unit VTAccessibilityFactory;

// class to create IAccessibles for the tree passed into it.
// If not already assigned, creates IAccessibles for the tree itself
// and the focused item
// the tree accessible is returned when the tree receives an WM_GETOBJECT message
// the AccessibleItem is returned when the Accessible is being asked for the first child
// To create your own IAccessibles, use the VTStandardAccessible unit as a reference,
// and assign your Accessibles to the variables in the unit's initialization.
// You only need to add the unit to your project, and voilá, you have an accessible string tree!
//
// Written by Marco Zehe. (c) 2007

interface

uses
  {$if CompilerVersion >= 18}
    oleacc, // MSAA support in Delphi 2006 or higher
  {$ifend}
  Classes, VirtualTrees;

type
  IVTAccessibleProvider = interface
    function CreateIAccessible(ATree: TBaseVirtualTree): IAccessible;
  end;

  TVTAccessibilityFactory = class(TObject)
  private
    FAccessibleProviders: TInterfaceList;
  public
    constructor Create;
    destructor Destroy; override;
    function CreateIAccessible(ATree: TBaseVirtualTree): IAccessible;
    procedure RegisterAccessibleProvider(AProvider: IVTAccessibleProvider);
    procedure UnRegisterAccessibleProvider(AProvider: IVTAccessibleProvider);
  end;

function GetAccessibilityFactory: TVTAccessibilityFactory;

implementation

var
  VTAccessibleFactory: TVTAccessibilityFactory = nil;
  AccessibilityAvailable: Boolean = False;

{ TVTAccessibilityFactory }

constructor TVTAccessibilityFactory.Create;
begin
  inherited Create;
  FAccessibleProviders := TInterfaceList.Create;
  FAccessibleProviders.Clear;
end;

function TVTAccessibilityFactory.CreateIAccessible(
  ATree: TBaseVirtualTree): IAccessible;
var
  I: Integer;
  TmpIAccessible: IAccessible;
// returns an IAccessible.
// 1. If the Accessible property of the passed-in tree is nil,
// the first registered element will be returned.
// Usually, this is the IAccessible that provides information about the tree itself.
// If it is not nil, we'll check whether the AccessibleItem is nil.
// If it is, we'll look in the registered IAccessibles for the appropriate one.
// Each IAccessibleProvider will check the tree for properties to determine whether it is responsible.
// We'll work top to bottom, from the most complicated to the most simple.
// The index for these should all be greater than 0, e g the IAccessible for the tree itself should always be registered first, then any IAccessible items.
begin
  Result := nil;
  if ATree <> nil then
  begin
    if ATree.Accessible = nil then
    begin
      if FAccessibleProviders.Count > 0 then
      begin
        Result := IVTAccessibleProvider(FAccessibleProviders.Items[0]).CreateIAccessible(ATree);
        Exit;
      end;
    end;
    if ATree.AccessibleItem = nil then
    begin
      if FAccessibleProviders.Count > 0 then
      begin
        for I := FAccessibleProviders.Count - 1 downto 1 do
        begin
          TmpIAccessible := IVTAccessibleProvider(FAccessibleProviders.Items[I]).CreateIAccessible(ATree);
          if TmpIAccessible <> nil then
          begin
            Result := TmpIAccessible;
            Break;
          end;
        end;
        if TmpIAccessible = nil then
        begin
          Result := IVTAccessibleProvider(FAccessibleProviders.Items[0]).CreateIAccessible(ATree);
        end;
      end;
    end
    else
      Result := ATree.AccessibleItem;
  end;
end;

destructor TVTAccessibilityFactory.Destroy;
begin
  FAccessibleProviders.Free;
  FAccessibleProviders := nil;
  inherited Destroy;
end;

procedure TVTAccessibilityFactory.RegisterAccessibleProvider(
  AProvider: IVTAccessibleProvider);
// Ads a provider if it is not already registered
begin
  if FAccessibleProviders.IndexOf(AProvider) < 0 then
    FAccessibleProviders.Add(AProvider)
end;

procedure TVTAccessibilityFactory.UnRegisterAccessibleProvider(
  AProvider: IVTAccessibleProvider);
// Unregisters/removes an IAccessible provider if it is present
begin
  if FAccessibleProviders.IndexOf(AProvider) >= 0 then
    FAccessibleProviders.Remove(AProvider);
end;

function GetAccessibilityFactory: TVTAccessibilityFactory;

// Accessibility helper function to create a singleton class that will create or return
// the IAccessible interface for the tree and the focused node.

begin
  // first, check if we've loaded the library already
  if not AccessibilityAvailable then
    AccessibilityAvailable := True;
  if AccessibilityAvailable then
  begin
    // Check to see if the class has already been created.
    if VTAccessibleFactory = nil then
      VTAccessibleFactory := TVTAccessibilityFactory.Create;
    Result := VTAccessibleFactory;
  end
  else
    Result := nil;
end;

initialization

finalization
  VTAccessibleFactory.Free;

end.
