{
* This Source Code Form is subject to the terms of the Mozilla Public License,
* v. 2.0. If a copy of the MPL was not distributed with this file, You can
* obtain one at http://mozilla.org/MPL/2.0/
*
* Copyright (C) 2005-2012, Peter Johnson (www.delphidabbler.com).
*
* $Rev$
* $Date$
*
* Class that records and retrieves history of page accesses.
}
unit UHistory;
interface
uses
// Delphi
Generics.Collections,
// Project
UView;
type
/// <summary>Class that records and retrieves history of displayed view
/// items.</summary>
/// <remarks>Access to two lists is provided: the 'back' list of items that
/// preceed the currently selected item in the history and the 'forward' list
/// of items that follow the currently selected item.</remarks>
THistory = class(TObject)
strict private
type
/// <summary>Class that implementes list of views in history.</summary>
THistoryList = TViewList;
var
/// <summary>List of views in history.</summary>
fItems: THistoryList;
/// <summary>Index of current history item in list</summary>
fCursor: Integer;
const
/// <summary>Max number if items stored in history.</summary>
cMaxHistoryItems = 50;
strict private
/// <summary>Returns current view item in history.</summary>
function GetCurrent: IView;
/// <summary>Handles database change events by updating the history as
/// necessary.</summary>
/// <param name="Sender">TObject [in] Object that triggered event. Not
/// used.</param>
/// <param name="EvtInfo">IInterface [in] Object that carries information
/// about the database change event.</param>
procedure DBChangeHandler(Sender: TObject; const EvtInfo: IInterface);
public
/// <summary>Constructs and initialises history object.</summary>
constructor Create;
/// <summary>Destroys history object.</summary>
destructor Destroy; override;
/// <summary>Clears history.</summary>
procedure Clear;
/// <summary>Creates new history item for given view and adds it to history
/// list.</summary>
/// <remarks>Any items in history after insertion position for this new
/// item are discarded.</remarks>
procedure NewItem(ViewItem: IView);
/// <summary>Selects given view item in history list.</summary>
procedure SelectItem(ViewItem: IView);
/// <summary>Moves backward in history and returns new current item or nil
/// if at start of list when method called.</summary>
function GoBack: IView;
/// <summary>Stores list of views in 'back' list in given list object.
/// </summary>
procedure BackList(const List: TViewList);
/// <summary>Returns number of items in 'back' list.</summary>
function BackListCount: Integer;
/// <summary>Moves forward in history and returns new current item or nil
/// if at end of list when method called.</summary>
function GoForward: IView;
/// <summary>Stores list of views in 'forward' list in given list object.
/// </summary>
procedure ForwardList(const List: TViewList);
/// <summary>Returns number of items in 'forward' list.</summary>
function ForwardListCount: Integer;
/// <summary>Reference to currently selected view item in history or nil if
/// there is no current item.</summary>
property Current: IView read GetCurrent;
end;
implementation
uses
// Delphi
SysUtils,
// Project
DB.UMain, UExceptions;
{ THistory }
procedure THistory.BackList(const List: TViewList);
var
Idx: Integer; // loops thru "back" list
begin
List.Clear;
// Loop backward thru history list from just before current position, adding
// each item to given list
for Idx := Pred(fCursor) downto 0 do
List.Add(fItems[Idx]);
end;
function THistory.BackListCount: Integer;
begin
// We have entries in back list if current item index > 0
if fCursor > 0 then
Result := fCursor
else
Result := 0;
end;
procedure THistory.Clear;
begin
fItems.Clear; // frees objects in list
fCursor := -1; // there is no current item
end;
constructor THistory.Create;
begin
inherited;
fItems := THistoryList.Create;
Clear;
Database.AddChangeEventHandler(DBChangeHandler);
end;
procedure THistory.DBChangeHandler(Sender: TObject; const EvtInfo: IInterface);
var
EventInfo: IDatabaseChangeEventInfo; // information about the event
begin
EventInfo := EvtInfo as IDatabaseChangeEventInfo;
// Clear history if snippet or category changed or removed
case EventInfo.Kind of
evSnippetDeleted, evSnippetChanged,
evCategoryDeleted, evCategoryChanged:
Clear;
evSnippetAdded, evCategoryAdded:
NewItem(TViewFactory.CreateDBItemView(EventInfo.Info));
end;
end;
destructor THistory.Destroy;
begin
Database.RemoveChangeEventHandler(DBChangeHandler);
fItems.Free;
inherited;
end;
procedure THistory.ForwardList(const List: TViewList);
var
Idx: Integer; // loops thru forward list
begin
List.Clear;
// Loop forward thru history list from just after current position, adding
// each item to given list
for Idx := Succ(fCursor) to Pred(fItems.Count) do
List.Add(fItems[Idx]);
end;
function THistory.ForwardListCount: Integer;
begin
// Number of items in list is count of those following cursor position
Result := Pred(fItems.Count) - fCursor;
if Result < 0 then
Result := 0;
end;
function THistory.GetCurrent: IView;
begin
if (fCursor >= 0) and (fCursor < fItems.Count) then
Result := fItems[fCursor]
else
Result := nil;
end;
function THistory.GoBack: IView;
begin
if fCursor >= 0 then
Dec(fCursor);
Result := Current;
end;
function THistory.GoForward: IView;
begin
if fCursor < fItems.Count then
Inc(fCursor);
Result := Current;
end;
procedure THistory.NewItem(ViewItem: IView);
var
I: Integer; // loops thru history list
ClonedItem: IView; // cloned copy of view item
begin
Assert(fCursor <= fItems.Count, ClassName + '.NewItem: fCursor too large');
Assert(fCursor >= -1, ClassName + '.NewItem: fCursor too small');
// Create copy of given view item
if Supports(ViewItem, INulView) then
Exit; // don't record nul views
ClonedItem := TViewFactory.Clone(ViewItem);
// Increment cursor if possible - it will reference new item
if fCursor < fItems.Count then
Inc(fCursor);
// Store new item in list at required position
if fCursor = fItems.Count then
// cursor beyond end of list so append item to list
fCursor := fItems.Add(ClonedItem)
else
begin
// cursor within list so store new item at current position ...
fItems[fCursor] := ClonedItem;
// ... and delete all items in list after new position
for I := Pred(fItems.Count) downto Succ(fCursor) do
fItems.Delete(I); // this frees associated view item
end;
// Delete first item in list if list is too large and adjust cursor
if fItems.Count > cMaxHistoryItems then
begin
fItems.Delete(0);
Dec(fCursor);
end;
end;
procedure THistory.SelectItem(ViewItem: IView);
var
Idx: Integer; // index of view item in history list
begin
Idx := fItems.IndexOf(ViewItem);
if Idx = -1 then
raise EBug.Create(
ClassName + '.SelectItem: Selected view item not in history list'
);
fCursor := Idx;
end;
end.
