pythonnative/src/pythonnative/preview.py at main · andevsr/pythonnative · GitHub

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"""Desktop preview runtime, the engine behind ``pn preview``.

``pn preview`` renders a PythonNative app in a real OS window using the

Tkinter backend ([`pythonnative.native_views.desktop`][pythonnative.native_views.desktop]),

with **instant Fast Refresh** on every file save. It exists to make the

inner development loop fast: see your UI and iterate in seconds without

booting a simulator or deploying to a device.

Architecture

------------

- A single Tk window holds one *stage* frame. Every screen on the

navigation stack gets its own child container inside the stage; the

desktop view handlers create widgets under the active container.

- [`DesktopApp`][pythonnative.preview.DesktopApp] owns the navigation

stack of [`screen`][pythonnative.screen] hosts and the push/pop/reset

primitives the declarative navigators call through ``host._push`` /

``host._pop``.

- The Tk event loop runs on the main thread. A lightweight poll

(`~60 Hz`) drains (a) UI work marshaled from the asyncio runtime

thread via [`runtime.call_on_main_thread`][pythonnative.runtime.call_on_main_thread],

(b) re-renders requested off-thread, and (c) file-change reloads.

- A background [`FileWatcher`][pythonnative.hot_reload.FileWatcher]

detects ``.py`` edits and enqueues a reload onto the main thread.

This module imports ``tkinter`` and is only imported by the

``pn preview`` command, which sets ``PN_PLATFORM=desktop`` first.

"""

from __future__ import annotations

import os

import queue

import sys

import tkinter as tk

import traceback

from typing import Any, Callable, List, Optional, Tuple

# iPhone-ish logical-point window so layouts that assume a phone-sized

# viewport look right out of the box; resizable at runtime.

DEFAULT_WIDTH = 390

DEFAULT_HEIGHT = 844

_POLL_INTERVAL_MS = 16

_WATCH_INTERVAL_S = 0.4

def _publish_desktop_color_scheme() -> None:

"""Publish the host OS appearance so `use_color_scheme` works in preview.

``PN_COLOR_SCHEME=light|dark`` forces a value (handy for testing

both appearances); otherwise macOS is asked via ``defaults read``

(the key only exists when dark mode is on). Other platforms default

to light.

"""

from . import appearance

forced = os.environ.get("PN_COLOR_SCHEME")

if forced in ("light", "dark"):

appearance.set_system_color_scheme(forced)

return

if sys.platform == "darwin":

import subprocess

try:

result = subprocess.run(

["defaults", "read", "-g", "AppleInterfaceStyle"],

capture_output=True,

text=True,

timeout=2,

)

is_dark = result.returncode == 0 and result.stdout.strip() == "Dark"

appearance.set_system_color_scheme("dark" if is_dark else "light")

except Exception:

pass

class DesktopApp:

"""Navigation-stack controller for the desktop preview window.

One instance backs a preview session. It is handed to each

[`screen`][pythonnative.screen] host as the ``native_instance`` so

hosts can drive navigation (``push_screen`` / ``pop_screen`` /

``reset_to_root``), report the viewport size, and set the window

title, mirroring the role a ``UIViewController`` / ``Activity``

plays on device.

"""

def __init__(self, root: Any, stage: Any, width: float, height: float) -> None:

self._root = root

self._stage = stage

self._width = float(width)

self._height = float(height)

self._stack: List[Any] = []

self._error_widget: Any = None

self._mount_failed = False

self._component_path = ""

# -- queried by the screen host -----------------------------------

def viewport_size(self) -> Tuple[float, float]:

"""Return the current stage size in points (host viewport)."""

return (self._width, self._height)

def set_title(self, title: str) -> None:

"""Set the preview window title (called from screen options)."""

try:

self._root.title(title)

except Exception:

pass

# -- container management -----------------------------------------

def _new_container(self) -> Any:

frame = tk.Frame(self._stage, highlightthickness=0, bd=0, background="#ffffff")

frame.place(x=0, y=0, relwidth=1.0, relheight=1.0)

frame.lift()

return frame

def _show_container(self, host: Any) -> None:

container = getattr(host, "_pn_container", None)

if container is not None:

try:

container.place(in_=self._stage, x=0, y=0, relwidth=1.0, relheight=1.0)

container.lift()

except Exception:

pass

def _forget_container(self, host: Any) -> None:

container = getattr(host, "_pn_container", None)

if container is not None:

try:

container.place_forget()

except Exception:

pass

def _activate(self, host: Any) -> None:

"""Make ``host`` the rendering target and reflow it to the viewport."""

from .native_views import desktop as desktop_backend

desktop_backend.set_root_container(getattr(host, "_pn_container", None))

self._show_container(host)

# -- lifecycle ----------------------------------------------------

def _make_host(self, component_path: str, args: Optional[dict] = None) -> Any:

from . import screen as screen_module

from .native_views import desktop as desktop_backend

container = self._new_container()

desktop_backend.set_root_container(container)

host = screen_module.create_screen(component_path, self)

host._pn_container = container

if args:

host.set_args(args)

return host

def mount_root(self, component_path: str) -> None:

"""Mount the initial screen as the base of the navigation stack.

Import-time failures (a missing dependency, a syntax error the

developer is mid-fix on) are shown as an error overlay and flagged

so the next successful reload remounts cleanly, rather than

crashing the preview process.

"""

self._component_path = component_path

self._clear_error()

try:

host = self._make_host(component_path)

except Exception:

self._mount_failed = True

self._show_error(traceback.format_exc())

return

self._stack.append(host)

try:

host.on_create()

host.on_resume()

self._mount_failed = False

except Exception:

self._mount_failed = True

self._show_error(traceback.format_exc())

def push_screen(self, component_path: str, args: Optional[dict] = None) -> None:

"""Push a new screen, suspending the current one (declarative nav)."""

if self._stack:

current = self._stack[-1]

try:

current.on_pause()

except Exception:

pass

self._forget_container(current)

try:

host = self._make_host(component_path, args)

except Exception:

self._show_error(traceback.format_exc())

return

self._stack.append(host)

try:

host.on_create()

host.on_resume()

except Exception:

self._show_error(traceback.format_exc())

def pop_screen(self) -> None:

"""Pop the top screen and restore the one beneath it."""

if len(self._stack) <= 1:

return

top = self._stack.pop()

self._teardown(top)

restored = self._stack[-1]

self._activate(restored)

try:

restored.on_resume()

restored.set_viewport_size(self._width, self._height)

except Exception:

pass

def reset_to_root(self) -> None:

"""Pop every screen above the root (declarative ``reset`` / tab root)."""

while len(self._stack) > 1:

self._teardown(self._stack.pop())

if self._stack:

root_host = self._stack[0]

self._activate(root_host)

try:

root_host.on_resume()

root_host.set_viewport_size(self._width, self._height)

except Exception:

pass

def _teardown(self, host: Any) -> None:

try:

host.on_pause()

except Exception:

pass

# ``on_destroy`` unmounts the host's reconciler: effect cleanups

# run, native widgets are destroyed, and event registrations are

# released (see ``screen._destroy_host``).

try:

host.on_destroy()

except Exception:

pass

container = getattr(host, "_pn_container", None)

if container is not None:

try:

container.destroy()

except Exception:

pass

def teardown_all(self) -> None:

"""Destroy every screen on the stack (preview window closing)."""

while self._stack:

self._teardown(self._stack.pop())

def back_pressed(self) -> None:

"""Route a desktop back gesture (Escape) like a hardware back press.

``use_back_handler`` subscribers on the active screen get the

first chance to consume the event; otherwise the stack pops

(matching Android's default back behavior). At the root the

event is ignored.

"""

host = self.active_host()

if host is None:

return

try:

if host.on_back_pressed():

return

except Exception:

pass

self.pop_screen()

# -- viewport / resize --------------------------------------------

def resize(self, width: float, height: float) -> None:

"""Propagate a window resize to the active screen's reconciler."""

if width <= 0 or height <= 0:

return

self._width = float(width)

self._height = float(height)

host = self.active_host()

if host is not None:

try:

host.set_viewport_size(self._width, self._height)

except Exception:

pass

def active_host(self) -> Any:

"""Return the top-of-stack host, or ``None`` if nothing is mounted."""

return self._stack[-1] if self._stack else None

# -- hot reload ---------------------------------------------------

def reload(self, changed_modules: Optional[List[str]] = None) -> None:

"""Apply a hot reload across every mounted screen.

If the initial mount failed (e.g. a syntax error the developer

is now fixing), this re-attempts a fresh mount so the preview

recovers without a restart. Otherwise each host on the stack

performs Fast Refresh against the reloaded modules.

"""

from .native_views import desktop as desktop_backend

if self._mount_failed or not self._stack:

self._remount_root()

return

self._clear_error()

for host in self._stack:

desktop_backend.set_root_container(getattr(host, "_pn_container", None))

try:

host.reload(changed_modules)

except Exception:

self._show_error(traceback.format_exc())

active = self.active_host()

if active is not None:

desktop_backend.set_root_container(getattr(active, "_pn_container", None))

def _remount_root(self) -> None:

for host in list(self._stack):

self._teardown(host)

self._stack.clear()

self.mount_root(self._component_path)

# -- error overlay ------------------------------------------------

def _show_error(self, message: str) -> None:

self._clear_error()

widget = tk.Text(

self._stage,

wrap="word",

background="#1c1c1e",

foreground="#ff6b6b",

insertbackground="#ffffff",

borderwidth=0,

highlightthickness=0,

padx=16,

pady=16,

)

widget.insert("1.0", "PythonNative preview error\n\n" + message)

widget.configure(state="disabled")

widget.place(x=0, y=0, relwidth=1.0, relheight=1.0)

widget.lift()

self._error_widget = widget

print(message, file=sys.stderr)

def _clear_error(self) -> None:

if self._error_widget is not None:

try:

self._error_widget.destroy()

except Exception:

pass

self._error_widget = None

def _resolve_paths(component_path: str, project_root: Optional[str], watch_dir: Optional[str]) -> Tuple[str, str]:

"""Return ``(project_root, watch_dir)`` with sensible defaults.

The project root (which must be importable for ``component_path`` to

resolve) is prepended to ``sys.path``; the watch dir defaults to the

top-level package directory of ``component_path`` under the root.

"""

root = os.path.abspath(project_root or os.getcwd())

if root not in sys.path:

sys.path.insert(0, root)

if watch_dir is None:

top_package = component_path.split(".", 1)[0]

candidate = os.path.join(root, top_package)

watch_dir = candidate if os.path.isdir(candidate) else root

return root, os.path.abspath(watch_dir)

def run_preview(

component_path: str,

project_root: Optional[str] = None,

watch_dir: Optional[str] = None,

width: int = DEFAULT_WIDTH,

height: int = DEFAULT_HEIGHT,

title: str = "PythonNative Preview",

hot_reload: bool = True,

) -> None:

"""Open the preview window for ``component_path`` and run until closed.

Args:

component_path: Module path (``"app.main"`` → its ``App``) or a

dotted ``module.Component`` path, same convention as

[`create_screen`][pythonnative.create_screen].

project_root: Directory added to ``sys.path`` so the component

imports. Defaults to the current working directory.

watch_dir: Directory watched for ``.py`` changes. Defaults to

the component's top-level package (e.g. ``app/``).

width: Initial window width in points.

height: Initial window height in points.

title: Window title.

hot_reload: Watch for file changes and Fast Refresh on save.

Raises:

RuntimeError: If ``PN_PLATFORM=desktop`` was not set before

PythonNative was imported (``pn preview`` sets it for you).

"""

from . import runtime as runtime_module

from .native_views import desktop as desktop_backend

from .utils import IS_DESKTOP

if not IS_DESKTOP:

raise RuntimeError(

"run_preview() requires the desktop backend. Set PN_PLATFORM=desktop "

"before importing pythonnative (the `pn preview` command does this)."

)

root_dir, watched = _resolve_paths(component_path, project_root, watch_dir)

_publish_desktop_color_scheme()

# The preview is inherently a development surface: turn on dev

# diagnostics (validation warnings, hook-order checks, RedBox).

from . import diagnostics

diagnostics.set_dev_mode(True)

root = tk.Tk()

root.title(title)

root.geometry(f"{int(width)}x{int(height)}")

root.minsize(240, 320)

stage = tk.Frame(root, background="#ffffff", highlightthickness=0, bd=0)

stage.pack(fill="both", expand=True)

desktop_backend.set_root_container(stage)

app = DesktopApp(root, stage, width, height)

# Marshal asyncio-thread UI work (animations, alerts) onto the Tk

# main thread by funneling it through this queue, drained in _poll.

main_queue: "queue.Queue[Callable[[], None]]" = queue.Queue()

runtime_module.set_desktop_main_dispatch(main_queue.put)

app.mount_root(component_path)

def _on_configure(event: Any) -> None:

if event.widget is stage:

app.resize(event.width, event.height)

stage.bind("<Configure>", _on_configure)

# Escape acts as the desktop stand-in for the hardware back button:

# ``use_back_handler`` subscribers can intercept it, and otherwise

# the navigation stack pops.

root.bind("<Escape>", lambda _event: app.back_pressed())

watcher = _build_watcher(watched, root_dir, app, main_queue) if hot_reload else None

if watcher is not None:

watcher.start()

print(f"[pn preview] watching {watched} for changes", file=sys.stderr)

from . import screen as screen_module

def _poll() -> None:

for _ in range(128):

try:

job = main_queue.get_nowait()

except queue.Empty:

break

try:

job()

except Exception:

traceback.print_exc()

try:

screen_module.drain_desktop_scheduled_renders()

except Exception:

traceback.print_exc()

try:

root.after(_POLL_INTERVAL_MS, _poll)

except Exception:

pass

def _on_close() -> None:

if watcher is not None:

try:

watcher.stop()

except Exception:

pass

# Unmount every screen first so effect cleanups (timers, tasks,

# subscriptions) run before the Tk interpreter goes away.

try:

app.teardown_all()

except Exception:

pass

runtime_module.set_desktop_main_dispatch(None)

desktop_backend.clear_root_container()

try:

root.destroy()

except Exception:

pass

root.protocol("WM_DELETE_WINDOW", _on_close)

root.after(_POLL_INTERVAL_MS, _poll)

print(f"[pn preview] {component_path} ({int(width)}x{int(height)})", file=sys.stderr)

try:

root.mainloop()

except KeyboardInterrupt:

pass

finally:

if watcher is not None:

try:

watcher.stop()

except Exception:

pass

runtime_module.set_desktop_main_dispatch(None)

desktop_backend.clear_root_container()

def _build_watcher(

watch_dir: str,

base_dir: str,

app: DesktopApp,

main_queue: "queue.Queue[Callable[[], None]]",

) -> Any:

"""Create a file watcher that enqueues reloads onto the main thread.

The watcher runs on its own daemon thread; because Tkinter is not

thread-safe, the ``on_change`` callback only *enqueues* the reload

(translated from changed file paths into dotted module names), which

[`run_preview`][pythonnative.preview.run_preview]'s poll loop runs

on the Tk main thread.

"""

from .hot_reload import FileWatcher, ModuleReloader

def _on_change(changed_files: List[str]) -> None:

modules: List[str] = []

for path in changed_files:

module = ModuleReloader.file_to_module(path, base_dir)

if module:

modules.append(module)

def _apply() -> None:

print(f"[pn preview] reloading: {', '.join(modules) or 'app'}", file=sys.stderr)

app.reload(modules)

main_queue.put(_apply)

return FileWatcher(watch_dir, _on_change, interval=_WATCH_INTERVAL_S)

pythonnative/src/pythonnative/preview.py at main · andevsr/pythonnative · GitHub

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