Tree views, a styles example, and the Readme for all of it

A tree node is a TreeViewItem: a header and items, patched by key as a
list's items are. The chosen node is chosen by its key, once the nodes are
there, and a click sends the key. SelectedItem is a direct property, which
a clear does not reset, so no choice is set as null.

The Readme now covers menus and keys, dialogs, styles and themes, and the
new controls. docs/styles.png is examples/styles.bjo.

🤖 Generated with [ECA](https://eca.dev) (anthropic/claude-opus-4-8)

Co-Authored-By: eca-agent <git@eca.dev>

Readme.org

@@ -68,6 +68,16 @@ being changed, and they are dropped, so only the user's events are messages.
 | ~(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~,
@@ -122,6 +132,53 @@ a line around a view, with round corners if ~#:radius~ is more than zero.
 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.
+
+* Styles and themes
+
+[[file:docs/styles.png]]
+
+~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.
+
+#+BEGIN_SRC scheme
+(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")))
+#+END_SRC
+
+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.
+[[file:examples/styles.bjo][examples/styles.bjo]] is the picture above.
+
+* Menus and keys
+
+#+BEGIN_SRC scheme
+(menu-bar
+  (submenu "File"
+    (menu-item "New" New #:shortcut "Ctrl+N")
+    (menu-separator)
+    (menu-item "Quit" Quit #:shortcut "Ctrl+Q")))
+#+END_SRC
+
+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
@@ -159,10 +216,42 @@ 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                |
+
+#+BEGIN_SRC scheme
+(Delete (next m (confirm "Delete it?" Deleted?)))
+((Deleted? #t) (next (remove m)))
+((Deleted? #f) (next m))
+#+END_SRC
+
+~#: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 #:title #:width #:height)~
+- ~(run-app init gui-update view #:subscriptions #:keys #:theme #:styles
-  opens a window and runs until it is closed.
+  #: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
@@ -176,6 +265,7 @@ cancelled, and the program ends.
   - ~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-model~ reads the model, and ~app-control-at~ finds a control by its
     path of child indices.
 
@@ -196,15 +286,21 @@ 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 examples/counter.bjo
 bjo run examples/stopwatch.bjo
 bjo run examples/todo.bjo
+bjo run examples/styles.bjo
 #+END_SRC
 
 * Not yet
 
-- Menus and dialogs.
+- A data grid, and lists that make controls only for the items on screen.
-- Styles of one's own, with classes, and colours for the pointer-over and
+- Dialogs of one's own, with a view and a state.
-  pressed states.
+- The macOS menu bar.
+- Pointer-over and pressed colours for kinds other than buttons.
 - A moved control is taken out of the panel and put back, so it may lose the
   keyboard focus.

examples/styles.bjo

@@ -0,0 +1,38 @@
+;; This Source Code Form is subject to the terms of the Mozilla Public
+;; License, v. 2.0. If a copy of the MPL was not distributed with this
+;; file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+;; bjogui/examples/styles.bjo — styles, classes and the dark theme.
+;;
+;;     bjo run examples/styles.bjo
+;;
+;; The picture in the Readme is this program, taken with
+;;
+;;     BJOGUI_SCREENSHOT=docs/styles.png bjo run examples/styles.bjo
+(import (std prelude))
+(import (bjogui view))
+(import (bjogui app))
+
+(type (: Msg (Union Nothing)))
+
+(defun (main)
+  (run-app 0 (fun (msg n) (next n))
+           (fun (n)
+             (column #:spacing 12.0 #:margin 20.0
+               (classed (text "Styled with classes") "title")
+               (row #:spacing 8.0
+                 (classed (button "Save" Nothing) "primary")
+                 (classed (button "Delete" Nothing) "danger")
+                 (button "Plain" Nothing))
+               (classed (border (text "a card") #:padding 10.0) "card")
+               (toggle-switch "Dark mode" #t (fun (b) Nothing))
+               (number-input 3.0 (fun (d) Nothing) #:width 160.0)
+               (date-picker "2026-10-25" (fun (d) Nothing))))
+           #:theme (fun (n) "dark")
+           #: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")
+                          (style "button" #:class "danger" #:background "#c0392b" #:colour "white"
+                                 #:hover-background "#e74c3c" #:radius 12.0)
+                          (style "border" #:class "card" #:background "#34495e" #:radius 8.0))
+           #:title "styles" #:width 360.0 #:height 330.0))

src/app.bjo

@@ -99,6 +99,8 @@
   (WrapPanel         (: Avalonia.Controls.WrapPanel (-> WrapPanel)))
   (DockPanel         (: Avalonia.Controls.DockPanel (-> DockPanel)))
   (DockEdge          (: Avalonia.Controls.Dock))
+  (TreeView          (: Avalonia.Controls.TreeView (-> TreeView)))
+  (TreeViewItem      (: Avalonia.Controls.TreeViewItem (-> TreeViewItem)))
   (PropertyChange    (: Avalonia.AvaloniaPropertyChangedEventArgs))
   (Classes           (: Avalonia.Controls.Classes))
   (StyledElement     (: Avalonia.StyledElement))
@@ -220,6 +222,10 @@
   (auto-watermark-property (: Avalonia.Controls.AutoCompleteBox.WatermarkProperty #:get))
   (auto-text            (: Avalonia.Controls.AutoCompleteBox.Text (-> AutoCompleteBox string) #:get))
   (expanded?            (: Avalonia.Controls.Expander.IsExpanded (-> Expander bool) #:get))
+  (tree-header-property (: Avalonia.Controls.Primitives.HeaderedItemsControl.HeaderProperty #:get))
+  (tree-expanded-property (: Avalonia.Controls.TreeViewItem.IsExpandedProperty #:get))
+  (tree-selected-property (: Avalonia.Controls.TreeView.SelectedItemProperty #:get))
+  (tree-selected        (: Avalonia.Controls.TreeView.SelectedItem (-> TreeView Object) #:get))
   (classes-of           (: Avalonia.StyledElement.Classes (-> StyledElement Classes) #:get))
   (classes-clear!       (: Avalonia.Controls.Classes.Clear (-> Classes void)))
   (classes-add!         (: Avalonia.Controls.Classes.Add (-> Classes string void)))
@@ -378,6 +384,8 @@
     (MenuBarKind #t)
     (MenuItemKind #t)
     (ContextMenuKind #t)
+    (TreeKind #t)
+    (TreeNodeKind #t)
     (_ #f)))
 
 ;; A kind whose children each have a place of their own: the content of a
@@ -430,6 +438,7 @@
        (ToggleSwitchKind (Some (cast AvaloniaProperty content-property)))
        (RadioKind (Some (cast AvaloniaProperty content-property)))
        (ExpanderKind (Some (cast AvaloniaProperty header-property)))
+       (TreeNodeKind (Some (cast AvaloniaProperty tree-header-property)))
        (TabKind (Some (cast AvaloniaProperty header-property)))
        (MenuItemKind (Some (cast AvaloniaProperty menu-header-property)))
        (_ None)))
@@ -484,7 +493,12 @@
     ((PropRadius _) (when-kind (= kind BorderKind) (cast AvaloniaProperty corner-radius-property)))
     ((PropShortcut _) (when-kind (= kind MenuItemKind) (cast AvaloniaProperty input-gesture-property)))
     ((PropTip _) (Some (cast AvaloniaProperty tip-property)))
-    ((PropExpanded _) (when-kind (= kind ExpanderKind) (cast AvaloniaProperty expanded-property)))
+    ((PropExpanded _)
+     (cond ((= kind ExpanderKind) (Some (cast AvaloniaProperty expanded-property)))
+           ((= kind TreeNodeKind) (Some (cast AvaloniaProperty tree-expanded-property)))
+           (else None)))
+    ;; Set after the children, by `sync-tree-choice!`.
+    ((PropChosenKey _) None)
     ((PropDockSide _) (Some (cast AvaloniaProperty dock-property)))
     ((PropClasses _) None)
     ((PropDate _) (when-kind (= kind DateKind) (cast AvaloniaProperty date-property)))
@@ -573,6 +587,7 @@
     ((PropExpanded b) (cast Object b))
     ((PropDate s) (text s))
     ((PropClasses _) (cast Object 0))
+    ((PropChosenKey s) (text s))
     ((PropDockSide i)
      (cast Object (match i (0 DockEdge.Left) (1 DockEdge.Top) (2 DockEdge.Right) (_ DockEdge.Bottom))))))
 
@@ -589,7 +604,7 @@
     ((PropRows _) 20) ((PropRow _) 21) ((PropColumn _) 22) ((PropRowSpan _) 23) ((PropColumnSpan _) 24)
     ((PropBackground _) 25) ((PropForeground _) 26) ((PropBorderColour _) 27) ((PropBorderWidth _) 28)
     ((PropRadius _) 29) ((PropShortcut _) 30) ((PropTip _) 31) ((PropExpanded _) 32) ((PropDate _) 33)
-    ((PropDockSide _) 34) ((PropClasses _) 35)))
+    ((PropDockSide _) 34) ((PropClasses _) 35) ((PropChosenKey _) 36)))
 
 (: (-> Control AvaloniaProperty Object void))
 (defun (put! control property value)
@@ -1009,6 +1024,13 @@
                (fun ()
                  (def d (range-value (cast RangeBase control)))
                  (fun (on) (match on ((OnSlide f) (Some (f d))) (_ None))))))
+    (TreeKind
+     (on-event (cast RoutedEvent selection-changed-event)
+               (fun ()
+                 (def key (match (tree-node-key live (tree-selected (cast TreeView control)))
+                            ((Some k) k)
+                            (None "")))
+                 (fun (on) (match on ((OnPick f) (Some (f key))) (_ None))))))
     ((or DropdownKind ListBoxKind TabsKind)
      (on-event (cast RoutedEvent selection-changed-event)
                (fun ()
@@ -1060,6 +1082,8 @@
     (SeparatorKind (cast Control (Separator.)))
     (ContextKind (cast Control (Border.)))
     (ContextMenuKind (cast Control (ContextMenu.)))
+    (TreeKind (cast Control (TreeView.)))
+    (TreeNodeKind (cast Control (TreeViewItem.)))
     (ToggleSwitchKind (cast Control (ToggleSwitch.)))
     (RadioKind (cast Control (RadioButton.)))
     (NumberKind (cast Control (NumericUpDown.)))
@@ -1119,6 +1143,7 @@
              (mutablevec-add! (.children live) c)
              (adopt! kind control (- (mutablevec-length (.children live)) 1) (.control c))))
   (sync-selection! control kind (view-props v))
+  (sync-tree-choice! live)
   live)
 
 ;; Changes the controls of `live` to show `next`. Gives `live` itself, or new
@@ -1136,8 +1161,51 @@
               ((or (panel? kind) (items? kind)) (patch-children! app live kind (view-children next)))
               (else unit))
         (sync-selection! (.control live) kind (view-props next))
+        (sync-tree-choice! live)
         live)))
 
+;; The node of a tree whose control is `control`, among the live nodes below
+;; `live`, and its key.
+(: (-> (Live %m) Object (Option string)))
+(defun (tree-node-key live control)
+  (: (-> (Live %m) (Option string)))
+  (defun (walk l)
+    (if (and (= (view-kind (.view l)) TreeNodeKind) (eq? (cast Object (.control l)) control))
+        (view-key (.view l))
+        (loop (:for c (mutablevec->vec (.children l)))
+              (:when-let (Some k) (walk c))
+              (:finish #t (Some k))
+              => None)))
+  (walk live))
+
+;; The control of the node with `key` below `live`.
+(: (-> (Live %m) string (Option Control)))
+(defun (tree-node-control live key)
+  (if (and (= (view-kind (.view live)) TreeNodeKind) (= (view-key (.view live)) (Some key)))
+      (Some (.control live))
+      (loop (:for c (mutablevec->vec (.children live)))
+            (:when-let (Some found) (tree-node-control c key))
+            (:finish #t (Some found))
+            => None)))
+
+;; Chooses the node of a tree whose key the view gives, once the nodes are
+;; there, if it is not chosen already.
+(: (-> (Live %m) void))
+(defun (sync-tree-choice! live)
+  (when (= (view-kind (.view live)) TreeKind)
+    (def control (.control live))
+    (loop (:for p (view-props (.view live)))
+          (:do (match p
+                 ((PropChosenKey key)
+                  (match (if (string-empty? key) None (tree-node-control live key))
+                    ((Some node)
+                     (unless (eq? (tree-selected (cast TreeView control)) (cast Object node))
+                       (put! control (cast AvaloniaProperty tree-selected-property) (cast Object node))))
+                    ;; Null and not a clear: SelectedItem is a direct property,
+                    ;; which a clear does not reset.
+                    (None (put! control (cast AvaloniaProperty tree-selected-property) (clr-default)))))
+                 (_ unit))))))
+
 ;; The children of a kind whose children each have a place of their own.
 (: (-> (App %model %m) (Live %m) Kind (Vec (View %m)) void))
 (defun (patch-single! app live kind views)

src/view.bjo

@@ -22,6 +22,7 @@
   list-box tabs tab
   menu-bar submenu menu-item menu-separator
   toggle-switch radio number-input date-picker auto-complete expander
+  tree-view tree-node
   column row grid scroll border flow dock
   ;; Wrappers.
   keyed at paint with-context-menu tooltip at-top at-bottom at-left at-right classed
@@ -44,7 +45,7 @@
                      ListBoxKind TabsKind TabKind
                      MenuBarKind MenuItemKind SeparatorKind ContextKind ContextMenuKind
                      ToggleSwitchKind RadioKind NumberKind DateKind AutoCompleteKind
-                     ExpanderKind WrapKind DockKind
+                     ExpanderKind WrapKind DockKind TreeKind TreeNodeKind
                      ColumnKind RowKind GridKind ScrollKind BorderKind)))
 
 ;; A property of a control. A kind takes only some of them; the app ignores
@@ -84,14 +85,16 @@
                      (: PropExpanded bool)          ; whether an expander shows its content
                      (: PropDate string)            ; a date picker's date, as "2026-10-25"
                      (: PropDockSide int)           ; where a view is in its dock: 0 to 3
-                     (: PropClasses (Vec string)))))  ; the classes that styles match
+                     (: PropClasses (Vec string))   ; the classes that styles match
+                     (: PropChosenKey string))))    ; the key of a tree's chosen node
 
 ;; An event, and the message it sends.
 (type (: (On %m) (Union (: OnClick %m)
                         (: OnInput (-> string %m))
                         (: OnToggle (-> bool %m))
                         (: OnSlide (-> double %m))
-                        (: OnSelect (-> int %m)))))
+                        (: OnSelect (-> int %m))
+                        (: OnPick (-> string %m)))))
 
 ;; One control and what is in it.
 ;;
@@ -393,6 +396,28 @@
         (children [child])
         (key None)))
 
+;; A tree of nodes made by `tree-node`, with the node whose key is `chosen`
+;; chosen, or none for "". A click on a node sends `(on-pick key)`.
+(: (-> string (-> string %m) (#:width double) (#:height double) (#:margin double)
+       #:rest (View %m) (View %m)))
+(defun (tree-view chosen on-pick #:width 0.0 #:height 0.0 #:margin 0.0 #:rest nodes)
+  (View (kind TreeKind)
+        (props (with-layout [(PropChosenKey chosen)] width height margin 0.0))
+        (events [(OnPick on-pick)])
+        (children (array->vec nodes))
+        (key None)))
+
+;; A node of a tree, known by `key`, which must be one of its own in the
+;; tree. It opens to show `children`, more nodes, and is open at first when
+;; `#:expanded`; after that it is opened and closed by the user.
+(: (-> string string (#:expanded bool) #:rest (View %m) (View %m)))
+(defun (tree-node key header #:expanded #f #:rest children)
+  (View (kind TreeNodeKind)
+        (props [(PropLabel header) (PropExpanded expanded)])
+        (events [])
+        (children (array->vec children))
+        (key (Some key))))
+
 ;; A checkbox that is `checked?`. A click sends `(on-toggle checked)`, with
 ;; the new state.
 (: (-> string bool (-> bool %m) (#:enabled bool) (#:margin double) (View %m)))

tests/tree.bjo

@@ -0,0 +1,84 @@
+;; This Source Code Form is subject to the terms of the Mozilla Public
+;; License, v. 2.0. If a copy of the MPL was not distributed with this
+;; file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+;; bjogui/tests/tree.bjo — tree views.
+;;
+;;     bjo run tests/tree.bjo
+;;
+;; No window is opened.
+
+(import (std prelude))
+(import (std simpletest))
+(import (std interop))
+(import (bjogui view))
+(import (bjogui app))
+
+(import/class
+  (Object           (: System.Object))
+  (Disposable       (: System.IDisposable))
+  (AvaloniaObject   (: Avalonia.AvaloniaObject))
+  (AvaloniaProperty (: Avalonia.AvaloniaProperty))
+  (Priority         (: Avalonia.Data.BindingPriority))
+  (ItemsControl     (: Avalonia.Controls.ItemsControl))
+  (TreeView         (: Avalonia.Controls.TreeView))
+  (TreeViewItem     (: Avalonia.Controls.TreeViewItem))
+  (Headered         (: Avalonia.Controls.Primitives.HeaderedItemsControl)))
+
+(import/extern
+  (set-value      (: Avalonia.AvaloniaObject.SetValue
+                     (-> AvaloniaObject AvaloniaProperty Object Priority Disposable)))
+  (selected-property (: Avalonia.Controls.TreeView.SelectedItemProperty #:get))
+  (selected       (: Avalonia.Controls.TreeView.SelectedItem (-> TreeView Object) #:get))
+  (item-count     (: Avalonia.Controls.ItemsControl.ItemCount (-> ItemsControl int) #:get))
+  (header         (: Avalonia.Controls.Primitives.HeaderedItemsControl.Header (-> Headered Object) #:get))
+  (expanded?      (: Avalonia.Controls.TreeViewItem.IsExpanded (-> TreeViewItem bool) #:get))
+  (same?          (: System.Object.ReferenceEquals (-> Object Object bool))))
+
+(type (: Msg (Union (: Chose string) Reverse)))
+(type (: Model (Record (: chosen string) (: fruit (List string)))))
+
+(: (-> Msg Model (Tuple Model (List (Event Msg)))))
+(defun (gui-update msg m)
+  (next (match msg
+          ((Chose k) (record-set m (chosen k)))
+          (Reverse (record-set m (fruit (list-reverse (.fruit m))))))))
+
+(: (-> Model (View Msg)))
+(defun (view m)
+  (tree-view (.chosen m) Chose
+    (apply tree-node "fruit" "Fruit" #:expanded #t
+           (vec->array (vec-map (fun (f) (tree-node f (string-upcase f))) (list->vec (.fruit m)))))
+    (tree-node "veg" "Vegetables"
+      (tree-node "kale" "KALE"))))
+
+(defun (main)
+  (def app (start-app (Model (chosen "pear") (fruit '("apple" "pear")))
+                      gui-update view (fun (c) unit)))
+  (defun (ctl path) (app-control-at app path))
+  (def tree (cast TreeView (ctl '())))
+  (defun (chosen-is? path) (same? (selected tree) (cast Object (ctl path))))
+
+  (expect "the top nodes" (item-count (cast ItemsControl tree)) 2)
+  (expect "a node's nodes" (item-count (cast ItemsControl (ctl '(0)))) 2)
+  (expect "a header" (.ToString (header (cast Headered (ctl '(0 1))))) "PEAR")
+  (expect "open at first" (expanded? (cast TreeViewItem (ctl '(0)))) #t)
+  (expect "closed at first" (expanded? (cast TreeViewItem (ctl '(1)))) #f)
+  (expect "the chosen node is chosen once the nodes are there" (chosen-is? '(0 1)) #t)
+
+  (ignore (set-value (cast AvaloniaObject tree) (cast AvaloniaProperty selected-property)
+                     (cast Object (ctl '(1 0))) Priority.LocalValue))
+  (expect "a node chosen sends its key" (.chosen (app-model app)) "kale")
+
+  (app-dispatch! app (Chose "apple"))
+  (expect "the model chooses the node" (chosen-is? '(0 0)) #t)
+
+  (def apple (ctl '(0 0)))
+  (app-dispatch! app Reverse)
+  (expect "nodes moved by key keep their controls" (same? (cast Object (ctl '(0 1))) (cast Object apple)) #t)
+  (expect "and the choice" (chosen-is? '(0 1)) #t)
+
+  (app-dispatch! app (Chose ""))
+  (expect "no choice" (same? (selected tree) (clr-default)) #t)
+
+  (println "bjogui tree ok"))