LCL Accessibility

From Lazarus wiki
Jump to navigationJump to search

Accessibility means planning and developing applications in a way in which everyone will be able to use them, including blind, persons with low vision, with motion disorders, etc.

LCL Accessibility Introduction

The LCL should automatically provide accessibility for all of its standard controls, or allow the underlying widgetset to provide its own accessibility for the controls utilized directly from the widgetset. LCL users need to provide accessibility only for TCustomControl descendents introduced in the application. LCL developers need to provide accessibility for all TCustomControl descendents in the LCL and also for all non-windowed controls in the LCL (such as TLabel).

The accessibility information provided is a tree of accessible objects which represents the controls hierarchy in the application and which can cover both windowed and non-windowed controls. What the screen reader does exactly with this information is theorically up to him to decide, but of course that it is necessary to test to verify if the most popular screen readers are actually using the information provided in the expected way. The LCL is responsible for making this bridges, since it provides a quite simple accessibility interface and platform APIs, in special the one from Mac OS X, are more complex.

TControl will automatically create an accessible object and place it correctly in the accessible objects tree, so it is not necessary to manually create accessible objects unless a control has sub-items which should be represented individually and are not TControl-descendents. One typical example are the items of the TTreeView control. They are not controls, but can be selected individually and therefore each item should be an accessible object on its own.


 TLazAccessibilityRole = (
   larAnimation, // An object that displays an animation.
   larButton, // A button.
   larCell, // A cell in a table.
   larChart, // An object that displays a graphical representation of data.
   larCheckBox, // An object that can be checked or unchecked, or sometimes in an intermediary state
   larClock, // A clock displaying time.
   larColorPicker, // A control which allows selecting a color.
   larComboBox, // A list of choices that the user can select from.
   larDateField, // A controls which displays and possibly allows to choose a date.
   larGrid, // A grid control which displays cells
   larGroup, // A control which groups others, such as a TGroupBox.
   larIgnore, // Something to be ignored. For example a blank space between other objects.
   larImage, // A graphic or picture or an icon.
   larLabel, // A text label as usually placed near other widgets.
   larListBox, // A list of items, from which the user can select one or more items.
   larListItem, // An item in a list of items.
   larMenuBar, // A main menu bar.
   larMenuItem, // A item in a menu.
   larProgressIndicator, // A control which shows a progress indication.
   larResizeGrip, // A grip that the user can drag to change the size of widgets.
   larScrollBar, // A control to scroll another one
   larSpinner, // A control which allows to increment / decrement a value.
   larTabControl, // A control with tabs, like TPageControl.
   larTextEditorMultiline, // A multi-line text editor (for example: TMemo, SynEdit)
   larTextEditorSingleline, // A single-line text editor (for example: TEdit)
   larTrackBar, // A control which allows to drag a slider.
   larTreeView, // A list of items in a tree structure.
   larTreeItem, // An item in a tree structure.
   larWindow // A top level window.
 // The Child Accessible Objects are designed for non-TControl children
 // of a TCustomControl descendent, for example the items of a TTreeView
 { TLazAccessibleObject }
 TLazAccessibleObject = class
   FHandle: PtrInt;
   function GetHandle: PtrInt;
   FChildren: TFPList; // of TLazAccessibleObject
   class procedure WSRegisterClass; virtual;//override;
   // Primary information
   AccessibleDescription: TCaption;
   AccessibleValue: TCaption;
   AccessibleRole: TLazAccessibilityRole;
   // Secondary information for notifications
   SelectedText: string;
   OwnerControl: TControl;
   Parent: TLazAccessibleObject;
   DataObject: TObject; // Available to be used to connect to an object
   SecondaryHandle: PtrInt; // Available for Widgetsets to use
   constructor Create(AOwner: TControl); virtual;
   destructor Destroy; override;
   procedure SetAccesibilityFields(const ADescription, AValue: string; const ARole: TLazAccessibilityRole);
   function FindOwnerWinControl: TWinControl;
   function AddChildAccessibleObject: TLazAccessibleObject; virtual;
   procedure ClearChildAccessibleObjects;
   procedure RemoveChildAccessibleObject(AObject: TLazAccessibleObject);
   function GetChildAccessibleObject(AIndex: Integer): TLazAccessibleObject;
   function GetChildAccessibleObjectWithDataObject(ADataObject: TObject): TLazAccessibleObject;
   function GetChildAccessibleObjectsCount: Integer;
   function GetSelectedChildAccessibleObject: TLazAccessibleObject; virtual;
   function GetChildAccessibleObjectAtPos(APos: TPoint): TLazAccessibleObject; virtual;
   procedure SendNotification(ANotification: TLazAccessibilityNotification);
   property Handle: PtrInt read GetHandle;
 TControl = class(TLCLComponent)


   // accessibility
   procedure SetAccesibilityFields(const ADescription, AValue: string; const ARole: TLazAccessibilityRole);
   function GetAccessibleObject: TLazAccessibleObject;
   function CreateAccessibleObject: TLazAccessibleObject; virtual;
   function GetSelectedChildAccessibleObject: TLazAccessibleObject; virtual;
   function GetChildAccessibleObjectAtPos(APos: TPoint): TLazAccessibleObject; virtual;
   // standard properties, which should be supported by all descendants
   property AccessibleDescription: TCaption read GetAccessibleDescription write SetAccessibleDescription;
   property AccessibleValue: TCaption read GetAccessibleValue write SetAccessibleValue;
   property AccessibleRole: TLazAccessibilityRole read GetAccessibleRole write SetAccessibleRole;




Screen Readers=

The first step in testing screen reader support is installing one and being able to use it to check what users will see when using your application.

Mac OS X VoiceOver

The screen reader for the vast majority of users in Mac OS X is VoiceOver which comes pre-installed in Mac OS X. To activate it go to Settings->Universal Access and turn VoiceOver on. It couldnt be any simpler. Using it is also mostly easy and intuitive.

Orca for Linux

Orca is currently the most popular screen reader in Linux for both GNOME and KDE. See:

Accessibility Inspector Tools

Accessibility inspectors can help a lot at checking if the application is giving correct information.

Mac OS X

In /Developer/Applications/Utilities/Accessibility Tools/ there is an Accessibility Inspector which will be very useful when developing in this platform.

Accessibility APIs


Search Google for: carbon accessibility


Microsoft Active Accessibility



AT-SPI for Linux






Concepts mapping

TLazAccessibleObject Qt Mac OS X MSAA AT-SPI Comments
AccessibleRole QAccessible::Role AXRole Role ? -
AccessibleDescription QAccessibleWidget::setDescription AXRoleDescription and AXDescription ? ? -
AccessibleValue QAccessibleWidget::setValue AXTitle, AXValue Value, Name, State ? -
Position ? AXPosition ? ? -
Size ? AXSize ? ? -
Handle ? AXUIElementRef ? ? -

See Also