delphi - 如何使用 "for/in"查找另一个组件内部的组件？

Question

我正在重构组件代码，发现以下代码:

procedure TMenuToolbarButton.ClearActivation;
var
  i: Integer;
begin
  for i := 0 to Self.Parent.ComponentCount -1 do
  begin
    if (Self.Parent.Components[i] is TMenuToolbarButton) then
    begin
      (Self.Parent.Components[i] as TMenuToolbarButton).FActivatedImage.Visible := False;
      (Self.Parent.Components[i] as TMenuToolbarButton).FActivatedImageGrowLeft.Visible := False;
    end;
  end;
end;

我今天工作得很好，但我想在这个方法中使用 for/in，如下所示:

procedure TMenuToolbarButton.ClearActivation;
var
  MyMenuToolbarButton: TMenuToolbarButton;
begin
  for MyMenuToolbarButton in Self.Parent do
  begin
    MyMenuToolbarButton.FActivatedImage.Visible         := False;
    MyMenuToolbarButton.FActivatedImageGrowLeft.Visible := False;
  end;
end;

我已经尝试使用 Generics.Collections 类型转换 Self.Parent像这样:TObjectList<TMenuToolbarButton>(Self.Parent)

所以，我想知道是否有更好的方法让工作代码更“优雅”

Answer 1

Parent 是 TWinControl，而不是 TObjectList，因此您尝试的类型转换无效。

您不能直接将 for.. in 循环与 Components 属性一起使用，因为它不是满足任何 documented requirements 的可迭代容器。 :

Delphi supports for-element-in-collection style iteration over containers. The following container iteration patterns are recognized by the compiler:

• for Element in ArrayExpr do Stmt;

• for Element in StringExpr do Stmt;

• for Element in SetExpr do Stmt;

• for Element in CollectionExpr do Stmt;

• for Element in Record do Stmt;

Components 属性不是数组、字符串、集合、集合或记录，因此无法通过 for..in 进行迭代> 循环。

但是，TComponent 本身满足可迭代集合的记录要求:

To use the for-in loop construct on a class or interface, the class or interface must implement a prescribed collection pattern. A type that implements the collection pattern must have the following attributes:

• The class or interface must contain a public instance method called GetEnumerator(). The GetEnumerator() method must return a class, interface, or record type.

• The class, interface, or record returned by GetEnumerator() must contain a public instance method called MoveNext(). The MoveNext() method must return a Boolean. The for-in loop calls this method first to ensure that the container is not empty.

• The class, interface, or record returned by GetEnumerator() must contain a public instance, read-only property called Current. The type of the Current property must be the type contained in the collection.

TComponent 有一个公共(public) GetEnumerator()返回 TComponentEnumerator 的方法在内部迭代 Components 属性的对象。但是，由于该属性处理 TComponent 对象，因此您仍然需要在循环内手动对它们进行类型转换。

试试这个:

procedure TMenuToolbarButton.ClearActivation;
var
  //i: Integer;
  Comp: TComponent;
  Btn: TMenuToolbarButton;
begin
  //for i := 0 to Self.Parent.ComponentCount -1 do
  for Comp in Self.Parent do
  begin
    //Comp := Self.Parent.Components[i];
    if Comp is TMenuToolbarButton then
    begin
      Btn := TMenuToolbarButton(Comp);
      Btn.FActivatedImage.Visible := False;
      Btn.FActivatedImageGrowLeft.Visible := False;
    end;
  end;
end;

因此，在这种情况下，使用 for..in 循环并不会比传统的 for..to 循环真正获得任何有用的东西。