#+TITLE: bjogui

Desktop programs in Bjolang, written as a model, a ~gui-update~ and a ~view~,
and shown with [[https://avaloniaui.net][Avalonia]].

#+BEGIN_SRC scheme
(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"))
#+END_SRC

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                       |
| ~(column child ...)~, ~(row child ...)~              | ~StackPanel~  | nothing                       |
| ~(grid #:columns #:rows child ...)~                  | ~Grid~        | nothing                       |
| ~(scroll child)~                                     | ~ScrollViewer~ | nothing                      |

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~.

#+BEGIN_SRC scheme
(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)
#+END_SRC

An image that cannot be read shows nothing.

* 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.

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

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.

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

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.

* Running: (bjogui app)

- ~(run-app init gui-update view #:subscriptions #:title #:width #:height)~
  opens a window and runs until it is closed.
- ~(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-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

#+BEGIN_SRC sh
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 examples/counter.bjo
bjo run examples/stopwatch.bjo
#+END_SRC

* Not yet

- A list box with a selection, tabs, menus and dialogs.
- Colours, borders and styles of one's own.
- A moved control is taken out of the panel and put back, so it may lose the
  keyboard focus.