Delphi Informant Complete 1995 - 2000/Delphi Informant Complete 1995 to 2000.iso/Delphi Informant Magazine Complete Works SOURCE CODE 1998.rar/1998/May/di9805rs/Llist.pas

Pascal/Delphi Source File | 1997-12-31 | 6KB | 226 lines

unit LList;

interface

uses
  Windows, Messages, SysUtils, Graphics, Controls,
  Forms, Dialogs, Menus, StdCtrls, Classes;

type
    // The linked list cells.
    PCell = ^TCell;
    TCell = record
        Value    : String[20]; // The data.
        NextCell : PCell;      // The next cell in the list.
    end;

  TLinkedListForm = class(TForm)
    ValueText: TEdit;
    CmdInsertAfter: TButton;
    CmdRemoveAfter: TButton;
    procedure FormCreate(Sender: TObject);
    procedure CmdInsertAfterClick(Sender: TObject);
    procedure CmdRemoveAfterClick(Sender: TObject);
    procedure ValueTextChange(Sender: TObject);
    procedure DrawList;
    procedure FormPaint(Sender: TObject);
    procedure FormMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure EnableButtons;
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
    sentinel  : TCell;
    num_items : Integer; // For convenience.
    selected  : Integer; // Selected cell index. 0 = sentinel.
  public
    { Public declarations }
  end;

var
  LinkedListForm: TLinkedListForm;

implementation

{$R *.DFM}

const
    HGT = 14;
    GAP = 3;

// Initialize the empty linked list.
procedure TLinkedListForm.FormCreate(Sender: TObject);
begin
    sentinel.NextCell := nil;
    sentinel.Value := '<sentinel>';
    num_items := 0;

    // Start with no item selected.
    selected := -1;
end;

// Add a new cell after the selected cell.
procedure TLinkedListForm.CmdInsertAfterClick(Sender: TObject);
var
    after_me, new_cell : PCell;
    i                  : Integer;
begin
    // Find the insertion position.
    after_me := @sentinel;
    for i := 1 to selected do
        after_me := after_me^.NextCell;

    // Create the new cell.
    New(new_cell);
    new_cell^.Value := ValueText.Text;

    // Insert the cell in the list.
    new_cell^.NextCell := after_me^.NextCell;
    after_me^.NextCell := new_cell;
    num_items := num_items + 1;

    // Select the new cell and redisplay the list.
    selected := selected + 1;
    DrawList;
    EnableButtons;

    ValueText.Text := '';
    ValueText.SetFocus;
end;

// Remove the cell after the selected cell.
procedure TLinkedListForm.CmdRemoveAfterClick(Sender: TObject);
var
    after_me, target : PCell;
    i                : Integer;
begin
    // Find the cell before the target cell.
    after_me := @sentinel;
    for i := 1 to selected do
        after_me := after_me^.NextCell;

    // Remove the next cell from the list.
    target := after_me^.NextCell;
    after_me^.NextCell := target^.NextCell;

    // Free the target cell's memory.
    Dispose(target);
    num_items := num_items - 1;

    // Redisplay the list.
    DrawList;
    EnableButtons;
    ValueText.SetFocus;
end;

// Enable the appropriate buttons.
procedure TLinkedListForm.ValueTextChange(Sender: TObject);
begin
    CmdInsertAfter.Enabled := ((selected >= 0) and
        (ValueText.Text <> ''));
end;

// Display the list, highlighting the selected item.
procedure TLinkedListForm.DrawList;
var
    cell_ptr : PCell;
    i, x, y  : Integer;
    rect     : TRect;
begin
    // Clear the form.
    rect.Left := 0;
    rect.Top := 0;
    rect.Right := ClientWidth;
    rect.Bottom := ClientHeight;
    Canvas.Brush.Color := clLtGray;
    Canvas.FillRect(rect);

    // Display the list items.
    x := ValueText.Left + ValueText.Width + 5;
    y := ValueText.Top;
    rect.Left := x;
    rect.Right := ClientWidth;
    cell_ptr := @sentinel;
    for i := 0 to num_items do
    begin
        if (i = selected) then
        begin
            Canvas.Brush.Color := clBlack;
            Canvas.Font.Color := clLtGray;
        end else begin
            Canvas.Brush.Color := clLtGray;
            Canvas.Font.Color := clBlack;
        end;
        // Fill in the item's background.
        rect.Top := y;
        rect.Bottom := y + HGT;
        Canvas.FillRect(rect);

        // Display the text.
        Canvas.TextOut(x, y, cell_ptr^.Value);
        y := y + HGT + GAP;

        // Move to the next cell.
        cell_ptr := cell_ptr^.NextCell;
    end;
end;

// Redraw the list.
procedure TLinkedListForm.FormPaint(Sender: TObject);
begin
    DrawList;
end;

// Select the item clicked.
procedure TLinkedListForm.FormMouseUp(Sender: TObject;
  Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
var
    i : Longint;
begin
    // Do nothing if the mouse is too far left.
    if ((X < ValueText.Left + ValueText.Width + 5) or
        (Y < ValueText.Top))
    then
        selected := -1
    else begin
        // See which item number it is.
        i := (y - ValueText.Top) div (HGT + GAP);
        if ((i >= 0) and (i <= num_items)) then
            selected := i
        else
            selected := -1;
    end;

    // Redraw the list.
    DrawList;

    // Enable the appropriate buttons.
    EnableButtons;
end;

procedure TLinkedListForm.EnableButtons;
begin
    CmdInsertAfter.Enabled := ((selected >= 0) and
        (ValueText.Text <> ''));
    CmdRemoveAfter.Enabled := ((selected >= 0) and
        (selected < num_items));
end;

// Free all the linked list memory.
// This doesn't matter for this example program. It would
// be important if the program created and destroyed
// many forms.
procedure TLinkedListForm.FormDestroy(Sender: TObject);
var
    target : PCell;
begin
    while (sentinel.NextCell <> nil) do
    begin
        target := sentinel.NextCell;
        sentinel.NextCell := target^.NextCell;
        Dispose(target);
    end;
end;

end.