear;
  if cols > 0 then
    for col := 0 to cols - 1 do
      with Tree.Header.Columns.Add do
        begin
          Text:= 'H' + IntToStr(col);
          Width:= 80;
        end;

  Tree.Clear;
  Tree.BeginUpdate;

  if (cols> 0) and (rows > 0) then
    for row := 0 to rows - 1 do
      begin
        Node:= Tree.AddChild(nil,nil);
        Tree.ValidateNode(Node,False);

        LTreeData:= Tree.GetNodeData(Node);
        SetLength(LTreeData^.Fields, cols);

        if cols > 0 then
          for col := 0 to cols - 1 do
            LTreeData^.Fields[col]:= '[' + inttostr(col) + ',' + inttostr(row) + ']';
      end;

  Tree.EndUpdate;
end;

procedure TForm2.treeGetNodeDataSize(Sender: TBaseVirtualTree;
  var NodeDataSize: Integer);
begin
  NodeDataSize:= szVirtualTree;
end;

procedure TForm2.treeGetText(Sender: TBaseVirtualTree; Node: PVirtualNode;
  Column: TColumnIndex; TextType: TVSTTextType; var CellText: string);
var
  LTreeData: PTreeData;
begin
  LTreeData:= Sender.GetNodeData(Node);
  CellText:= LTreeData^.Fields[Column];
  showmessage(CellText);
end;

end.

按下按钮后，我收到的消息是:

[0,0] 
[0,0] 
[0,0] 
[0,0] 
[1,0]
[1,0]
[1,0]
[1,0] 
[0,1]
[0,1]
[0,1]
[0,1] 
[1,1]
[1,1]
[1,1]
[1,1]

基本上，每个节点和每列都会触发 GetText 4 次。

为什么不只触发一次？像这样:[0,0] [1,0] [0,1] [1,1]

Answer 1

该控件使用所谓的虚拟范例。作者 Mike Lischke explains this like so :

As the name already indicates, this control uses a different paradigm for tree management than other controls of this kind. It does not know anything about the data it manages (except its size), not even the captions of a node. Everything is retrieved from the application via events (or descendants via overridden methods).

该控件不跟踪节点标题。每当它需要知道它们时，它就会触发 OnGetText 方法。该控件不缓存，不记住节点标题。因此，如果控制代码的多个部分依赖于标题，则同一节点的事件将被多次调用。

您观察到的行为是完全正常且完全可以预料的。 OnGetText 处理程序的工作是生成节点的文本。如果同一问题被问两次，请不要担心。只要确保每次都给出相同的答案即可!