context_manager.py

from input.input import InputProxy
from output.output import OutputProxy

from functools import wraps
from threading import Thread, Lock

from helpers import setup_logger

logger = setup_logger(__name__, "info")

def context_target_wrapper(context, func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        try:
            func(*args, **kwargs)
        except:
            raise
        finally:
            logger.info("Context {} finished, signalling".format(context.name))
            context.signal_finished()
    return wrapper


class ContextError(Exception):
    pass


class Context(object):
    thread = None
    target = None
    threaded = True
    i = None
    o = None

    def __init__(self, name, event_callback):
        self.name = name
        self.event_cb = event_callback

    def set_target(self, target):
        """
        Sets a function to be called once context is switched. This way,
        you can define a custom callback to be used for your app.
        """
        self.target = target

    def set_io(self, i, o):
        """
        Saves references to proxy IO objects in the context object
        (this function is used by the context manager)
        """
        self.i = i
        self.o = o

    def get_io(self):
        """
        Returns proxy IO objects used by the app that the context belongs to.
        """
        return self.i, self.o

    def activate(self):
        """
        Starts the context - if it's threaded, starts the thread, otherwise it assumes
        that there's already a running thread and does nothing. In latter case, throws
        an exception if it detects that a supposedly non-threaded context has the ``threaded``
        flag set.
        """
        if self.is_threaded():
            if not self.thread_is_active():
                self.verify_target()
                self.start_thread()
            else:
                logger.debug("A thread for the {} context seems to already be active, not doing anything".format(self.name))
        else:
            if self.name == "main":
                logger.debug("Main context does not have thread target, that is to be expected")
            elif self.threaded:
                logger.warning("Context {} does not have a target! Raising an exception".format(self.name))
                raise ContextError("Context {} does not have a target!".format(self.name))

    def verify_target(self):
        """
        Checks whether a valid target is set - at the very least, a callable.
        """
        if not callable(self.target):
            raise ContextError("Context {} expected callable target, got {}!".format(self.name, type(self.target)))

    def is_threaded(self):
        """
        Tells whether the context is threaded or not - checking whether target exists and
        ``threaded`` flag is set.
        """
        return self.threaded and self.target is not None

    def thread_is_active(self):
        """
        Tells whether there's a currently-running thread associated with the context.
        """
        return self.thread and self.thread.isAlive()

    def start_thread(self):
        """
        Actually launches the context thread (based on the target function,
        wrapped into the ``context_target_wrapper()``.
        """
        wrapped_target = context_target_wrapper(self, self.target)
        if self.thread: del self.thread
        self.thread = Thread(target=wrapped_target)
        self.thread.daemon = True
        self.thread.start()

    def signal_finished(self):
        """
        Signals to the ContextManager that the application has finished running,
        as well as clears the display (to avoid a bug from happening, where last
        contents of the display are shortly shown on the next reactivation).
        """
        self.o._clear()
        return self.event_cb(self.name, "finished")

    def signal_background(self):
        """
        Signals to the ContextManager that the application wants to go into background.
        Currently, has the same effect as ``signal_finished``.
        """
        return self.event_cb(self.name, "background")

    def request_switch(self):
        """
        Requests ContextManager to switch to the context in question. If switch is done,
        returns True, otherwise returns False.
        """
        return self.event_cb(self.name, "request_switch")

    def is_active(self):
        """
        Tells whether this context is the one active.
        """
        return self.event_cb(self.name, "is_active")

    def get_previous_context_image(self):
        """
        Useful for making screenshots (mainly, for ZeroMenu). Might get deprecated in
        the future, once a better way to do this is found.
        """
        return self.event_cb(self.name, "get_previous_context_image")

    def request_global_keymap(self, keymap):
        """
        Requests ContextManager to set callbacks into the global keymap.
        Returns a dictionary with results for each set key (with keys as dict. keys):
        if a callback for the key was set successfully, the value is True;
        otherwise, the value will be an exception raised in the process.

        Might get deprecated in the future, once a better way to do this is found.
        """
        return self.event_cb(self.name, "request_global_keymap", keymap)


class ContextManager(object):

    current_context = None
    fallback_context = "main"
    initial_contexts = ["main"]

    def __init__(self):
        self.contexts = {}
        self.previous_contexts = {}
        self.switching_contexts = Lock()

    def init_io(self, input_processor, screen):
        """
        Saves references to hardware IO objects and creates initial contexts.
        """
        self.input_processor = input_processor
        self.screen = screen
        self.create_initial_contexts()

    def create_initial_contexts(self):
        """
        Creates contexts specified in ``self.initial_contexts`` - since
        targets aren't set, marks them as non-threaded.
        """
        for context_alias in self.initial_contexts:
            c = self.create_context(context_alias)
            c.threaded = False

    def get_context_names(self):
        """
        Gets names of all contexts available.
        """
        return self.contexts.keys()

    def get_current_context(self):
        """
        Returns the alias (name) of the current context.
        """
        return self.current_context

    def register_context_target(self, context_alias, target):
        """
        A context manager-side function that sets the target for a context.
        """
        logger.debug("Registering a thread target for the {} context".format(context_alias))
        self.contexts[context_alias].set_target(target)

    def switch_to_context(self, context_alias):
        """
        Lets you switch to another context by its alias.
        """
        # Saving the current context alias in the "previous context" storage
        if context_alias != self.current_context:
            self.previous_contexts[context_alias] = self.current_context
            with self.switching_contexts:
                try:
                    self.unsafe_switch_to_context(context_alias)
                except ContextError:
                    logger.exception("A ContextError was caught")
                    self.previous_contexts.pop(context_alias)
                    return False
                else:
                    return True

    def unsafe_switch_to_context(self, context_alias):
        """
        This is a non-thread-safe context switch function. Not to be used directly
        - is only for internal usage. In case an exception is raised, sets things as they
        were before and re-raises the exception - in the worst case, 
        """
        logger.info("Switching to {} context".format(context_alias))
        previous_context = self.current_context
        self.current_context = context_alias
        # First, activating IO - if it fails, restoring the previous context's IO
        try:
            self.activate_context_io(context_alias)
        except:
            logger.exception("Switching to the {} context failed - couldn't activate IO!".format(context_alias))
            try:
                self.activate_context_io(previous_context)
            except:
                logger.exception("Also couldn't activate IO for the previous context: {}!".format(previous_context))
                self.failsafe_switch_to_fallback_context()
                raise
            self.current_context = previous_context
            # Passing the exception back to the caller
            raise 
        # Activating the context - restoring everything if it fails
        try:
            self.contexts[context_alias].activate()
        except:
            logger.exception("Switching to the {} context failed - couldn't activate the context!".format(context_alias))
            # Activating IO of the previous context
            try:
                self.activate_context_io(previous_context)
            except:
                logger.exception("Also couldn't activate IO for the previous context: {}!".format(previous_context))
                self.failsafe_switch_to_fallback_context()
                raise
            # Activating the previous context itself
            try:
                self.contexts[previous_context].activate()
            except:
                logger.exception("Also couldn't activate context for the previous context: {}!".format(previous_context))
                self.failsafe_switch_to_fallback_context()
                raise
            self.current_context = previous_context
            # Passing the exception back to the caller
            raise
        else:
            logger.debug("Switched to {} context!".format(context_alias))

    def failsafe_switch_to_fallback_context(self):
        """
        Last resort function for the ``unsafe_switch_to_context`` to use
        when both switching to the context and failsafe switching fails.
        """
        logger.warning("Some contexts broke, switching to fallback context: {}".format(self.fallback_context))
        self.current_context = self.fallback_context
        # In case something fucked up in the previous context dictionary, fixing that
        # More like - working around, preventing the user from making more mistakes
        self.previous_contexts[self.fallback_context] = self.fallback_context
        self.activate_context_io(self.current_context)
        self.contexts[self.current_context].activate()
        logger.info("Fallback switched to {} - proceed with caution".format(self.current_context))

    def activate_context_io(self, context_alias):
        """
        This method activates input and output objects associated with a context.
        """
        logger.debug("Activating IO for {} context".format(context_alias))
        proxy_i, proxy_o = self.contexts[context_alias].get_io()
        if not isinstance(proxy_i, InputProxy) or not isinstance(proxy_o, OutputProxy):
            raise ContextError("Non-proxy IO objects for the context {}".format(context_alias))
        self.input_processor.attach_new_proxy(proxy_i)
        self.screen.attach_new_proxy(proxy_o)

    def create_context(self, context_alias):
        """
        Creates a context object (with IO) and saves it internally
        (by the given context alias).
        """
        logger.debug("Creating {} context".format(context_alias))
        context = Context(context_alias, self.signal_event)
        context.set_io(*self.create_io_for_context(context_alias))
        self.contexts[context_alias] = context
        return context

    def create_io_for_context(self, context_alias):
        """
        Creates IO objects for the context, registers them and returns them.
        """
        proxy_i = InputProxy(context_alias)
        proxy_o = OutputProxy(context_alias)
        self.input_processor.register_proxy(proxy_i)
        self.screen.init_proxy(proxy_o)
        return proxy_i, proxy_o

    def get_io_for_context(self, context_alias):
        """
        Returns the IO objects for the context.
        """
        return self.contexts[context_alias].get_io()

    def get_previous_context(self, context_alias, pop=False):
        """
        Returns name of the previous context for a given context. If ``pop``
        is set to True, also removes the name from the internal dictionary.
        """
        if pop:
            prev_context = self.previous_contexts.pop(context_alias, self.fallback_context)
        else:
            prev_context = self.previous_contexts.get(context_alias, self.fallback_context)
        # If previous context is, by some chance, the same, let's fall back
        if prev_context == context_alias:
            prev_context = self.fallback_context
        return prev_context

    def signal_event(self, context_alias, event, *args, **kwargs):
        """
        A callback for context objects to use to signal/receive events - 
        providing an interface for apps to interact with the context manager.
        This function will, at some point in the future, be working through
        RPC.
        """
        if event == "finished" or event == "background":
            # For now, those two events are handled the same - later on,
            # there will be differences.
            with self.switching_contexts:
                #Locking to avoid a check-then-do race condition
                if self.current_context == context_alias:
                    #Current context is the active one, switching to the previous context
                    next_context = self.get_previous_context(context_alias, pop=True)
                    logger.debug("Next context: {}".format(next_context))
                    try:
                        self.unsafe_switch_to_context(next_context)
                    except ContextError:
                        logger.exception("A ContextError was caught")
                        self.previous_contexts[context_alias] = next_context
                        return False
                    return True
                else:
                    return False
        elif event == "get_previous_context_image":
            # This is a special-case function for screenshots. I'm wondering
            # if there's a better way to express this.
            previous_context = self.get_previous_context(context_alias)
            return self.contexts[previous_context].get_io()[1].get_current_image()
        elif event == "is_active":
            return context_alias == self.current_context
        elif event == "request_switch":
            # As usecases appear, we will likely want to do some checks here
            logger.info("Context switch requested by {} app".format(context_alias))
            return self.switch_to_context(context_alias)
        elif event == "request_global_keymap":
            results = {}
            keymap = args[0]
            for key, cb in keymap.items():
                try:
                    self.input_processor.set_global_callback(key, cb)
                except Exception as e:
                    logger.warning("Context {} couldn't set a global callback on {} (function: {})".format(context_alias, key, cb.__name__))
                    results[key] = e
                else:
                    logger.warning("Context {} set a global callback on {} (function: {})".format(context_alias, key, cb.__name__))
                    results[key] = True
            return results
        else:
            logger.warning("Unknown event: {}!".format(event))