On this page:
accept-tab-focus
get-scroll-page
get-scroll-pos
get-scroll-range
get-view-start
get-virtual-size
init-auto-scrollbars
init-manual-scrollbars
on-paint
on-scroll
scroll
set-scroll-page
set-scroll-pos
set-scroll-range
show-scrollbars
swap-gl-buffers
with-gl-context
Inherited methods:
from canvas<%>
get-canvas-background
get-dc
min-client-height
min-client-width
on-char
on-event
on-tab-in
set-canvas-background
set-resize-corner
warp-pointer
from window<%>
accept-drop-files
client->screen
enable
focus
get-client-size
get-cursor
get-handle
get-height
get-label
get-plain-label
get-size
get-width
get-x
get-y
has-focus?
is-enabled?
is-shown?
on-drop-file
on-focus
on-move
on-size
on-subwindow-char
on-subwindow-event
on-superwindow-enable
on-superwindow-show
popup-menu
refresh
screen->client
set-cursor
set-label
show
from area<%>
get-graphical-min-size
get-parent
get-top-level-window
min-height
min-width
stretchable-height
stretchable-width
from subarea<%>
horiz-margin
vert-margin

canvas% : class?

  superclass: object%

  extends: canvas<%>
A canvas% object is a general-purpose window for drawing and handling events.

(new canvas% [parent parent] 
  [[style style] 
  [paint-callback paint-callback] 
  [label label] 
  [gl-config gl-config] 
  [enabled enabled] 
  [vert-margin vert-margin] 
  [horiz-margin horiz-margin] 
  [min-width min-width] 
  [min-height min-height] 
  [stretchable-width stretchable-width] 
  [stretchable-height stretchable-height]]) 
  (is-a?/c canvas%)
  parent : 
(or/c (is-a?/c frame%) (is-a?/c dialog%)
      (is-a?/c panel%) (is-a?/c pane%))
  style : 
(listof (one-of/c 'border 'control-border 'combo
                  'vscroll 'hscroll 'resize-corner
                  'gl 'no-autoclear 'transparent
                  'no-focus 'deleted))
   = null
  paint-callback : ((is-a?/c canvas%) (is-a?/c dc<%>) . -> . any)
   = void
  label : (or/c label-string? false/c) = #f
  gl-config : (or/c (is-a?/c gl-config%) false/c) = #f
  enabled : any/c = #t
  vert-margin : (integer-in 0 1000) = 0
  horiz-margin : (integer-in 0 1000) = 0
  min-width : (integer-in 0 10000) = graphical-minimum-width
  min-height : (integer-in 0 10000) = graphical-minimum-height
  stretchable-width : any/c = #t
  stretchable-height : any/c = #t
The style argument indicates one or more of the following styles:

The 'hscroll and 'vscroll styles create a canvas with an initially inactive scrollbar. The scrollbars are activated with either init-manual-scrollbars or init-auto-scrollbars, and they can be hidden and re-shown with show-scrollbars.

The paint-callback argument is called by the default on-paint method, using the canvas and the DC returned by get-dc as the argument.

The label argument names the canvas for get-label, but it is not displayed with the canvas.

The gl-config argument determines properties of an OpenGL context for this canvas, as obtained through the canvas’s drawing context. See also get-dc and get-gl-context in dc<%>.

For information about the enabled argument, see window<%>. For information about the horiz-margin and vert-margin arguments, see subarea<%>. For information about the min-width, min-height, stretchable-width, and stretchable-height arguments, see area<%>.

(send a-canvas accept-tab-focus)  boolean?
(send a-canvas accept-tab-focus on?)  void?
  on? : any/c
Gets or sets whether tab-focus is enabled for the canvas (assuming that the canvas is not created with the 'no-focus style). When tab-focus is enabled, the canvas can receive the keyboard focus when the user navigates among a frame or dialog’s controls with the Tab and arrow keys. By default, tab-focus is disabled.

When tab-focus is enabled for a canvas, Tab, arrow, and Enter keyboard events are consumed by a frame’s default on-traverse-char method. (In addition, a dialog’s default method consumes Escape key events.) Otherwise, on-traverse-char allows the keyboard events to be propagated to the canvas.

(send a-canvas get-scroll-page which)
  (integer-in 1 1000000000)
  which : (one-of/c 'horizontal 'vertical)
Get the current page step size of a manual scrollbar. The result is 0 if the scrollbar is not active or it is automatic.

The which argument is either 'horizontal or 'vertical, indicating whether to get the page step size of the horizontal or vertical scrollbar, respectively.

See also init-manual-scrollbars.

(send a-canvas get-scroll-pos which)
  (integer-in 0 1000000000)
  which : (one-of/c 'horizontal 'vertical)
Gets the current value of a manual scrollbar. The result is always 0 if the scrollbar is not active or it is automatic.

The which argument is either 'horizontal or 'vertical, indicating that the value of the horizontal or vertical scrollbar should be returned, respectively.

See also init-manual-scrollbars.

(send a-canvas get-scroll-range which)
  (integer-in 0 1000000000)
  which : (one-of/c 'horizontal 'vertical)
Gets the current maximum value of a manual scrollbar. The result is always 0 if the scrollbar is not active or it is automatic.

The which argument is either 'horizontal or 'vertical, indicating whether to get the maximum value of the horizontal or vertical scrollbar, respectively.

See also init-manual-scrollbars.

(send a-canvas get-view-start)
  
(integer-in 0 10000) (integer-in 0 10000)
Get the location at which the visible portion of the canvas starts, based on the current values of the horizontal and vertical scrollbars if they are initialized as automatic (see init-auto-scrollbars). Combined with get-client-size, an application can efficiently redraw only the visible portion of the canvas. The values are in pixels.

If the scrollbars are disabled or initialized as manual (see init-manual-scrollbars), the result is (values 0 0).

(send a-canvas get-virtual-size)
  (value (integer-in 0 10000) (integer-in 0 10000))
Gets the size in device units of the scrollable canvas area (as opposed to the client size, which is the area of the canvas currently visible). This is the same size as the client size (as returned by get-client-size) unless scrollbars are initialized as automatic (see init-auto-scrollbars).

(send a-canvas init-auto-scrollbars horiz-pixels    
  vert-pixels    
  h-value    
  v-value)  void?
  horiz-pixels : (or/c (integer-in 1 1000000000) false/c)
  vert-pixels : (or/c (integer-in 1 1000000000) false/c)
  h-value : (real-in 0.0 1.0)
  v-value : (real-in 0.0 1.0)
Enables and initializes automatic scrollbars for the canvas. A horizontal or vertical scrollbar can be activated only in a canvas that was created with the 'hscroll or 'vscroll style flag, respectively.

With automatic scrollbars, the programmer specifies the desired virtual size of the canvas, and the scrollbars are automatically handled to allow the user to scroll around the virtual area. The scrollbars are not automatically hidden if they are unneeded; see show-scrollbars.

See also init-manual-scrollbars for information about manual scrollbars. The horizontal and vertical scrollbars are always either both manual or both automatic, but they are independently enabled. Automatic scrollbars can be re-initialized as manual, and vice-versa.

If either horiz-pixels or vert-pixels is #f, the scrollbar is not enabled in the corresponding direction, and the canvas’s virtual size in that direction is the same as its client size.

The h-value and v-value arguments specify the initial values of the scrollbars as a fraction of the scrollbar’s range. A 0.0 value initializes the scrollbar to its left/top, while a 1.0 value initializes the scrollbar to its right/bottom.

See also on-scroll and get-virtual-size.

(send a-canvas init-manual-scrollbars h-length    
  v-length    
  h-page    
  v-page    
  h-value    
  v-value)  void?
  h-length : (or/c (integer-in 0 1000000000) false/c)
  v-length : (or/c (integer-in 0 1000000000) false/c)
  h-page : (integer-in 1 1000000000)
  v-page : (integer-in 1 1000000000)
  h-value : (integer-in 0 1000000000)
  v-value : (integer-in 0 1000000000)
Enables and initializes manual scrollbars for the canvas. A horizontal or vertical scrollbar can be activated only in a canvas that was created with the 'hscroll or 'vscroll style flag, respectively.

With manual scrollbars, the programmer is responsible for managing all details of the scrollbars, and the scrollbar state has no effect on the canvas’s virtual size. Instead, the canvas’s virtual size is the same as its client size.

See also init-auto-scrollbars for information about automatic scrollbars. The horizontal and vertical scrollbars are always either both manual or both automatic, but they are independently enabled. Automatic scrollbars can be re-initialized as manual, and vice-versa.

The h-length and v-length arguments specify the length of each scrollbar in scroll steps (i.e., the maximum value of each scrollbar). If either is #f, the scrollbar is disabled in the corresponding direction.

The h-page and v-page arguments set the number of scrollbar steps in a page, i.e., the amount moved when pressing above or below the value indicator in the scrollbar control.

The h-value and v-value arguments specify the initial values of the scrollbars.

If h-value is greater than h-length or v-value is greater than v-length, an exn:fail:contract exception is raised. (The page step may be larger than the total size of a scrollbar.)

See also on-scroll and get-virtual-size.

(send a-canvas on-paint)  void?
Overrides on-paint in canvas<%>.
Calls the procedure supplied as the paint-callback argument when the canvas% was created.

(send a-canvas on-scroll event)  void?
  event : (is-a?/c scroll-event%)
Called when the user changes one of the canvas’s scrollbars. A scroll-event% argument provides information about the scroll action.

This method is called only when manual scrollbars are changed, not automatic scrollbars; for automatic scrollbars, the on-paint method is called, instead.

(send a-canvas scroll h-value v-value)  void?
  h-value : (or/c (real-in 0.0 1.0) false/c)
  v-value : (or/c (real-in 0.0 1.0) false/c)
Sets the values of automatic scrollbars. (This method has no effect on manual scrollbars.)

If either argument is #f, the scrollbar value is not changed in the corresponding direction.

The h-value and v-value arguments each specify a fraction of the scrollbar’s movement. A 0.0 value sets the scrollbar to its left/top, while a 1.0 value sets the scrollbar to its right/bottom. A 0.5 value sets the scrollbar to its middle. In general, if the canvas’s virtual size is v, its client size is c, and (> v c), then scrolling to p sets the view start to (floor (* p (- v c))).

See also init-auto-scrollbars and get-view-start.

(send a-canvas set-scroll-page which value)  void?
  which : (one-of/c 'horizontal 'vertical)
  value : (integer-in 1 1000000000)
Set the current page step size of a manual scrollbar. (This method has no effect on automatic scrollbars.)

The which argument is either 'horizontal or 'vertical, indicating whether to set the page step size of the horizontal or vertical scrollbar, respectively.

See also init-manual-scrollbars.

(send a-canvas set-scroll-pos which value)  void?
  which : (one-of/c 'horizontal 'vertical)
  value : (integer-in 0 1000000000)
Sets the current value of a manual scrollbar. (This method has no effect on automatic scrollbars.)

The which argument is either 'horizontal or 'vertical, indicating whether to set the value of the horizontal or vertical scrollbar set, respectively.

The value of the canvas’s scrollbar can be changed by the user scrolling, and such changes do not go through this method; use on-scroll to monitor scrollbar value changes.

See also init-manual-scrollbars and scroll.

(send a-canvas set-scroll-range which    
  value)  void?
  which : (one-of/c 'horizontal 'vertical)
  value : (integer-in 0 1000000000)
Sets the current maximum value of a manual scrollbar. (This method has no effect on automatic scrollbars.)

The which argument is either 'horizontal or 'vertical, indicating whether to set the maximum value of the horizontal or vertical scrollbar, respectively.

See also init-manual-scrollbars.

(send a-canvas show-scrollbars show-horiz?    
  show-vert?)  void?
  show-horiz? : any/c
  show-vert? : any/c
Shows or hides the scrollbars as indicated by show-horiz? and show-vert?. If show-horiz? is true and the canvas was not created with the 'hscroll style, an exn:fail:contract exception is raised. Similarly, if show-vert? is true and the canvas was not created with the 'vscroll style, an exn:fail:contract exception is raised.

The horizontal scrollbar can be shown only if the canvas was created with the 'hscroll style, and the vertical scrollbar can be shown only if the canvas was created with the 'vscroll style. See also init-auto-scrollbars and init-manual-scrollbars.

(send a-canvas swap-gl-buffers)  void?
Calls swap-buffers on the result of get-gl-context for this canvas’s DC as returned by get-dc.

The swap-buffers method acquires a re-entrant lock, so nested calls to with-gl-context on different threads or OpenGL contexts can block or deadlock.

(send a-canvas with-gl-context thunk)  any
  thunk : (-> any)
Passes the given thunk to call-as-current of the result of get-gl-context for this canvas’s DC as returned by get-dc.

The call-as-current method acquires a re-entrant lock, so nested calls to with-gl-context on different threads or OpenGL contexts can block or deadlock.