Prevent duplicate launch from shortcut

TPO

@shinyformica, I have the same issue and have come up with a solution based on a simple protocol. When applications follow the protocol, they will not "pile up on top of one another" when launched from the iOS home screen:

When an app is already active in Pythonista and it is launched again with its home screen shortcut, the new instance of the app detects the situation and exits, leaving the already active instance of the app on screen.
When an app is launched from its home screen shortcut and a previous app is already active in Pythonista, the previous app will be notified that it should terminate, its main UI view will be closed, and the new app will be launched.

Protocol

An application should create an instance of class AppSingleLaunch, and use it to test if the application is already active, using the is_active() method. If yes, the application should simply exit. If not, the application should declare its main UI view, using the will_present() method, and present the view. Here is an example:
```
 import app_single_launch

 app = AppSingleLaunch("MyApp")
 if not app.is_active():
     view = MyAppView(app)
     app.will_present(view)
     view.present()
```

An application should make a call to the AppSingleLaunch.will_close() method, from the will_close() method of its main UI view:

 class MyAppView(ui.View):

     def __init__(self, app):
         self.app = app

     def will_close(self):
       self.app.will_close()

Demo

Save the code for app_single_launch.py and the two demo apps in the same directory.

Define home screen shortcuts for the two demo apps.

Launch demo app 1 using its home screen shortcut, type some text in the text field, then relaunch the app using its home screen shortcut : the text typed previously is still showing, meaning we are using the first launched instance, not the second one. Closing the app brings us back to the Pythonista IDE, not to previously piled up instances of the app.

Launch demo app 1 using its home screen shortcut, then Launch demo app 2 using its home screen shortcut, then close it : the Pythonista IDE shows, not a piled up instance of demo app 1.

Et voilà !

app_single_launch.py

""" Ensure a Pythonista script can only be launched once from the iOS home screen.

This module provides a solution to the problem stated by shinyformica, which I
have also encountered (https://forum.omz-software.com/topic/5440/prevent-
duplicate-launch-from-shortcut):
    "Is there a good, canonical way to prevent a script from launching again if
    there is already an instance running? Specifically, I want to prevent the
    scenario where a user launches the script from the home screen via an app
    shortcut, then goes back to the home screen and launches it again.""

The solution is based on a simple protocol, which applications need to adhere
to. When this is the case, applications will not "pile up on top of one another"
when launched from the iOS home screen:
- When an app is already active in Pythonista and it is launched again with its
  home screen shortcut, the new instance of the app detects the situation and
  exits, leaving the already active instance of the app on screen.
- When an app is launched from its home screen shortcut and a previous app is
  already active in Pythonista, the previous app will be notified that it
  should terminate, its main UI view will be closed, and the new app will be
  launched.

Protocol:
1) An application should create an instance of class AppSingleLaunch, and use
   it to test if the application is already active, using the is_active()
   method. If yes, the application should simply exit. If not, the application
   should declare its main UI view, using the will_present() method, and
   present the view. Here is an example:

        import app_single_launch

        app = AppSingleLaunch("MyApp")
        if not app.is_active():
            view = MyAppView(app)
            app.will_present(view)
            view.present()

2) An application should make a call to the AppSingleLaunch.will_close()
   method, from the will_close() method of its main UI view:

        class MyAppView(ui.View):

            def __init__(self, app):
                self.app = app

            def will_close(self):
              self.app.will_close()

Implementation: in order to achieve the desired result, we need to remember the
last application launched according to the protocol, to determine if it is
still active, and, if it is, to close it. This is achieved by storing into a
lock file some information about the last application launched:
- Its name, as passed to single_launch.launching()
- The id of the ui.View instance for its main view, as passed to single_launch.
  launching(). This is later used to determine if the view is still on screen
  (when an object is still associated with the id), and to close the app's view.
  After several tests, it turns out we must use an ui.View object for this
  purpose, as they seem to persist better than other objects after the cleanup
  pykit-preflight.py does when an app is launched from the home screen.
The location of the lock file is defined by global variable LOCK_PATH. The
default location is in the 'site-packages' directory.

Known issue:
- When an app is on screen, then launched again from its home screen shortcut,
  some issues may happen with inline import statements (rare, would need to be
  qualified further).

26-Feb-2019 TPO - Created this module
28-Feb-2019 TPO - Initial release
 3-Mar-2019 TPO - Wrapped the code into the AppSingleLaunch class """


import gc
import json
from pathlib import Path
import time
from typing import Any

import ui


__all__ = [
    'AppSingleLaunch',
]


DEBUG = False
LOCK_PATH = '~/Documents/site-packages/single_launch.lock'


def _object_for_id(id_: int) -> Any:
    """ Return an object, given its id. """

    # Do a complete garbage collect, to avoid false positives in case the
    # object was still in use recently. In the context of AppSingleLaunch,
    # this would happen if an app was closed, then launched again immediately.
    gc.collect()
    for obj in gc.get_objects():
        if id(obj) == id_:
            return obj
    return None


class AppSingleLaunch:
    """ Wrapper class for all module functionnality. """

    def __init__(self, app: str):
        """ Initialize an AppSingleLaunch instance.

        Arguments:
        - app: application name, which should be unique (but this is not
        enforced). """
        self.app = app

    def is_active(self) -> bool:
        """ Test if the application is already active.

        Returns:
        - True if the application is already running, in which case the caller
          should do nothing and exit.
        - False if the application is not already running, in which case the
          caller should launch the application in a normal way, and declare its
          main view by calling the will_present() method."""
        if DEBUG:
            print(f"is_active(), app = {self.app}")
        lock_path = Path(LOCK_PATH).expanduser()
        if lock_path.exists():
            with open(lock_path) as lock_file:
                (lock_app, lock_view_id) = tuple(json.load(lock_file))
            lock_view = _object_for_id(lock_view_id)
            if DEBUG:
                print("- Lock file =", lock_app, lock_view_id,
                      "valid" if lock_view else "invalid")
            if lock_app == self.app and lock_view:
                if DEBUG:
                    print(f"- App {self.app} already active")
                return True
        if DEBUG:
            print(f"- App {self.app} not active")
        return False

    def will_present(self, view: ui.View) -> None:
        """ Declare that the application is about to present its main view.

        Arguments:
        - view: ui.View instance for the app's main view. """
        if DEBUG:
            print(f"will_present({id(view)}), app = {self.app}")
        lock_path = Path(LOCK_PATH).expanduser()
        if lock_path.exists():
            with open(lock_path) as lock_file:
                (lock_app, lock_view_id) = tuple(json.load(lock_file))
            lock_view = _object_for_id(lock_view_id)
            if DEBUG:
                print("- Lock file =", lock_app, lock_view_id,
                      "valid" if lock_view else "invalid")
            if lock_app == self.app and lock_view:
                raise ValueError(f"App {self.app} is already active, cannot "
                                 f"call will_present() against it.")
            else:
                if lock_view and isinstance(lock_view, ui.View):
                    if DEBUG:
                        print(f"- Closing app {lock_app}")
                    lock_view.close()
                    time.sleep(1)  # Required for view to close properly
                # else: lock is a leftover from a previous Pythonista session
                #       and can be safely ignored.
        with open(lock_path, 'w') as lock_file:
            json.dump([self.app, id(view)], lock_file)
        if DEBUG:
            print(f"- Launching app {self.app}\n- Lock file =", self.app, id(view))

    def will_close(self) -> None:
        """ Declare that the application is about to close its main view. """
        lock_path = Path(LOCK_PATH).expanduser()
        if lock_path.exists():
            with open(lock_path) as lock_file:
                (lock_app, lock_view_id) = tuple(json.load(lock_file))
            if lock_app != self.app:
                raise ValueError(f"App {self.app} if not active, "
                                 f"{lock_app} is active")
            lock_path.unlink()

single_launch_demo1.py

""" app_single_launch demo script #1. """
from app_single_launch import AppSingleLaunch
import ui


class MainView(ui.View):
    def __init__(self, app: AppSingleLaunch):
        self.app = app
        self.name = "Demo app 1"
        self.flex = 'WH'
        self.background_color = 'white'
        self.add_subview(ui.TextField(
            width=200,
            height=30,
            placeholder="Type some text"))

    def will_close(self) -> None:
        self.app.will_close()


if __name__ == '__main__':
    app = AppSingleLaunch("Demo app 1")
    if not app.is_active():
        view = MainView(app)
        app.will_present(view)
        view.present()

single_launch_demo2.py

""" app_single_launch demo script #2. """
from app_single_launch import AppSingleLaunch
import ui


class MainView(ui.View):
    def __init__(self, app: AppSingleLaunch):
        self.app = app
        self.name = "Demo app 2"
        self.flex = 'WH'
        self.background_color = 'white'
        self.add_subview(ui.TextField(
            width=200,
            height=30,
            placeholder="Type some text"))

    def will_close(self) -> None:
        self.app.will_close()


if __name__ == '__main__':
    app = AppSingleLaunch("Demo app 2")
    if not app.is_active():
        view = MainView(app)
        app.will_present(view)
        view.present()

shinyformica

Just revisiting this issue, since it's once again on my plate to try and find a working solution.

When we left off, I'd tried various attempts using different ideas discussed above. None of them are really successful. I can successfully prevent a new instance from trying to launch, keeping a token in the ui module which let's me know if an instance is already running, and which is cleared when the running instance is closed.

The trouble is still with what happens when a script is launched via a home screen shortcut.

In that case, the pykit_preflight.py script is in some way being run (it's not clear exactly when and how that occurs @omz...any hints?), and it clears out the sys.modules and globals before it runs the script. So even though the newly launched script sees that an instance is already running, and bails immediately, the previous instance is left in a broken state.

The clearing of the modules and globals means the existing instance is suddenly left with all of its module-level global data pointing at None, so it ends up raising an exception as soon as it tries to do anything.

So...I tried all sorts of ways to prevent or replace how pykit_preflight does it's thing to no avail. I even tried stashing away the globals for every module in my codebase and restoring them all. No luck.

Anyone have any ideas or better knowledge of how to prevent the clearing when a script is launched from the home screen shortcut, while pythonista is already open and running?

JonB

Iirc, @dgelessus figured out a way to bypass the preflight altogether. I can't find that thread right now, I think it involved hijacking a call that happened within preflight.. this might be an option in his pythonista_startup.

You probably know all this, but from another thread, here are some methods to avoid module and or variables getting cleared:

##ways to keep modules from clearing:

for modules, delete file
e.g

   import mymodule
   del mymodule.__file__

now mymodule is not removed from sys.modules

create name staring with dunder__

import mymodule
sys.modules['__mymodule']=sys.modules['mymodule']

set file access to nonwritable

chmod(mymodule.__file__,0xo444)

place module in site-packages

##ways to keep global vars from being cleared
1.start variable with __
__myglobalvar=4
2.add to pythonista_startup
myglobalvar=4
import pythonista_startup

pythonista_startup.myglobalvar=myglobalvar

for things like ui.Views, threads, which needs to reference itself, but does not need to be refernced externally, etc it is sufficient to simply add the var to an existing builtin module

try:
ui.mysavedviews.append(myview)
except NameError:
ui.mysavedviews=[myview]

retain_global is another way.

other thoughts

For modules getting cleared out from under you, the only practical solution iirc is that every function must import what it needs. That's what stash does, and it can survive anything (I think including pythonista://). It is really tedious-- basically anything that can get called after your initial launch cannot rely on modules being imported at the file/module level. Instead, each def must import modules it uses. There is not a great way to check that you remembered everything.

JonB

Take a look at (here](https://github.com/dgelessus/pythonista_startup/blob/master/_preflight_hook_experiment.py)
And also
Here
at line 39
The first let's you register hooks that get called very early in preflight. I suppose you could check for your UI, and if present, raise an Exception, thus killing preflight. (I'm not 100% sure that's work).

Option 2 uses the DirAllTheGlobals trick to patch pythonista_startup's dir to return everything in main -- since preflight checks if each variable is in pythonista_startup.

I'd think you could also create a custom importer, that forced all imports into pythonista_startup where it would be safe, or maybe have a system to "register" required imports/globals associated with an app, that can be cleared when app closes

shinyformica

@JonB thanks!

Amazingly, yanking out just the "disable global clearing" part of that pythonista_startup magic worked to allow relaunch from the home shortcut.

So, the ugly tinkering is relatively short: replace the existing pythonista startup module with a new module object containing the globals for the current pythonista_startup module, and which returns all the globals in main when asked for its contents.

This will effectively short-circuit the preflight system from clearing out any globals, since it skips any which are inside the pythonista startup module itself.

Here's the shortened code just to solve this one issue, if anyone else is encountering it:

print(u"Executing pythonista_startup...")

try:
    print(u"Preventing globals clearing...")
    
    import sys
    import types
    
    class SaveGlobals(types.ModuleType):
        import __main__
        
        def __dir__(self):
            return dir(type(self).__main__)
    
    # THESE LINES MUST COME LAST.
    # Anything past this point is executed in the context of the old
    # pythonista_startup module, which may already be partially
    # garbage-collected.
    saved = SaveGlobals(__name__, __doc__)
    vars(saved).update(vars(sys.modules["pythonista_startup"]))
    sys.modules["pythonista_startup"] = saved
    
    del sys
    del types
    del SaveGlobals
    del saved
    
    print(u"Done preventing globals clearing.")
except: # Catch everything
    import sys
    import traceback
    
    print(u"Swallowed exception:", file=sys.stderr)
    traceback.print_exc()
    
    print(u"Attempt to re-raise", file=sys.stderr)
    del sys
    del traceback
    raise

print(u"Done executing pythonista_startup.")

jmv38

@shinyformica hello!
how do you make a homescreen shortcut to a pythonista script?
thanks!

cvp

@jmv38 here or here

jmv38

@cvp thank you!
when you say:

Altough last version of Pythonista offers an easy way to add an home screen shortcut for an edited script

do you mean the first link above?

cvp

@jmv38 no, via wrench icon
Problem is that beta expires in 2 days...

jmv38

@cvp actually it does work on the regular version.
Not sure why i didnt see that before.
Thanks.

nfmusician

@TPO Do you have this on a Github anywhere? It's useful.

TPO

@nfmusician there you go : https://github.com/TPO-POMGOM/Pythonista-utilities

(I currently lack time to migrate to Github other utilities and modules I have developped for Pythonista, will do it as soon as time allows)