Application suspension

CHANGELOG.md

@@ -12,7 +12,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Breaking change: Significant changes to `TextArea.__init__` default values/behaviour https://github.com/Textualize/textual/pull/3933
   - `soft_wrap=True` - soft wrapping is now enabled by default.
   - `show_line_numbers=False` - line numbers are now disabled by default.
-  - `tab_behaviour="focus"` - pressing the tab key now switches focus instead of indenting by default. 
+  - `tab_behaviour="focus"` - pressing the tab key now switches focus instead of indenting by default.
 - Breaking change: `DOMNode.has_pseudo_class` now accepts a single name only https://github.com/Textualize/textual/pull/3970
 - Made `textual.cache` (formerly `textual._cache`) public https://github.com/Textualize/textual/pull/3976
 - `Tab.label` can now be used to change the label of a tab https://github.com/Textualize/textual/pull/3979
@@ -30,6 +30,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `Query.blur` and `Query.focus` https://github.com/Textualize/textual/pull/4012
 - Added `MessagePump.message_queue_size` https://github.com/Textualize/textual/pull/4012
 - Added `TabbedContent.active_pane` https://github.com/Textualize/textual/pull/4012
+- Added `App.suspend` https://github.com/Textualize/textual/pull/4064
+- Added `App.action_suspend_process` https://github.com/Textualize/textual/pull/4064
 
 ### Fixed
 

docs/api/signal.md

@@ -0,0 +1 @@
+::: textual.signal

docs/guide/app.md

@@ -240,6 +240,66 @@ if __name__ == "__main__"
     sys.exit(app.return_code or 0)
 ```
 
+## Suspending
+
+A Textual app can be suspended; this means that app input and output will be paused and the terminal display will be returned to its previous state.
+When the app is resumed the display will be restored and all input and output will resume.
+
+!!! info "Compatible Environments"
+
+    App suspension is only available in the following environments:
+
+    - GNU/Lnux
+    - macOS
+    - Windows
+
+    It is currently not available when an application is being served via Textual Web.
+
+### Suspending in code
+
+To suspend your application use the [App.suspend](/api/app/#textual.app.App.suspend) context manager.
+For example, here is an application that has a button that will open an external editor, suspending the application before running it:
+
+```python hl_lines="14-15"
+from os import system
+
+from textual import on
+from textual.app import App, ComposeResult
+from textual.widgets import Button
+
+class SuspendingApp(App[None]):
+
+    def compose(self) -> ComposeResult:
+        yield Button("Open the editor", id="edit")
+
+    @on(Button.Pressed, "#edit")
+    def run_external_editor(self) -> None:
+        with self.suspend():    # (1)!
+            system("vim")
+
+if __name__ == "__main__":
+    SuspendingApp().run()
+```
+
+1. All code in the body of the `with` statement will be run while the app is suspended.
+
+### Suspending from foreground
+
+On Unix and Unix-like systems (GNU/Linux, macOS, etc) Textual has support for the user pressing a key combination to suspend the application as the foreground process.
+Ordinarily this key combination is <kbd>Ctrl</kbd>+<kbd>Z</kbd>;
+in a Textual application this is disabled by default, but an action is provided ([`action_suspend_process`](/api/app/#textual.app.App.action_suspend_process)) that you can bind in the usual way.
+For example:
+
+```python
+BINDINGS = [
+    Binding("ctrl+z", "suspend_process")
+]
+```
+
+!!! note
+
+    If `suspend_process` is called on Windows, or when your application is being hosted under Textual Web, the call will be ignored.
+
 ## CSS
 
 Textual apps can reference [CSS](CSS.md) files which define how your app and widgets will look, while keeping your Python code free of display related code (which tends to be messy).

src/textual/app.py

@@ -13,6 +13,7 @@
 import io
 import os
 import platform
+import signal
 import sys
 import threading
 import warnings
@@ -38,6 +39,7 @@
     Generator,
     Generic,
     Iterable,
+    Iterator,
     List,
     Sequence,
     Type,
@@ -56,7 +58,17 @@
 from rich.protocol import is_renderable
 from rich.segment import Segment, Segments
 
-from . import Logger, LogGroup, LogVerbosity, actions, constants, events, log, messages
+from . import (
+    Logger,
+    LogGroup,
+    LogVerbosity,
+    actions,
+    constants,
+    events,
+    log,
+    messages,
+    on,
+)
 from ._animator import DEFAULT_EASING, Animatable, Animator, EasingFunction
 from ._ansi_sequences import SYNC_END, SYNC_START
 from ._callback import invoke
@@ -98,6 +110,7 @@
     ScreenResultType,
     _SystemModalScreen,
 )
+from .signal import Signal
 from .widget import AwaitMount, Widget
 from .widgets._toast import ToastRack
 from .worker import NoActiveWorker, get_current_worker
@@ -201,6 +214,14 @@ class ActiveModeError(ModeError):
     """Raised when attempting to remove the currently active mode."""
 
 
+class SuspendNotSupported(Exception):
+    """Raised if suspending the application is not supported.
+
+    This exception is raised if [`App.suspend`][textual.app.App.suspend] is called while
+    the application is running in an environment where this isn't supported.
+    """
+
+
 ReturnType = TypeVar("ReturnType")
 
 CSSPathType = Union[
@@ -371,7 +392,7 @@ class MyApp(App[None]):
     """Indicates if the app has focus.
 
     When run in the terminal, the app always has focus. When run in the web, the app will
-    get focus when the terminal widget has focus.        
+    get focus when the terminal widget has focus.
     """
 
     def __init__(
@@ -573,6 +594,24 @@ def __init__(
         self._original_stderr = sys.__stderr__
         """The original stderr stream (before redirection etc)."""
 
+        self.app_suspend_signal = Signal(self, "app-suspend")
+        """The signal that is published when the app is suspended.
+
+        When [`App.suspend`][textual.app.App.suspend] is called this signal
+        will be [published][textual.signal.Signal.publish];
+        [subscribe][textual.signal.Signal.subscribe] to this signal to
+        perform work before the suspension takes place.
+        """
+        self.app_resume_signal = Signal(self, "app-resume")
+        """The signal that is published when the app is resumed after a suspend.
+
+        When the app is resumed after a
+        [`App.suspend`][textual.app.App.suspend] call this signal will be
+        [published][textual.signal.Signal.publish];
+        [subscribe][textual.signal.Signal.subscribe] to this signal to
+        perform work after the app has resumed.
+        """
+
         self.set_class(self.dark, "-dark-mode")
         self.set_class(not self.dark, "-light-mode")
 
@@ -3287,3 +3326,82 @@ def action_command_palette(self) -> None:
         """Show the Textual command palette."""
         if self.use_command_palette and not CommandPalette.is_open(self):
             self.push_screen(CommandPalette(), callback=self.call_next)
+
+    def _suspend_signal(self) -> None:
+        """Signal that the application is being suspended."""
+        self.app_suspend_signal.publish()
+
+    @on(Driver.SignalResume)
+    def _resume_signal(self) -> None:
+        """Signal that the application is being resumed from a suspension."""
+        self.app_resume_signal.publish()
+
+    @contextmanager
+    def suspend(self) -> Iterator[None]:
+        """A context manager that temporarily suspends the app.
+
+        While inside the `with` block, the app will stop reading input and
+        emitting output. Other applications will have full control of the
+        terminal, configured as it was before the app started running. When
+        the `with` block ends, the application will start reading input and
+        emitting output again.
+
+        Example:
+            ```python
+            with self.suspend():
+                os.system("emacs -nw")
+            ```
+
+        Raises:
+            SuspendNotSupported: If the environment doesn't support suspending.
+
+        !!! note
+            Suspending the application is currently only supported on
+            Unix-like operating systems and Microsoft Windows. Suspending is
+            not supported in Textual Web.
+        """
+        if self._driver is None:
+            return
+        if self._driver.can_suspend:
+            # Publish a suspend signal *before* we suspend application mode.
+            self._suspend_signal()
+            self._driver.suspend_application_mode()
+            # We're going to handle the start of the driver again so mark
+            # this next part as such; the reason for this is that the code
+            # the developer may be running could be in this process, and on
+            # Unix-like systems the user may `action_suspend_process` the
+            # app, and we don't want to have the driver auto-restart
+            # application mode when the application comes back to the
+            # foreground, in this context.
+            with self._driver.no_automatic_restart(), redirect_stdout(
+                sys.__stdout__
+            ), redirect_stderr(sys.__stderr__):
+                yield
+            # We're done with the dev's code so resume application mode.
+            self._driver.resume_application_mode()
+            # ...and publish a resume signal.
+            self._resume_signal()
+        else:
+            raise SuspendNotSupported(
+                "App.suspend is not supported in this environment."
+            )
+
+    def action_suspend_process(self) -> None:
+        """Suspend the process into the background.
+
+        Note:
+            On Unix and Unix-like systems a `SIGTSTP` is sent to the
+            application's process. Currently on Windows and when running
+            under Textual Web this is a non-operation.
+        """
+        # Check if we're in an environment that permits this kind of
+        # suspend.
+        if not WINDOWS and self._driver is not None and self._driver.can_suspend:
+            # First, ensure that the suspend signal gets published while
+            # we're still in application mode.
+            self._suspend_signal()
+            # With that out of the way, send the SIGTSTP signal.
+            os.kill(os.getpid(), signal.SIGTSTP)
+            # NOTE: There is no call to publish the resume signal here, this
+            # will be handled by the driver posting a SignalResume event
+            # (see the event handler on App._resume_signal) above.

src/textual/driver.py

@@ -2,7 +2,8 @@
 
 import asyncio
 from abc import ABC, abstractmethod
-from typing import TYPE_CHECKING
+from contextlib import contextmanager
+from typing import TYPE_CHECKING, Iterator
 
 from . import events
 from .events import MouseUp
@@ -34,12 +35,19 @@ def __init__(
         self._loop = asyncio.get_running_loop()
         self._down_buttons: list[int] = []
         self._last_move_event: events.MouseMove | None = None
+        self._auto_restart = True
+        """Should the application auto-restart (where appropriate)?"""
 
     @property
     def is_headless(self) -> bool:
         """Is the driver 'headless' (no output)?"""
         return False
 
+    @property
+    def can_suspend(self) -> bool:
+        """Can this driver be suspended?"""
+        return False
+
     def send_event(self, event: events.Event) -> None:
         """Send an event to the target app.
 
@@ -118,5 +126,39 @@ def disable_input(self) -> None:
     def stop_application_mode(self) -> None:
         """Stop application mode, restore state."""
 
+    def suspend_application_mode(self) -> None:
+        """Suspend application mode.
+
+        Used to suspend application mode and allow uninhibited access to the
+        terminal.
+        """
+        self.stop_application_mode()
+
+    def resume_application_mode(self) -> None:
+        """Resume application mode.
+
+        Used to resume application mode after it has been previously
+        suspended.
+        """
+        self.start_application_mode()
+
+    class SignalResume(events.Event):
+        """Event sent to the app when a resume signal should be published."""
+
+    @contextmanager
+    def no_automatic_restart(self) -> Iterator[None]:
+        """A context manager used to tell the driver to not auto-restart.
+
+        For drivers that support the application being suspended by the
+        operating system, this context manager is used to mark a body of
+        code as one that will manage its own stop and start.
+        """
+        auto_restart = self._auto_restart
+        self._auto_restart = False
+        try:
+            yield
+        finally:
+            self._auto_restart = auto_restart
+
     def close(self) -> None:
         """Perform any final cleanup."""