GenericsTList (Delphi)

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Description

This example requires a button and two TListboxes on a form. The Generics.Collections objects are created dynamically. Notice that you can call the TList Sort method with the TComparer as a parameter, or create the TList with the TComparer and just call Sort. Also, do not free reverseComp after associating it with the sort list.

Code

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, Generics.Defaults,
  Generics.Collections;   // Must appear after Classes, so that it can use the correct TCollectionNotification.

type
  TForm2 = class(TForm)
    Button1: TButton;
    ListBox1: TListBox;
    ListBox2: TListBox;
    Label1: TLabel;
    Label2: TLabel;
    Label3: TLabel;
    ListBox3: TListBox;
    ListBox4: TListBox;
    Label4: TLabel;
    ListBox5: TListBox;
    Label5: TLabel;
    ListBox6: TListBox;
    Label6: TLabel;
    procedure Button1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    Dictionary: TDictionary<Integer, Integer>;
    procedure OnDictAdd(
      Sender: TObject; const Item: Integer; Action: TCollectionNotification);
  end;
  TReverseIntComparer = class(TComparer<Integer>)
    function Compare(const Left, Right: Integer): Integer; override;
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}
{$APPTYPE CONSOLE}


function TReverseIntComparer.Compare(const Left, Right: Integer): Integer;
begin
  if Left < Right then
    Result := 1
  else if Left > Right then
    Result := -1
  else
    Result := 0;
end;

procedure TForm2.OnDictAdd(Sender: TObject; const Item: Integer; Action: TCollectionNotification);
begin
  if Action = cnAdded then
    WriteLn('TDictionary Key has been added to the dictionary');
  if Action = cnRemoved then
    WriteLn('TDictionary Key has been removed from the dictionary');
end;

procedure TForm2.Button1Click(Sender: TObject);
var
  mylist, sortlist: TList<Integer>;
  reverseComp: TReverseIntComparer;
  myarray: TArray;
  values: array of Integer;
  i: Integer;
begin
  myarray:= TArray.Create;
  Dictionary.Add(0, 0);
  try
    Randomize;
    mylist:= Generics.Collections.TList<Integer>.Create;
    reverseComp:= TReverseIntComparer.Create;
    for i := 0 to 100 - 1 do
      mylist.Add(Random(100));
    for i := 0 to mylist.Count - 1 do
      Listbox1.Items.Add(IntToStr(mylist[i]));
    mylist.Sort;
    for i := 0 to mylist.Count - 1 do
      Listbox2.Items.Add(IntToStr(mylist[i]));
    mylist.Delete(0);
    mylist.Delete(2);
    mylist.Delete(4);
    for i := 0 to mylist.Count - 1 do
      Listbox3.Items.Add(IntToStr(mylist[i]));
    mylist.Sort(reverseComp);
    for i := 0 to mylist.Count - 1 do
      Listbox4.Items.Add(IntToStr(mylist[i]));
    sortlist:= Generics.Collections.TList<Integer>.Create(reverseComp);
    for i := 0 to 100 - 1 do
      sortlist.Add(Random(100));
    for i := 0 to mylist.Count - 1 do
      Listbox5.Items.Add(IntToStr(sortlist[i]));
    sortlist.Sort;
    for i := 0 to mylist.Count - 1 do
      Listbox6.Items.Add(IntToStr(mylist[i]));
  finally
//    reverseComp.Free;
    sortlist.Free;
    mylist.Free;
    Dictionary.Free;
    myarray.Free;
  end;
end;

procedure TForm2.FormCreate(Sender: TObject);
begin
  Dictionary := TDictionary<Integer, Integer>.Create;
  Dictionary.OnKeyNotify := Form2.OnDictAdd;
end;

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