Merge pull request #912 from Textualize/mount-wait

wait for mount

docs/getting_started.md

@@ -47,3 +47,7 @@ textual --help
 ```
 
 See [devtools](guide/devtools.md) for more about the `textual` command.
+
+## Need help?
+
+See the [help](./help.md) page for how to get help with Textual, or to report bugs.

docs/guide/events.md

@@ -183,7 +183,7 @@ Let's look at an example which looks up word definitions from an [api](https://d
 
 === "Output"
 
-    ```{.textual path="docs/examples/events/dictionary.py" press="tab,t,e,x,t,_,_,_,_,_,_,_,_,_,_,_"}
+    ```{.textual path="docs/examples/events/dictionary.py" press="t,e,x,t,_,_,_,_,_,_,_,_,_,_,_"}
     ```
 
 Note the highlighted line in the above code which calls `asyncio.create_task` to run a coroutine in the background. Without this you would find typing in to the text box to be unresponsive.

docs/help.md

@@ -0,0 +1,20 @@
+---
+hide:
+  - navigation
+---
+
+# Help
+
+Here's where to go if you need help with Textual.
+
+## Bugs and feature requests
+
+Report bugs via GitHub on the Textual [issues](https://github.com/Textualize/textual/issues) page. You can also post feature requests via GitHub issues, but see the [roadmap](./roadmap.md) first.
+
+## Forum
+
+Visit the [Textual forum](https://community.textualize.io/) for Textual (and Rich) discussions.
+
+## Discord Server
+
+For more realtime feedback or chat, join our discord server to connect with the [Textual community](https://discord.gg/Enf6Z3qhVr).

docs/index.md

@@ -70,7 +70,7 @@ Textual is a framework for building applications that run within your terminal.
 ```
 
 
-```{.textual path="docs/examples/events/dictionary.py" columns="100" lines="30" press="tab,_,t,e,x,t,_,_,_,_,_,_,_,_,_,_,_,_,_"}
+```{.textual path="docs/examples/events/dictionary.py" columns="100" lines="30" press="_,t,e,x,t,_,_,_,_,_,_,_,_,_,_,_,_,_"}
 ```
 
 

docs/roadmap.md

@@ -0,0 +1,64 @@
+# Roadmap
+
+We ([textualize.io](https://www.textualize.io/)) are actively building and maintaining Textual. 
+
+We have many new features in the pipeline. This page will keep track of that work.
+
+## Features
+
+High-level features we plan on implementing.
+
+- [ ] Command interface
+    * [ ] Command menu
+    * [ ] Fuzzy search
+- [ ] Configuration (.toml based extensible configuration format)
+- [x] Devtools
+    * [ ] Browser-inspired devtools interface with integrated DOM view, log, and REPL
+- [ ] Reactive state
+- [x] Themes 
+    * [ ] Customize via config 
+    * [ ] Builtin theme editor
+
+## Widgets
+
+Widgets are key to making user friendly interfaces. The builtin widgets should cover many common (and some uncommon) use-cases. The following is a list of the widgets we have built or are planning to build.
+
+- [x] Buttons
+    * [x] Error / warning variants
+- [ ] Color picker
+- [x] Checkbox
+- [ ] Content switcher
+- [x] DataTable
+    * [x] Cell select
+    * [ ] Row / Column select
+    * [ ] API to update cells / rows 
+    * [ ] Lazy loading API
+- [ ] Date picker
+- [ ] Drop-down menus
+- [ ] Form Widget
+    * [ ] Serialization / Deserialization
+    * [ ] Export to `attrs` objects
+    * [ ] Export to `PyDantic` objects
+- [ ] Image support
+  - [ ] Half block
+  - [ ] Braile
+  - [ ] Sixels, and other image extensions
+- [x] Input
+    * [ ] Validation
+    * [ ] Error / warning states
+    * [ ] Template types: IP address, physical units (weight, volume), currency, credit card etc
+- [ ] Markdown viewer (more dynamic than Rich markdown, with scrollable code areas / collapseable sections)
+- [ ] Plots
+  - [ ] bar chart
+  - [ ] line chart
+  - [ ] Candlestick chars
+- [ ] Progress bars
+    * [ ] Style variants (solid, thin etc)
+- [ ] Radio boxes
+- [ ] Sparklines
+- [ ] Tabs
+- [ ] TextArea (multi-line input)
+    * [ ] Basic controls
+    * [ ] Syntax highlighting
+    * [ ] Indentation guides
+    * [ ] Smart features for various languages

examples/code_browser.py

@@ -30,7 +30,7 @@ class CodeBrowser(App):
 
     def watch_show_tree(self, show_tree: bool) -> None:
         """Called when show_tree is modified."""
-        self.set_class(show_tree, "-show-tree")
+        self.set_class(show_tree, "-show-tree")        
 
     def compose(self) -> ComposeResult:
         """Compose our UI."""

examples/dictionary.css

@@ -15,7 +15,7 @@ Input {
 
 #results-container {
     background: $background 50%;
-    margin: 0;
+    margin: 0 0 1 0;
     height: 100%;
     overflow: hidden auto;
     border: tall $background;

examples/dictionary.py

@@ -23,6 +23,11 @@ class DictionaryApp(App):
         yield Input(placeholder="Search for a word")
         yield Content(Static(id="results"), id="results-container")
 
+    def on_mount(self) -> None:
+        """Called when app starts."""
+        # Give the input focus, so we can start typing straight away
+        self.query_one(Input).focus()
+
     async def on_input_changed(self, message: Input.Changed) -> None:
         """A coroutine to handle a text changed message."""
         if message.value:

mkdocs.yml

@@ -6,7 +6,7 @@ edit_uri: edit/css/docs/
 nav:
   - Introduction:
     - "index.md"
-    - "getting_started.md"
+    - "getting_started.md"  
   - "tutorial.md"
   - Guide:
     - "guide/index.md"
@@ -23,6 +23,7 @@ nav:
     - "guide/widgets.md"
     - "guide/animation.md"
     - "guide/screens.md"
+  - "roadmap.md"
   - How to:
     - "how-to/index.md"
     - "how-to/animation.md"
@@ -116,6 +117,8 @@ nav:
     - "reference/static.md"
     - "reference/timer.md"
     - "reference/widget.md"
+  - "help.md"
+
 
 
 markdown_extensions:

sandbox/will/mount.py

@@ -0,0 +1,35 @@
+from textual.app import App, ComposeResult
+
+from textual.containers import Container
+from textual.widget import Widget
+from textual.widgets import Static
+
+
+class MountWidget(Widget):
+    def on_mount(self) -> None:
+        print("Widget mounted")
+
+
+class MountContainer(Container):
+    def compose(self) -> ComposeResult:
+        yield Container(MountWidget(id="bar"))
+
+    def on_mount(self) -> None:
+        bar = self.query_one("#bar")
+        print("MountContainer got", bar)
+
+
+class MountApp(App):
+    def compose(self) -> ComposeResult:
+        yield MountContainer(id="foo")
+
+    def on_mount(self) -> None:
+        foo = self.query_one("#foo")
+        print("foo is", foo)
+        static = self.query_one("#bar")
+        print("App got", static)
+
+
+if __name__ == "__main__":
+    app = MountApp()
+    app.run()

sandbox/will/screens.py

@@ -7,6 +7,9 @@ class ModalScreen(Screen):
         yield Pretty(self.app.screen_stack)
         yield Footer()
 
+    def on_mount(self) -> None:
+        pretty = self.query_one("Pretty")
+
     def on_screen_resume(self):
         self.query_one(Pretty).update(self.app.screen_stack)
 

src/textual/app.py

@@ -37,7 +37,7 @@ from .css.stylesheet import Stylesheet
 from .design import ColorSystem
 from .devtools.client import DevtoolsClient, DevtoolsConnectionError, DevtoolsLog
 from .devtools.redirect_output import StdoutRedirector
-from .dom import DOMNode, NoScreen
+from .dom import DOMNode
 from .driver import Driver
 from .drivers.headless_driver import HeadlessDriver
 from .features import FeatureFlag, parse_features
@@ -47,7 +47,7 @@ from .messages import CallbackType
 from .reactive import Reactive
 from .renderables.blank import Blank
 from .screen import Screen
-from .widget import Widget
+from .widget import AwaitMount, Widget
 
 PLATFORM = platform.system()
 WINDOWS = PLATFORM == "Windows"
@@ -144,6 +144,10 @@ class App(Generic[ReturnType], DOMNode):
     _BASE_PATH: str | None = None
     CSS_PATH: CSSPathType = None
 
+    title: Reactive[str] = Reactive("Textual")
+    sub_title: Reactive[str] = Reactive("")
+    dark: Reactive[bool] = Reactive(True)
+
     def __init__(
         self,
         driver_class: Type[Driver] | None = None,
@@ -228,10 +232,6 @@ class App(Generic[ReturnType], DOMNode):
         )
         self._screenshot: str | None = None
 
-    title: Reactive[str] = Reactive("Textual")
-    sub_title: Reactive[str] = Reactive("")
-    dark: Reactive[bool] = Reactive(True)
-
     def animate(
         self,
         attribute: str,
@@ -696,21 +696,24 @@ class App(Generic[ReturnType], DOMNode):
         self._require_stylesheet_update.add(self.screen if node is None else node)
         self.check_idle()
 
-    def mount(self, *anon_widgets: Widget, **widgets: Widget) -> None:
+    def mount(self, *anon_widgets: Widget, **widgets: Widget) -> AwaitMount:
         """Mount widgets. Widgets specified as positional args, or keywords args. If supplied
         as keyword args they will be assigned an id of the key.
 
         """
-        self._register(self.screen, *anon_widgets, **widgets)
+        mounted_widgets = self._register(self.screen, *anon_widgets, **widgets)
+        return AwaitMount(mounted_widgets)
 
-    def mount_all(self, widgets: Iterable[Widget]) -> None:
+    def mount_all(self, widgets: Iterable[Widget]) -> AwaitMount:
         """Mount widgets from an iterable.
 
         Args:
             widgets (Iterable[Widget]): An iterable of widgets.
         """
-        for widget in widgets:
+        mounted_widgets = list(widgets)
+        for widget in mounted_widgets:
             self._register(self.screen, widget)
+        return AwaitMount(mounted_widgets)
 
     def is_screen_installed(self, screen: Screen | str) -> bool:
         """Check if a given screen has been installed.
@@ -1008,23 +1011,36 @@ class App(Generic[ReturnType], DOMNode):
             self.set_interval(0.25, self.css_monitor, name="css monitor")
             self.log.system("[b green]STARTED[/]", self.css_monitor)
 
-        process_messages = super()._process_messages
-
         async def run_process_messages():
-            compose_event = events.Compose(sender=self)
-            await self._dispatch_message(compose_event)
-            mount_event = events.Mount(sender=self)
-            await self._dispatch_message(mount_event)
 
-            Reactive.initialize_object(self)
+            try:
+                await self._dispatch_message(events.Compose(sender=self))
+                await self._dispatch_message(events.Mount(sender=self))
+            finally:
+                self._mounted_event.set()
+
+            Reactive._initialize_object(self)
+
             self.title = self._title
             self.stylesheet.update(self)
             self.refresh()
+
             await self.animator.start()
             await self._ready()
             if ready_callback is not None:
                 await ready_callback()
-            await process_messages()
+
+            self._running = True
+
+            try:
+                await self._process_messages_loop()
+            except asyncio.CancelledError:
+                pass
+            finally:
+                self._running = False
+                for timer in list(self._timers):
+                    await timer.stop()
+
             await self.animator.stop()
             await self._close_all()
 
@@ -1059,6 +1075,9 @@ class App(Generic[ReturnType], DOMNode):
             if self.devtools.is_connected:
                 await self._disconnect_devtools()
 
+    async def _pre_process(self) -> None:
+        pass
+
     async def _ready(self) -> None:
         """Called immediately prior to processing messages.
 
@@ -1083,9 +1102,9 @@ class App(Generic[ReturnType], DOMNode):
 
         self.set_timer(screenshot_timer, on_screenshot, name="screenshot timer")
 
-    def _on_compose(self) -> None:
-        widgets = self.compose()
-        self.mount_all(widgets)
+    async def _on_compose(self) -> None:
+        widgets = list(self.compose())
+        await self.mount_all(widgets)
 
     def _on_idle(self) -> None:
         """Perform actions when there are no messages in the queue."""
@@ -1110,15 +1129,15 @@ class App(Generic[ReturnType], DOMNode):
 
     def _register(
         self, parent: DOMNode, *anon_widgets: Widget, **widgets: Widget
-    ) -> None:
-        """Mount widget(s) so they may receive events.
+    ) -> list[Widget]:
+        """Register widget(s) so they may receive events.
 
         Args:
             parent (Widget): Parent Widget
         """
         if not anon_widgets and not widgets:
-            return
-        name_widgets: Iterable[tuple[str | None, Widget]]
+            return []
+        name_widgets: list[tuple[str | None, Widget]]
         name_widgets = [*((None, widget) for widget in anon_widgets), *widgets.items()]
         apply_stylesheet = self.stylesheet.apply
 
@@ -1133,8 +1152,8 @@ class App(Generic[ReturnType], DOMNode):
                     self._register(widget, *widget.children)
                 apply_stylesheet(widget)
 
-        for _widget_id, widget in name_widgets:
-            widget.post_message_no_wait(events.Mount(sender=parent))
+        registered_widgets = [widget for _, widget in name_widgets]
+        return registered_widgets
 
     def _unregister(self, widget: Widget) -> None:
         """Unregister a widget.
@@ -1142,11 +1161,7 @@ class App(Generic[ReturnType], DOMNode):
         Args:
             widget (Widget): A Widget to unregister
         """
-        try:
-            widget.screen._reset_focus(widget)
-        except NoScreen:
-            pass
-
+        widget.reset_focus()
         if isinstance(widget._parent, Widget):
             widget._parent.children._remove(widget)
             widget._detach()
@@ -1164,9 +1179,16 @@ class App(Generic[ReturnType], DOMNode):
         """
         widget._attach(parent)
         widget._start_messages()
-        widget.post_message_no_wait(events.Mount(sender=parent))
 
     def is_mounted(self, widget: Widget) -> bool:
+        """Check if a widget is mounted.
+
+        Args:
+            widget (Widget): A widget.
+
+        Returns:
+            bool: True of the widget is mounted.
+        """
         return widget in self._registry
 
     async def _close_all(self) -> None:
@@ -1388,22 +1410,22 @@ class App(Generic[ReturnType], DOMNode):
 
     async def _on_resize(self, event: events.Resize) -> None:
         event.stop()
-        self.screen._screen_resized(event.size)
         await self.screen.post_message(event)
 
     async def _on_remove(self, event: events.Remove) -> None:
         widget = event.widget
         parent = widget.parent
-        if parent is not None:
-            parent.refresh(layout=True)
+
+        widget.reset_focus()
 
         remove_widgets = widget.walk_children(
             Widget, with_self=True, method="depth", reverse=True
         )
-        for child in remove_widgets:
-            self._unregister(child)
         for child in remove_widgets:
             await child._close_messages()
+            self._unregister(child)
+        if parent is not None:
+            parent.refresh(layout=True)
 
     async def action_press(self, key: str) -> None:
         await self.press(key)

src/textual/cli/previews/colors.py

@@ -68,9 +68,16 @@ class ColorsApp(App):
     BINDINGS = [("d", "toggle_dark", "Toggle dark mode")]
 
     def compose(self) -> ComposeResult:
-        yield Content(ColorButtons(), ColorsView())
+        yield Content(ColorButtons())
         yield Footer()
 
+    def on_mount(self) -> None:
+        self.call_later(self.update_view)
+
+    def update_view(self) -> None:
+        content = self.query_one("Content", Content)
+        content.mount(ColorsView())
+
     def on_button_pressed(self, event: Button.Pressed) -> None:
         self.query(ColorGroup).remove_class("-active")
         group = self.query_one(f"#group-{event.button.id}", ColorGroup)

src/textual/css/stylesheet.py

@@ -249,7 +249,7 @@ class Stylesheet:
                 css = css_file.read()
             path = os.path.abspath(filename)
         except Exception as error:
-            raise StylesheetError(f"unable to read {filename!r}; {error}")
+            raise StylesheetError(f"unable to read CSS file {filename!r}") from None
         self.source[str(path)] = CssSource(css, False, 0)
         self._require_parse = True
 

src/textual/dom.py

@@ -662,7 +662,7 @@ class DOMNode(MessagePump):
                 Defaults to None.
             with_self (bool, optional): Also yield self in addition to descendants. Defaults to True.
             method (Literal["breadth", "depth"], optional): One of "depth" or "breadth". Defaults to "depth".
-            reverse (bool, optional): Reverse the order (bottom up). Defaults to False
+            reverse (bool, optional): Reverse the order (bottom up). Defaults to False.
 
         Returns:
             Iterable[DOMNode | WalkType]: An iterable of nodes.

src/textual/message_pump.py

@@ -73,6 +73,7 @@ class MessagePump(metaclass=MessagePumpMeta):
         self._timers: WeakSet[Timer] = WeakSet()
         self._last_idle: float = time()
         self._max_idle: float | None = None
+        self._mounted_event = asyncio.Event()
 
     @property
     def task(self) -> Task:
@@ -278,18 +279,19 @@ class MessagePump(metaclass=MessagePumpMeta):
             await timer.stop()
         self._timers.clear()
         await self._message_queue.put(None)
-
         if self._task is not None and asyncio.current_task() != self._task:
             # Ensure everything is closed before returning
             await self._task
 
     def _start_messages(self) -> None:
         """Start messages task."""
-        Reactive.initialize_object(self)
         self._task = asyncio.create_task(self._process_messages())
 
     async def _process_messages(self) -> None:
         self._running = True
+
+        await self._pre_process()
+
         try:
             await self._process_messages_loop()
         except CancelledError:
@@ -299,6 +301,18 @@ class MessagePump(metaclass=MessagePumpMeta):
             for timer in list(self._timers):
                 await timer.stop()
 
+    async def _pre_process(self) -> None:
+        """Procedure to run before processing messages."""
+        # Dispatch compose and mount messages without going through loop
+        # These events must occur in this order, and at the start.
+        try:
+            await self._dispatch_message(events.Compose(sender=self))
+            await self._dispatch_message(events.Mount(sender=self))
+        finally:
+            # This is critical, mount may be waiting
+            self._mounted_event.set()
+        Reactive._initialize_object(self)
+
     async def _process_messages_loop(self) -> None:
         """Process messages until the queue is closed."""
         _rich_traceback_guard = True
@@ -331,11 +345,15 @@ class MessagePump(metaclass=MessagePumpMeta):
             except CancelledError:
                 raise
             except Exception as error:
+                self._mounted_event.set()
                 self.app._handle_exception(error)
                 break
             finally:
+
                 self._message_queue.task_done()
                 current_time = time()
+
+                # Insert idle events
                 if self._message_queue.empty() or (
                     self._max_idle is not None
                     and current_time - self._last_idle > self._max_idle

src/textual/reactive.py

@@ -20,6 +20,12 @@ if TYPE_CHECKING:
 ReactiveType = TypeVar("ReactiveType")
 
 
+class _NotSet:
+    pass
+
+
+_NOT_SET = _NotSet()
+
 T = TypeVar("T")
 
 
@@ -83,24 +89,31 @@ class Reactive(Generic[ReactiveType]):
         return cls(default, layout=False, repaint=False, init=True)
 
     @classmethod
-    def initialize_object(cls, obj: object) -> None:
-        """Call any watchers / computes for the first time.
+    def _initialize_object(cls, obj: object) -> None:
+        """Set defaults and call any watchers / computes for the first time.
 
         Args:
             obj (Reactable): An object with Reactive descriptors
         """
-
-        startswith = str.startswith
-        for key in obj.__class__.__dict__.keys():
-            if startswith(key, "_init_"):
-                name = key[6:]
-                if not hasattr(obj, name):
-                    default = getattr(obj, key)
-                    setattr(obj, name, default() if callable(default) else default)
+        if not hasattr(obj, "__reactive_initialized"):
+            startswith = str.startswith
+            for key in obj.__class__.__dict__:
+                if startswith(key, "_default_"):
+                    name = key[9:]
+                    # Check defaults
+                    if not hasattr(obj, name):
+                        # Attribute has no value yet
+                        default = getattr(obj, key)
+                        default_value = default() if callable(default) else default
+                        # Set the default vale (calls `__set__`)
+                        setattr(obj, name, default_value)
+        setattr(obj, "__reactive_initialized", True)
 
     def __set_name__(self, owner: Type[MessageTarget], name: str) -> None:
 
+        # Check for compute method
         if hasattr(owner, f"compute_{name}"):
+            # Compute methods are stored in a list called `__computes`
             try:
                 computes = getattr(owner, "__computes")
             except AttributeError:
@@ -108,31 +121,46 @@ class Reactive(Generic[ReactiveType]):
                 setattr(owner, "__computes", computes)
             computes.append(name)
 
+        # The name of the attribute
         self.name = name
+        # The internal name where the attribute's value is stored
         self.internal_name = f"_reactive_{name}"
         default = self._default
-
-        if self._init:
-            setattr(owner, f"_init_{name}", default)
-        else:
-            setattr(
-                owner, self.internal_name, default() if callable(default) else default
-            )
+        setattr(owner, f"_default_{name}", default)
 
     def __get__(self, obj: Reactable, obj_type: type[object]) -> ReactiveType:
-        return getattr(obj, self.internal_name)
+        value: _NotSet | ReactiveType = getattr(obj, self.internal_name, _NOT_SET)
+        if isinstance(value, _NotSet):
+            # No value present, we need to set the default
+            init_name = f"_default_{self.name}"
+            default = getattr(obj, init_name)
+            default_value = default() if callable(default) else default
+            # Set and return the value
+            setattr(obj, self.internal_name, default_value)
+            if self._init:
+                self._check_watchers(obj, self.name, default_value, first_set=True)
+            return default_value
+        return value
 
     def __set__(self, obj: Reactable, value: ReactiveType) -> None:
         name = self.name
-        current_value = getattr(obj, self.internal_name, None)
+        current_value = getattr(obj, name)
+        # Check for validate function
         validate_function = getattr(obj, f"validate_{name}", None)
+        # Check if this is the first time setting the value
         first_set = getattr(obj, f"__first_set_{self.internal_name}", True)
+        # Call validate, but not on first set.
         if callable(validate_function) and not first_set:
             value = validate_function(value)
+        # If the value has changed, or this is the first time setting the value
         if current_value != value or first_set:
+            # Set the first set flag to False
             setattr(obj, f"__first_set_{self.internal_name}", False)
+            # Store the internal value
             setattr(obj, self.internal_name, value)
+            # Check all watchers
             self._check_watchers(obj, name, current_value, first_set=first_set)
+            # Refresh according to descriptor flags
             if self._layout or self._repaint:
                 obj.refresh(repaint=self._repaint, layout=self._layout)
 
@@ -140,50 +168,77 @@ class Reactive(Generic[ReactiveType]):
     def _check_watchers(
         cls, obj: Reactable, name: str, old_value: Any, first_set: bool = False
     ) -> None:
+        """Check watchers, and call watch methods / computes
 
+        Args:
+            obj (Reactable): The reactable object.
+            name (str): Attribute name.
+            old_value (Any): The old (previous) value of the attribute.
+            first_set (bool, optional): True if this is the first time setting the value. Defaults to False.
+        """
+        # Get the current value.
         internal_name = f"_reactive_{name}"
         value = getattr(obj, internal_name)
 
         async def update_watcher(
             obj: Reactable, watch_function: Callable, old_value: Any, value: Any
         ) -> None:
+            """Call watch function, and run compute.
+
+            Args:
+                obj (Reactable): Reactable object.
+                watch_function (Callable): Watch method.
+                old_value (Any): Old value.
+                value (Any): new value.
+            """
             _rich_traceback_guard = True
+            # Call watch with one or two parameters
             if count_parameters(watch_function) == 2:
                 watch_result = watch_function(old_value, value)
             else:
                 watch_result = watch_function(value)
+            # Optionally await result
             if isawaitable(watch_result):
                 await watch_result
+            # Run computes
             await Reactive._compute(obj)
 
+        # Check for watch method
         watch_function = getattr(obj, f"watch_{name}", None)
         if callable(watch_function):
+            # Post a callback message, so we can call the watch method in an orderly async manner
             obj.post_message_no_wait(
                 events.Callback(
-                    obj,
+                    sender=obj,
                     callback=partial(
                         update_watcher, obj, watch_function, old_value, value
                     ),
                 )
             )
 
+        # Check for watchers set via `watch`
         watcher_name = f"__{name}_watchers"
         watchers = getattr(obj, watcher_name, ())
         for watcher in watchers:
             obj.post_message_no_wait(
                 events.Callback(
-                    obj,
+                    sender=obj,
                     callback=partial(update_watcher, obj, watcher, old_value, value),
                 )
             )
 
-        if not first_set:
-            obj.post_message_no_wait(
-                events.Callback(obj, callback=partial(Reactive._compute, obj))
-            )
+        # Run computes
+        obj.post_message_no_wait(
+            events.Callback(sender=obj, callback=partial(Reactive._compute, obj))
+        )
 
     @classmethod
     async def _compute(cls, obj: Reactable) -> None:
+        """Invoke all computes.
+
+        Args:
+            obj (Reactable): Reactable object.
+        """
         _rich_traceback_guard = True
         computes = getattr(obj, "__computes", [])
         for compute in computes:

src/textual/screen.py

@@ -387,12 +387,12 @@ class Screen(Widget):
 
     def _on_screen_resume(self) -> None:
         """Called by the App"""
-
         size = self.app.size
         self._refresh_layout(size, full=True)
 
     async def _on_resize(self, event: events.Resize) -> None:
         event.stop()
+        self._screen_resized(event.size)
 
     async def _handle_mouse_move(self, event: events.MouseMove) -> None:
         try:

src/textual/widget.py

@@ -1,10 +1,20 @@
 from __future__ import annotations
 
-from asyncio import Lock
+from asyncio import Lock, wait, create_task
 from fractions import Fraction
 from itertools import islice
 from operator import attrgetter
-from typing import TYPE_CHECKING, ClassVar, Collection, Iterable, NamedTuple, cast
+from typing import (
+    Awaitable,
+    Generator,
+    TYPE_CHECKING,
+    ClassVar,
+    Collection,
+    Iterable,
+    NamedTuple,
+    Sequence,
+    cast,
+)
 
 import rich.repr
 from rich.console import (
@@ -59,6 +69,28 @@ _JUSTIFY_MAP: dict[str, JustifyMethod] = {
 }
 
 
+class AwaitMount:
+    """An awaitable returned by mount() and mount_all().
+
+    Example:
+        await self.mount(Static("foo"))
+
+    """
+
+    def __init__(self, widgets: Sequence[Widget]) -> None:
+        self._widgets = widgets
+
+    def __await__(self) -> Generator[None, None, None]:
+        async def await_mount() -> None:
+            aws = [
+                create_task(widget._mounted_event.wait()) for widget in self._widgets
+            ]
+            if aws:
+                await wait(aws)
+
+        return await_mount().__await__()
+
+
 class _Styled:
     """Apply a style to a renderable.
 
@@ -316,7 +348,7 @@ class Widget(DOMNode):
         """Clear arrangement cache, forcing a new arrange operation."""
         self._arrangement = None
 
-    def mount(self, *anon_widgets: Widget, **widgets: Widget) -> None:
+    def mount(self, *anon_widgets: Widget, **widgets: Widget) -> AwaitMount:
         """Mount child widgets (making this widget a container).
 
         Widgets may be passed as positional arguments or keyword arguments. If keyword arguments,
@@ -327,9 +359,12 @@ class Widget(DOMNode):
             self.mount(Static("hello"), header=Header())
             ```
 
+        Returns:
+            AwaitMount: An awaitable that waits for widgets to be mounted.
+
         """
-        self.app._register(self, *anon_widgets, **widgets)
-        self.app.screen.refresh(layout=True)
+        mounted_widgets = self.app._register(self, *anon_widgets, **widgets)
+        return AwaitMount(mounted_widgets)
 
     def compose(self) -> ComposeResult:
         """Called by Textual to create child widgets.
@@ -1812,7 +1847,19 @@ class Widget(DOMNode):
             scroll_visible (bool, optional): Scroll parent to make this widget
                 visible. Defaults to True.
         """
-        self.screen.set_focus(self, scroll_visible=scroll_visible)
+
+        def set_focus(widget: Widget):
+            """Callback to set the focus."""
+            widget.screen.set_focus(self, scroll_visible=scroll_visible)
+
+        self.app.call_later(set_focus, self)
+
+    def reset_focus(self) -> None:
+        """Reset the focus (move it to the next available widget)."""
+        try:
+            self.screen._reset_focus(self)
+        except NoScreen:
+            pass
 
     def capture_mouse(self, capture: bool = True) -> None:
         """Capture (or release) the mouse.
@@ -1857,10 +1904,11 @@ class Widget(DOMNode):
         await self.action(binding.action)
         return True
 
+    async def _on_compose(self, event: events.Compose) -> None:
+        widgets = list(self.compose())
+        await self.mount(*widgets)
+
     def _on_mount(self, event: events.Mount) -> None:
-        widgets = self.compose()
-        self.mount(*widgets)
-        # Preset scrollbars if not automatic
         if self.styles.overflow_y == "scroll":
             self.show_vertical_scrollbar = True
         if self.styles.overflow_x == "scroll":
@@ -1932,7 +1980,7 @@ class Widget(DOMNode):
 
     def _on_hide(self, event: events.Hide) -> None:
         if self.has_focus:
-            self.screen._reset_focus(self)
+            self.reset_focus()
 
     def _on_scroll_to_region(self, message: messages.ScrollToRegion) -> None:
         self.scroll_to_region(message.region, animate=True)