The windowing toolbox provides the basic building blocks of GUI
programs, including frames (top-level windows), modal dialogs, menus,
buttons, check boxes, text fields, and radio buttons—
See Classes and Objects for an introduction to classes and interfaces in Racket.
To create a new top-level window, instantiate the frame% class:
; Make a frame by instantiating the frame% class (define frame (new frame% [label "Example"])) ; Show the frame by calling its show method (send frame show #t)
The built-in classes provide various mechanisms for handling GUI events. For example, when instantiating the button% class, supply an event callback procedure to be invoked when the user clicks the button. The following example program creates a frame with a text message and a button; when the user clicks the button, the message changes:
; Make a frame by instantiating the frame% class (define frame (new frame% [label "Example"])) ; Make a static text message in the frame (define msg (new message% [parent frame] [label "No events so far..."])) ; Make a button in the frame (new button% [parent frame] [label "Click Me"] ; Callback procedure for a button click: [callback (lambda (button event) (send msg set-label "Button click"))]) ; Show the frame by calling its show method (send frame show #t)
Programmers never implement the GUI event loop directly. Instead, the windowing system automatically pulls each event from an internal queue and dispatches the event to an appropriate window. The dispatch invokes the window’s callback procedure or calls one of the window’s methods. In the above program, the windowing system automatically invokes the button’s callback procedure whenever the user clicks Click Me.
If a window receives multiple kinds of events, the events are dispatched to methods of the window’s class instead of to a callback procedure. For example, a drawing canvas receives update events, mouse events, keyboard events, and sizing events; to handle them, derive a new class from the built-in canvas% class and override the event-handling methods. The following expression extends the frame created above with a canvas that handles mouse and keyboard events:
; Derive a new canvas (a drawing window) class to handle events (define my-canvas% (class canvas% ; The base class is canvas% ; Define overriding method to handle mouse events (define/override (on-event event) (send msg set-label "Canvas mouse")) ; Define overriding method to handle keyboard events (define/override (on-char event) (send msg set-label "Canvas keyboard")) ; Call the superclass init, passing on all init args (super-new))) ; Make a canvas that handles events in the frame (new my-canvas% [parent frame])
After running the above code, manually resize the frame to see the new canvas. Moving the cursor over the canvas calls the canvas’s on-event method with an object representing a motion event. Clicking on the canvas calls on-event. While the canvas has the keyboard focus, typing on the keyboard invokes the canvas’s on-char method.
The windowing system dispatches GUI events sequentially; that is, after invoking an event-handling callback or method, the windowing system waits until the handler returns before dispatching the next event. To illustrate the sequential nature of events, extend the frame again, adding a Pause button:
After the user clicks Pause, the entire frame becomes unresponsive for five seconds; the windowing system cannot dispatch more events until the call to sleep returns. For more information about event dispatching, see Event Dispatching and Eventspaces.
In addition to dispatching events, the GUI classes also handle the graphical layout of windows. Our example frame demonstrates a simple layout; the frame’s elements are lined up top-to-bottom. In general, a programmer specifies the layout of a window by assigning each GUI element to a parent container. A vertical container, such as a frame, arranges its children in a column, and a horizontal container arranges its children in a row. A container can be a child of another container; for example, to place two buttons side-by-side in our frame, create a horizontal panel for the new buttons:
(define panel (new horizontal-panel% [parent frame])) (new button% [parent panel] [label "Left"] [callback (lambda (button event) (send msg set-label "Left click"))]) (new button% [parent panel] [label "Right"] [callback (lambda (button event) (send msg set-label "Right click"))])
For more information about window layout and containers, see Geometry Management.
The content of a canvas is determined by its on-paint method, where the default on-paint calls the paint-callback function that is supplied when the canvas is created. The on-paint method receives no arguments and uses the canvas’s get-dc method to obtain a drawing context (DC) for drawing; the default on-paint method passes the canvas and this DC on to the paint-callback function. Drawing operations of the racket/draw toolbox on the DC are reflected in the content of the canvas onscreen.
For example, the following program creates a canvas that displays large, friendly letters:
(define frame (new frame% [label "Example"] [width 300] [height 300])) (new canvas% [parent frame] [paint-callback (lambda (canvas dc) (send dc set-scale 3 3) (send dc set-text-foreground "blue") (send dc draw-text "Don't Panic!" 0 0))]) (send frame show #t)
The background color of a canvas can be set through the set-canvas-background method. To make the canvas transparent (so that it takes on its parent’s color and texture as its initial content), supply 'transparent in the style argument when creating the canvas.
a frame is a top-level window that the user can move and resize.
a dialog is a modal top-level window; when a dialog is shown, other top-level windows are disabled until the dialog is dismissed.
a pane is a lightweight panel. It has no graphical representation or event-handling capabilities. The pane% class has three subclasses: vertical-pane%, horizontal-pane%, and grow-box-spacer-pane%.
a panel is a containee as well as a container.
a pane is a containee as well as a container.
a canvas is a subwindow for drawing on the screen.
a message is a static text field or bitmap with no user interaction.
a button is a clickable control.
a check box is a clickable control; the user clicks the control to set or remove its check mark.
a radio box is a collection of mutually exclusive radio buttons; when the user clicks a radio button, it is selected and the radio box’s previously selected radio button is deselected.
a choice item is a pop-up menu of text choices; the user selects one item in the control.
a list box is a scrollable lists of text choices; the user selects one or more items in the list (depending on the style of the list box).
a text field is a box for simple text entry.
a combo field combines a text field with a pop-up menu of choices.
a slider is a dragable control that selects an integer value within a fixed range.
a gauge is an output-only control (the user cannot change the value) for reporting an integer value within a fixed range.
Most areas are windows, but some are non-windows. A window, such as a panel, has a graphical representation, receives keyboard and mouse events, and can be disabled or hidden. In contrast, a non-window, such as a pane, is useful only for geometry management; a non-window does not receive mouse events, and it cannot be disabled or hidden.
Every area is an instance of the area<%> interface. Each container is also an instance of the area-container<%> interface, whereas each containee is an instance of subarea<%>. Windows are instances of window<%>. The area-container<%>, subarea<%>, and window<%> interfaces are subinterfaces of area<%>.
The following diagram shows more of the type hierarchy under area<%>:
| | |
|____ _______|__________ |
| | | |
| | |
The diagram below extends the one above to show the complete type hierarchy under area<%>. (Some of the types are represented by interfaces, and some types are represented by classes. In principle, every area type should be represented by an interface, but whenever the windowing toolbox provides a concrete implementation, the corresponding interface is omitted from the toolbox.) To avoid intersecting lines, the hierarchy is drawn for a cylindrical surface; lines from subarea<%> and subwindow<%> wrap from the left edge of the diagram to the right edge.
| | |
<<<____|____ _____|__________ __|___ ___________________<<<
| | | | | |
subwindow<%> | | | |
<<<______________|___________ | | | | _<<<
| | | | pane% |
|- button% | | | |
|- slider% | | |
|- gauge% | | __________________|
|- text-field% | | |
| |- top-level-window<%>
| |- frame%
Menu bars, menus, and menu items are graphical elements, but not areas (i.e., they do not have all of the properties that are common to areas, such as an adjustable graphical size). Instead, the menu classes form a separate container–containee hierarchy:
a menu bar is a top-level collection of menus that are associated with a frame.
a menu contains a set of menu items. The menu can appear in a menu bar, in a popup menu, or as a submenu in another menu.
a popup menu is a top-level menu that is dynamically displayed in a canvas or editor canvas.
a separator is an unselectable line in a menu or popup menu.
a plain menu item is a selectable text item in a menu. When the item is selected, its callback procedure is invoked.
a checkable menu item is a text item in a menu; the user selects a checkable menu item to toggle a check mark next to the item.
a menu is a menu item as well as a menu item container.
The following diagram shows the complete type hierarchy for the menu system:
|- separator-menu-item% _____|___
_________|_________ | |- popup-menu%
| | |
The windowing toolbox’s geometry management makes it easy to design windows that look right on all platforms, despite different graphical representations of GUI elements. Geometry management is based on containers; each container arranges its children based on simple constraints, such as the current size of a frame and the natural size of a button.
The built-in container classes include horizontal panels (and panes), which align their children in a row, and vertical panels (and panes), which align their children in a column. By nesting horizontal and vertical containers, a programmer can achieve most any layout. For example, to construct a dialog with the shape
| ------------------------------------- |
| Your name: | | |
| ------------------------------------- |
| -------- ---- |
| ( Cancel ) ( OK ) |
| -------- ---- |
with the following program:
; Create a dialog (define dialog (instantiate dialog% ("Example"))) ; Add a text field to the dialog (new text-field% [parent dialog] [label "Your name"]) ; Add a horizontal panel to the dialog, with centering for buttons (define panel (new horizontal-panel% [parent dialog] [alignment '(center center)])) ; Add Cancel and Ok buttons to the horizontal panel (new button% [parent panel] [label "Cancel"]) (new button% [parent panel] [label "Ok"]) (when (system-position-ok-before-cancel?) (send panel change-children reverse)) ; Show the dialog (send dialog show #t)
Each container arranges its children using the natural size of each child, which usually depends on instantiation parameters of the child, such as the label on a button or the number of choices in a radio box. In the above example, the dialog stretches horizontally to match the minimum width of the text field, and it stretches vertically to match the total height of the field and the buttons. The dialog then stretches the horizontal panel to fill the bottom half of the dialog. Finally, the horizontal panel uses the sum of the buttons’ minimum widths to center them horizontally.
As the example demonstrates, a stretchable container grows to fill its environment, and it distributes extra space among its stretchable children. By default, panels are stretchable in both directions, whereas buttons are not stretchable in either direction. The programmer can change whether an individual GUI element is stretchable.
The following subsections describe the container system in detail, first discussing the attributes of a containee in Containees, and then describing the attributes of a container in Containers. In addition to the built-in vertical and horizontal containers, programmers can define new types of containers as discussed in the final subsection, Defining New Types of Containers.
Each containee, or child, has the following properties:
horizontal and vertical margins.
A container arranges its children based on these four properties of each containee. A containee’s parent container is specified when the containee is created. A window containee can be hidden or deleted within its parent, and its parent can be changed by reparenting.
The graphical minimum size of a particular containee, as reported by get-graphical-min-size, depends on the platform, the label of the containee (for a control), and style attributes specified when creating the containee. For example, a button’s minimum graphical size ensures that the entire text of the label is visible. The graphical minimum size of a control (such as a button) cannot be changed; it is fixed at creation time. (A control’s minimum size is not recalculated when its label is changed.) The graphical minimum size of a panel or pane depends on the total minimum size of its children and the way that they are arranged.
To select a size for a containee, its parent container considers the containee’s requested minimum size rather than its graphical minimum size (assuming the requested minimum is larger than the graphical minimum). Unlike the graphical minimum, the requested minimum size of a containee can be changed by a programmer at any time using the min-width and min-height methods.
Unless a containee is stretchable (in a particular direction), it always shrinks to its minimum size (in the corresponding direction). Otherwise, containees are stretched to fill all available space in a container. Each containee begins with a default stretchability. For example, buttons are not initially stretchable, whereas a one-line text field is initially stretchable in the horizontal direction. A programmer can change the stretchability of a containee at any time using the stretchable-width and stretchable-height methods.
A margin is space surrounding a containee. Each containee’s margin is independent of its minimum size, but from the container’s point of view, a margin effectively increases the minimum size of the containee. For example, if a button has a vertical margin of 2, then the container must allocate enough room to leave two pixels of space above and below the button, in addition to the space that is allocated for the button’s minimum height. A programmer can adjust a containee’s margin with horiz-margin and vert-margin. The default margin is 2 for a control, and 0 for any other type of containee.
In practice, the requested minimum size and margin of a control are rarely changed, although they are often changed for a canvas. Stretchability is commonly adjusted for any type of containee, depending on the visual effect desired by the programmer.
A container has the following properties:
a list of (non-deleted) children containees;
a requested minimum width and a requested minimum height;
a spacing used between the children;
a border margin used around the total set of children;
horizontal and vertical stretchability (on or off); and
an alignment setting for positioning leftover space.
These properties are factored into the container’s calculation of its own size and the arrangement of its children. For a container that is also a containee (e.g., a panel), the container’s requested minimum size and stretchability are the same as for its containee aspect.
A containee’s parent container is specified when the containee is created. A containee window can be hidden or deleted within its parent container, and its parent can be changed by reparenting (but a non-window containee cannot be hidden, deleted, or reparented):
A hidden child is invisible to the user, but space is still allocated for each hidden child within a container. To hide or show a child, call the child’s show method.
A deleted child is hidden and ignored by container as it arranges its other children, so no space is reserved in the container for a deleted child. To make a child deleted or non-deleted, call the container’s delete-child or add-child method (which calls the child’s show method).
When a child is created, it is initially shown and non-deleted. A deleted child is subject to garbage collection when no external reference to the child exists. A list of non-deleted children (hidden or not) is available from a container through its get-children method.
The order of the children in a container’s non-deleted list is significant. For example, a vertical panel puts the first child in its list at the top of the panel, and so on. When a new child is created, it is put at the end of its container’s list of children. The order of a container’s list can be changed dynamically via the change-children method. (The change-children method can also be used to activate or deactivate children.)
The graphical minimum size of a container, as reported by get-graphical-min-size, is calculated by combining the minimum sizes of its children (summing them or taking the maximum, as appropriate to the layout strategy of the container) along with the spacing and border margins of the container. A larger minimum may be specified by the programmer using min-width and min-height methods; when the computed minimum for a container is larger than the programmer-specified minimum, then the programmer-specified minimum is ignored.
A container’s spacing determines the amount of space left between adjacent children in the container, in addition to any space required by the children’s margins. A container’s border margin determines the amount of space to add around the collection of children; it effectively decreases the area within the container where children can be placed. A programmer can adjust a container’s border and spacing dynamically via the border and spacing methods. The default border and spacing are 0 for all container types.
Because a panel or pane is a containee as well as a container, it has a containee margin in addition to its border margin. For a panel, these margins are not redundant because the panel can have a graphical border; the border is drawn inside the panel’s containee margin, but outside the panel’s border margin.
For a top-level-window container, such as a frame or dialog, the
container’s stretchability determines whether the user can resize the
window to something larger than its minimum size. Thus, the user
cannot resize a frame that is not stretchable. For other types of
containers (i.e., panels and panes), the container’s stretchability
is its stretchability as a containee in some other container. All
types of containers are initially stretchable in both
The alignment specification for a container determines how it positions its children when the container has leftover space. (A container can only have leftover space in a particular direction when none of its children are stretchable in that direction.) For example, when the container’s horizontal alignment is 'left, the children are left-aligned in the container and leftover space is accumulated to the right. When the container’s horizontal alignment is 'center, each child is horizontally centered in the container. A container’s alignment is changed with the set-alignment method.
Although nested horizontal and vertical containers can express most layout patterns, a programmer can define a new type of container with an explicit layout procedure. A programmer defines a new type of container by deriving a class from panel% or pane% and overriding the container-size and place-children methods. The container-size method takes a list of size specifications for each child and returns two values: the minimum width and height of the container. The place-children method takes the container’s size and a list of size specifications for each child, and returns a list of sizes and placements (in parallel to the original list).
An input size specification is a list of four values:
the child’s minimum width;
the child’s minimum height;
the child’s horizontal stretchability (#t means stretchable, #f means not stretchable); and
the child’s vertical stretchability.
For place-children, an output position and size specification is a list of four values:
the child’s new horizontal position (relative to the parent);
the child’s new vertical position;
the child’s new actual width;
the child’s new actual height.
The widths and heights for both the input and output include the children’s margins. The returned position for each child is automatically incremented to account for the child’s margin in placing the control.
Whenever the user moves the mouse, clicks or releases a mouse button, or presses a key on the keyboard, an event is generated for some window. The window that receives the event depends on the current state of the graphic display:
The receiving window of a mouse event is usually the window under the cursor when the mouse is moved or clicked. If the mouse is over a child window, the child window receives the event rather than its parent.
When the user clicks in a window, the window “grabs” the mouse, so that all mouse events go to that window until the mouse button is released (regardless of the location of the cursor). As a result, a user can click on a scrollbar thumb and drag it without keeping the cursor strictly inside the scrollbar control.
A mouse button-release event is normally generated for each mouse button-down event, but a button-release event might get dropped. For example, a modal dialog might appear and take over the mouse. More generally, any kind of mouse event can get dropped in principle, so avoid algorithms that depend on precise mouse-event sequences. For example, a mouse tracking handler should reset the tracking state when it receives an event other than a dragging event.
The receiving window of a keyboard event is the window that owns the keyboard focus at the time of the event. Only one window owns the focus at any time, and focus ownership is typically displayed by a window in some manner. For example, a text field control shows focus ownership by displaying a blinking caret.
Within a top-level window, only certain kinds of subwindows can have the focus, depending on the conventions of the platform. Furthermore, the subwindow that initially owns the focus is platform-specific. A user can moves the focus in various ways, usually by clicking the target window. A program can use the focus method to move the focus to a subwindow or to set the initial focus.
A key-press event may correspond to either an actual key press or an auto-key repeat. Multiple key-press events without intervening key-release events normally indicate an auto-key. Like any input event, however, key-release events sometimes get dropped (e.g., due to the appearance of a modal dialog).
Controls, such as buttons and list boxes, handle keyboard and mouse events automatically, eventually invoking the callback procedure that was provided when the control was created. A canvas propagates mouse and keyboard events to its on-event and on-char methods, respectively.
A mouse and keyboard event is delivered in a special way to its window. Each ancestor of the receiving window gets a chance to intercept the event through the on-subwindow-event and on-subwindow-char methods. See the method descriptions for more information.
The default on-subwindow-char method for a top-level window intercepts keyboard events to detect menu-shortcut events and focus-navigation events. See on-subwindow-char in frame% and on-subwindow-char in dialog% for details. Certain OS-specific key combinations are captured at a low level, and cannot be overridden. For example, on Windows and Unix, pressing and releasing Alt always moves the keyboard focus to the menu bar. Similarly, Alt-Tab switches to a different application on Windows. (Alt-Space invokes the system menu on Windows, but this shortcut is implemented by on-system-menu-char, which is called by on-subwindow-char in frame% and on-subwindow-char in dialog%.)
A graphical user interface is an inherently multi-threaded system: one thread is the program managing windows on the screen, and the other thread is the user moving the mouse and typing at the keyboard. GUI programs typically use an event queue to translate this multi-threaded system into a sequential one, at least from the programmer’s point of view. Each user action is handled one at a time, ignoring further user actions until the previous one is completely handled. The conversion from a multi-threaded process to a single-threaded one greatly simplifies the implementation of GUI programs.
Despite the programming convenience provided by a purely sequential event queue, certain situations require a less rigid dialog with the user:
Nested event handling: In the process of handling an event, it may be necessary to obtain further information from the user. Usually, such information is obtained via a modal dialog; in whatever fashion the input is obtained, more user events must be received and handled before the original event is completely handled. To allow the further processing of events, the handler for the original event must explicitly yield to the system. Yielding causes events to be handled in a nested manner, rather than in a purely sequential manner.
Asynchronous event handling: An application may consist of windows that represent independent dialogs with the user. For example, a drawing program might support multiple drawing windows, and a particularly time-consuming task in one window (e.g., a special filter effect on an image) should not prevent the user from working in a different window. Such an application needs sequential event handling for each individual window, but asynchronous (potentially parallel) event handling across windows. In other words, the application needs a separate event queue for each window, and a separate event-handling thread for each event queue.
An eventspace is a context for processing GUI events. Each eventspace maintains its own queue of events, and events in a single eventspace are dispatched sequentially by a designated handler thread. An event-handling procedure running in this handler thread can yield to the system by calling yield, in which case other event-handling procedures may be called in a nested (but single-threaded) manner within the same handler thread. Events from different eventspaces are dispatched asynchronously by separate handler threads.
When a frame or dialog is created without a parent, it is associated with the current eventspace as described in Creating and Setting the Eventspace. Events for a top-level window and its descendants are always dispatched in the window’s eventspace. Every dialog is modal; a dialog’s show method implicitly calls yield to handle events while the dialog is shown. (See also Eventspaces and Threads for information about threads and modal dialogs.) Furthermore, when a modal dialog is shown, the system disables key and mouse press/release events to other top-level windows in the dialog’s eventspace, but windows in other eventspaces are unaffected by the modal dialog. (Mouse motion, enter, and leave events are still delivered to all windows when a modal dialog is shown.)
In addition to events corresponding to user and windowing actions, such as button clicks, key presses, and updates, the system dispatches two kinds of internal events: timer events and explicitly queued events.
Timer events are created by instances of timer%. When a timer is started and then expires, the timer queues an event to call the timer’s notify method. Like a top-level window, each timer is associated with a particular eventspace (the current eventspace as described in Creating and Setting the Eventspace) when it is created, and the timer queues the event in its eventspace.
Explicitly queued events are created with queue-callback, which accepts a callback procedure to handle the event. The event is enqueued in the current eventspace at the time of the call to queue-callback, with either a high or low priority as specified by the (optional) second argument to queue-callback.
An eventspace’s event queue is actually a priority queue with events sorted according to their kind, from highest-priority (dispatched first) to lowest-priority (dispatched last):
The highest-priority events are high-priority events installed with queue-callback.
Timer events have the second-highest priority.
Graphical events, such as mouse clicks or window updates, have the second-lowest priority.
The lowest-priority events are low-priority events installed with queue-callback.
Although a programmer has no direct control over the order in which events are dispatched, a programmer can control the timing of dispatches by setting the event dispatch handler via the event-dispatch-handler parameter. This parameter and other eventspace procedures are described in more detail in Eventspaces.
When a new eventspace is created, a corresponding handler thread is created for the eventspace. When the system dispatches an event for an eventspace, it always does so in the eventspace’s handler thread. A handler procedure can create new threads that run indefinitely, but as long as the handler thread is running a handler procedure, no new events can be dispatched for the corresponding eventspace.
When a handler thread shows a dialog, the dialog’s show method implicitly calls yield for as long as the dialog is shown. When a non-handler thread shows a dialog, the non-handler thread simply blocks until the dialog is dismissed. Calling yield with no arguments from a non-handler thread has no effect. Calling yield with a semaphore from a non-handler thread is equivalent to calling semaphore-wait.
The make-eventspace procedure creates a new eventspace. The following example creates a new eventspace and a new frame in the eventspace (the parameterize syntactic form temporary sets a parameter value):
(let ([new-es (make-eventspace)]) (parameterize ([current-eventspace new-es]) (new frame% [label "Example"])))
When an eventspace is created, it is placed under the management of the current custodian. When a custodian shuts down an eventspace, all frames and dialogs associated with the eventspace are destroyed (without calling can-close? or on-close in top-level-window<%>), all timers in the eventspace are stopped, and all enqueued callbacks are removed. Attempting to create a new window, timer, or explicitly queued event in a shut-down eventspace raises the exn:misc exception.
An eventspace is a synchronizable event (not to be confused with a GUI event), so it can be used with sync. As a synchronizable event, an eventspace is in a blocking state when a frame is visible, a timer is active, a callback is queued, or a menu-bar% is created with a 'root parent. (Note that the blocking state of an eventspace is unrelated to whether an event is ready for dispatching.)
Whenever the system dispatches an event, the call to the handler is wrapped with a continuation prompt (see call-with-continuation-prompt) that delimits continuation aborts (such as when an exception is raised) and continuations captured by the handler. The delimited continuation prompt is installed outside the call to the event dispatch handler, so any captured continuation includes the invocation of the event dispatch handler.
For example, if a button callback raises an exception, than the abort performed by the default exception handler returns to the event-dispatch point, rather than terminating the program or escaping past an enclosing (yield). If with-handlers wraps a (yield) that leads to an exception raised by a button callback, however, the exception can be captured by the with-handlers.
Along similar lines, if a button callback captures a continuation (using the default continuation prompt tag), then applying the continuation re-installs only the work to be done by the handler up until the point that it returns; the dispatch machinery to invoke the button callback is not included in the continuation. A continuation captured during a button callback is therefore potentially useful outside of the same callback.
(struct gui-event (start end name) #:prefab)
The content of a canvas is buffered, so if a canvas must be redrawn, the on-paint method or paint-callback function usually does not need to be called again. To further reduce flicker, while the on-paint method or paint-callback function is called, the windowing system avoids flushing the canvas-content buffer to the screen.
Canvas content can be updated at any time by drawing with the result
of the canvas’s get-dc method, and drawing is
thread-safe. Changes to the canvas’s content are flushed to the screen
periodically (not necessarily on an event-handling boundary), but the
flush method immediately flushes to the screen—
For most animation purposes, suspend-flush, resume-flush, and flush can be used to avoid flicker and the need for an additional drawing buffer for animations. During an animation, bracket the construction of each animation frame with suspend-flush and resume-flush to ensure that partially drawn frames are not flushed to the screen. Use flush to ensure that canvas content is flushed when it is ready if a suspend-flush will soon follow, because the process of flushing to the screen can be starved if flushing is frequently suspend. The method refresh-now in canvas% conveniently encapsulates this sequence.