bjogui

• How it works
• The views: (bjogui view)
• Colours and borders
• Components: view-map
• Drawing: canvas
• Controls of your own: component and (bjogui draw)
• Styles and themes
• Menus and keys
• Work: commands and subscriptions
• Dialogs: (bjogui dialogs)
• Running: (bjogui app)
• Requirements
• Tests
• Not yet

Desktop programs in Bjolang, written as a model, a gui-update and a view, and shown with Avalonia.

The todo list above is examples/todo.bjo.

(import (bjogui view))
(import (bjogui app))

(type (: Msg (Union Inc Dec)))

(: gui-update (-> Msg int (Tuple int (List (Event Msg)))))
(defun (gui-update msg n)
  (next (match msg (Inc (+ n 1)) (Dec (- n 1)))))

(: view (-> int (View Msg)))
(defun (view n)
  (row #:spacing 8.0 #:margin 16.0
    (button "-" Dec)
    (text #"${n}")
    (button "+" Inc)))

(defun (main)
  (run-app 0 gui-update view #:title "Counter"))

A program never names an Avalonia type. A view is a Bjolang value, an event is a message, and the controls are made and changed by the package.

How it works

An event finds its message in the view its control was last given. gui-update makes the next model from the message, and view makes the next view. The next view is compared with the last one, control by control, and only what differs is changed. So a control keeps its focus, its caret and its scroll position when the model changes.

Children are matched by key, and those with no key by their place among the others with none. Give each item of a list that can change order a key with (keyed id view): a child that moves then keeps its control, and the control is moved rather than made again. A child whose kind changes gets a new control.

A control that is changed raises events of its own: a text input whose text is set says that its text changed. Those events come while the controls are being changed, and they are dropped, so only the user's events are messages.

The views: (bjogui view)

Constructor	Control	Sends
(text s)	TextBlock	nothing
(button label msg)	Button	msg on a click
(text-input value on-input)	TextBox	(on-input text) on a change
(text-area value on-input)	TextBox, several lines	(on-input text)
(checkbox label checked? on-toggle)	CheckBox	(on-toggle checked)
(slider value minimum maximum on-change)	Slider	(on-change value)
(progress-bar value)	ProgressBar	nothing
(dropdown options selected on-select)	ComboBox	(on-select index)
(image path)	Image	nothing
(list-box views selected on-select)	ListBox	(on-select index)
(tabs selected on-select (tab header view) ...)	TabControl	(on-select index)
(column child ...), (row child ...)	StackPanel	nothing
(grid #:columns #:rows child ...)	Grid	nothing
(scroll child)	ScrollViewer	nothing
(border child)	Border	nothing
(toggle-switch label on? on-toggle)	ToggleSwitch	(on-toggle on)
(radio label chosen? msg)	RadioButton	msg when it is chosen
(number-input value on-change)	NumericUpDown	(on-change value)
(date-picker "2026-10-25" on-change)	DatePicker	(on-change date)
(auto-complete value options on-input)	AutoCompleteBox	(on-input text)
(expander header expanded? on-toggle child)	Expander	(on-toggle expanded)
(tree-view chosen on-pick (tree-node key header ...) ...)	TreeView	(on-pick key)
(flow child ...)	WrapPanel	nothing
(dock child ...), with at-top, at-left …	DockPanel	nothing
(menu-bar (submenu label (menu-item label msg) ...))	Menu	msg when an item is chosen

They take keywords where they apply: #:size (the font size of a text), #:wrap (a text breaks its lines at its width), #:width, #:height, #:margin, #:padding, #:spacing (between the children of a column, a row or a grid), #:enabled, #:placeholder, #:step (a slider's arrow key) and #:maximum (a progress bar's full, 1 unless told otherwise). A size of zero, the default, leaves the size to the control.

A grid's #:columns and #:rows are sizes as Avalonia writes them: "Auto" fits the content, "*" shares what is left, "2*" takes two shares, and a number is pixels. (at row column view) puts a view in a cell, counting from 0, with #:row-span and #:column-span.

(grid #:columns "Auto,*" #:rows "Auto,Auto" #:spacing 6.0
  (at 0 0 (text "Volume")) (at 0 1 (slider (.volume m) 0.0 10.0 Volume))
  (at 1 0 (text "Colour")) (at 1 1 (dropdown ["red" "green" "blue"] (.colour m) Colour)))

(scroll (apply column (vec->array (vec-map (fun (t) (keyed (.id t) (text (.title t))))
                                           (.todos m))))
        #:height 200.0)

An image that cannot be read shows nothing.

A list box's items are views, so an item can be anything: a text, a row with a checkbox, a dropdown. Give them keys if the list can change order. Each item is a control, so a list of thousands makes thousands of controls.

(tabs (.tab m) Tab
  (tab "Todo" (list-box (vec-map (fun (t) (keyed (.id t) (text (.title t)))) (.todos m))
                        (.chosen m) Chose))
  (tab "Settings" (checkbox "Dark" (.dark m) Dark)))

Colours and borders

(paint view #:colour #:background) colours any view: #:colour is the text, #:background what is behind it. A colour is a name such as "red", or "#rgb", "#rrggbb" or "#aarrggbb". The empty string, the default, leaves the colour to the theme, and so does a colour that cannot be read.

(border child #:colour #:thickness #:radius #:background #:padding) draws a line around a view, with round corners if #:radius is more than zero.

(border (paint (text "Saved") #:colour "#c0392b")
        #:colour "#2980b9" #:radius 8.0 #:background "#ecf0f1" #:padding 8.0)

The Fluent theme gives a button its own colours when the pointer is over it or it is pressed, so a painted button shows the theme's colour then.

More wrappers: (tooltip view "text"), (with-context-menu view item ...) for a menu on the right button, and (classed view "name" ...) for styles.

A tree node is known by its key, which must be its own in the tree. A node is open at first when it says #:expanded; after that the user opens and closes it.

Components: view-map

A part of a program can have a message type, an update and a view of its own, and know nothing of the program it is in. (view-map f view) puts its view in a larger one: each message it sends is given to f first, which is usually the case of the larger message that carries it.

(type (: Msg (Union (: Colour PickerMsg) (: Width PickerMsg) ...)))

(column (view-map Colour (picker-view (.colour m)))
        (view-map Width (picker-view (.width m))))

((Colour pm) (next (record-set m (colour (picker-update pm (.colour m))))))

Drawing: canvas

(canvas width height shape ...) is a surface to draw on. A shape is placed by its own numbers, and any other view by (at-point x y view).

Shape	Takes
(rect x y width height)	#:fill, #:stroke, #:stroke-width, #:radius
(ellipse x y width height)	#:fill, #:stroke, #:stroke-width
(line x1 y1 x2 y2)	#:stroke, #:stroke-width
(path "M 10 10 L 50 10 L 30 40 Z")	#:fill, #:stroke, #:stroke-width

A path is written as SVG writes one. (on-press canvas f), (on-move canvas f) and (on-release canvas f) send (f x y), the place on the canvas, when a button is pressed, when the pointer moves, and when the button is let go. A press takes the pointer, so a drag that leaves the canvas still reaches it. Shapes are views like any other, so keyed ones are kept as their list changes. examples/sketch.bjo is the picture above, and uses two components of its own.

Controls of your own: component and (bjogui draw)

(component state draw #:input #:cursor #:focusable) makes a control that draws itself and hears the pointer and the keys. It keeps a state of its own, such as where a drag started or where the pointer hovers, which the model never sees. (drawn c value on-output) puts it in a view: it shows value, and each output it sends is made a message with on-output, as a slider's value is.

(import (bjogui draw))

;; Where a drag started, and the value then.
(type (: Drag (Union Idle (: Dragging double double))))

(: knob (Component double Drag double))
(def knob
  (component Idle
    (fun (pen w h s v)
      (fill-circle pen (/ w 2.0) (/ h 2.0) (/ w 2.0) "#2c3e50")
      (stroke-arc pen (/ w 2.0) (/ h 2.0) (- (/ w 2.0) 6.0) 2.4 (* v 4.7) "orange"
                  #:width 5.0 #:cap "round"))
    #:input (fun (in w h s v)
              (match (Tuple in s)
                ((Tuple (PointerDown p) _) (Tuple (Dragging (.y p) v) None))
                ((Tuple (PointerMove p) (Dragging y0 v0))
                 (Tuple s (Some (max 0.0 (min 1.0 (+ v0 (/ (- y0 (.y p)) 150.0)))))))
                ((Tuple (PointerUp _) _) (Tuple Idle None))
                (_ (Tuple s None))))
    #:cursor (fun (s v) "Hand")))

;; In a view:
(drawn knob (.volume model) Volume #:width 80.0 #:height 80.0)

• (draw pen width height state value) draws the component at its size. It is called again when the value, the state or the size changes, and not otherwise.
• (input in width height state value) gives the next state, and maybe an output. in is an Input: (PointerDown p), (PointerMove p), (PointerUp p), PointerEnter, PointerLeave, (Wheel p dx dy), (KeyDown "Ctrl+Z"), FocusIn and FocusOut. A Pointer has x and y from the component's corner, button ("left", "right", "middle"), clicks (2 for a double click) and mods ("Ctrl+Shift").
• A press takes the pointer, so a drag that leaves the component still reaches it. A #:focusable component takes the keys when it is pressed.
• An input that changes the state or sends an output is the component's, and goes no further. Any other goes on to the parents, so a key the component does not use still works as a shortcut.
• With no #:width or #:height, a component fills the space it is given, and is drawn again when that changes.
• Make a component once, at the top of a module. A view in the place of another component is a new control, whose state starts again.
• A component that sends nothing needs its type written, because its output type is not known: (: lamp (Component bool Unit Unit)).

(bjogui draw) is the pen. Its coordinates start at the component's top left corner, its colours are strings, and its angles are radians, clockwise from the right, as sin and cos take them.

• Shapes: fill-rect, stroke-rect (#:radius), fill-circle, stroke-circle, fill-ellipse, stroke-ellipse, stroke-line, stroke-polyline, fill-polygon, stroke-arc, and fill-path and stroke-path with SVG path data. Lines take #:width, #:cap and #:dash.
• Text and pictures: draw-text (#:size, #:bold, #:font, #:align, #:max-width), text-size to lay text out, and draw-image.
• translated, scaled, rotated, clipped and faded change what a function given to them draws.

The drawing is vectors, so it is sharp at any scale. examples/mixer.bjo has knobs and a timeline with a playhead, marks and a hover line. start-headless! sets Avalonia up with no screen, so that a test can draw components and read their pixels; see tests/components.bjo.

Styles and themes

run-app's #:theme gives "light", "dark", or anything else for the system's, for a model, so a setting can switch it. #:styles is a list of rules made by style: a kind, by the name of its constructor, a class given with classed, and the look.

(run-app init gui-update view
  #:theme (fun (m) (if (.dark m) "dark" "light"))
  #:styles (list (style "text" #:class "title" #:font-size 22.0 #:colour "#f1c40f")
                 (style "button" #:class "primary" #:background "#27ae60" #:colour "white"
                        #:hover-background "#2ecc71" #:pressed-background "#1e8449")))

A rule takes #:colour, #:background, #:font-size, #:padding and #:radius. A button's rule also takes #:hover-colour, #:hover-background, #:pressed-colour and #:pressed-background, which are set inside its template, as the Fluent theme sets its own; no other kind has them. examples/styles.bjo is the picture above.

Menus and keys

(menu-bar
  (submenu "File"
    (menu-item "New" New #:shortcut "Ctrl+N")
    (menu-separator)
    (menu-item "Quit" Quit #:shortcut "Ctrl+Q")))

A menu bar is a view, so it goes where the program puts it, usually at the top of a dock. A menu item's #:shortcut is shown beside it, and the keys send its message while it is in the view and enabled. run-app's #:keys gives more, for a model: (fun (m) (list (key "F5" Refresh))). Keys are the window's after the control that has the keyboard had its chance, so a text input keeps Ctrl+C.

Work: commands and subscriptions

gui-update is a plain function, so it cannot wait, and the compiler says so if it tries. Work is a value it returns, and the app runs it.

A command is an (Event %m): an event whose answer is a message. (next m command ...) is the next model and the commands to start; (next m) is the model alone. The app syncs each command in a fiber of its own and gives its message to gui-update. Any event will do: a timeout, a blocking call, a channel, a choose of several.

(Load (next (record-set m (loading #t))
            (wrap (blocking (fun () (file-read-text path))) Loaded)))

A subscription is a source of messages that runs while the model asks for it. subscriptions gives the list for a model. After each update the app starts those that are new and cancels those that are gone; the others keep running. A subscription is known by its key.

• (every duration msg) sends msg each time duration has passed.
• (subscription key next-event) is a source of your own: next-event gives the event of the next message, and is called again after each one. (subscription "jobs" (fun () (wrap (chan-recv jobs) Got))) sends what comes on a channel.

(: subscriptions (-> Model (List (Sub Msg))))
(defun (subscriptions m)
  (if (.running m) (list (every (milliseconds 100) Tick)) '()))

The fibers belong to the app. They post their messages to the UI thread, so gui-update and view always run there. When the window is closed they are cancelled, and the program ends.

Dialogs: (bjogui dialogs)

A dialog is a command, and its answer comes back as a message.

Command	Sends
(alert text msg)	msg when it is closed
(confirm text on-answer)	(on-answer #t) for OK, #f otherwise
(prompt text on-answer #:value)	(on-answer (Some text)), or None
(open-file on-answer #:filters)	(on-answer (Some path)), or None
(open-files on-answer)	(on-answer paths)
(save-file on-answer #:name)	(on-answer (Some path)), or None
(open-folder on-answer)	(on-answer (Some path)), or None
(quit)	nothing: it closes the windows

(Delete (next m (confirm "Delete it?" Deleted?)))
((Deleted? #t) (next (remove m)))
((Deleted? #f) (next m))

#:filters are kinds of file, each a name and patterns: (list (Tuple "Text" '("*.txt" "*.md"))). run-app's #:on-close msg makes the close button send msg instead of closing, and the program closes with (quit) when it is ready, say after asking about unsaved work.

A dialog belongs to the main window. With none, as under start-app, each answers as if it was cancelled. Avalonia's dialogs cannot be cancelled, so a command that is cancelled stops waiting, and the dialog stays until it is answered.

Running: (bjogui app)

• (run-app init gui-update view #:subscriptions #:keys #:theme #:styles #:on-close #:title #:width #:height) opens a window and runs until it is closed.
• #:screenshot path makes run-app save a PNG of the window once it is shown, at twice its size, and close it. It is the environment variable BJOGUI_SCREENSHOT unless the program says otherwise, so any program can be pictured with no change: BJOGUI_SCREENSHOT=docs/todo.png bjo run examples/todo.bjo made the picture above.

• (start-app init gui-update view on-root #:subscriptions) makes the controls with no window, and gives on-root the control to show. Its fibers belong to the scope it is started in.

  • app-dispatch! gives a message on the UI thread, and app-send! from any thread.
  • app-run-pending! runs what fibers have posted, where no loop of Avalonia's does.
  • app-stop! cancels the subscriptions and drops later messages.
  • app-press! presses keys, such as "Ctrl+S", as the window does.
  • app-input! gives an Input to the component at a path, as its control does.
  • app-model reads the model, and app-control-at finds a control by its path of child indices.

  tests/app.bjo and tests/effects.bjo use these to drive an app through its real controls.

Requirements

• Avalonia 11.3, which bjo restores from NuGet. On Linux it needs X11 or XWayland.
• A Bjolang with (std interop), which makes the delegates that Avalonia's events take.

Tests

bjo run tests/app.bjo       # prints "bjogui ok"
bjo run tests/effects.bjo   # prints "bjogui effects ok"
bjo run tests/controls.bjo  # prints "bjogui controls ok"
bjo run tests/lists.bjo     # prints "bjogui lists ok"
bjo run tests/inputs.bjo    # prints "bjogui inputs ok"
bjo run tests/menus.bjo     # prints "bjogui menus ok"
bjo run tests/dialogs.bjo   # prints "bjogui dialogs ok"
bjo run tests/tree.bjo      # prints "bjogui tree ok"
bjo run tests/canvas.bjo    # prints "bjogui canvas ok"
bjo run tests/components.bjo  # prints "bjogui components ok"
bjo run examples/counter.bjo
bjo run examples/stopwatch.bjo
bjo run examples/todo.bjo
bjo run examples/styles.bjo
bjo run examples/sketch.bjo
bjo run examples/mixer.bjo

Not yet

• A data grid, and lists that make controls only for the items on screen.
• Dialogs of one's own, with a view and a state.
• The macOS menu bar.
• Pointer-over and pressed colours for kinds other than buttons.
• Controls of Avalonia's or another package's that bjogui does not wrap.
• A component asks for no size of its own: it is given one, or fills the space it gets. Measuring itself, an accessibility tree, and text input inside a component would need a subclass.
• A moved control is taken out of the panel and put back, so it may lose the keyboard focus.