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135 lines
4.1 KiB
ReStructuredText
135 lines
4.1 KiB
ReStructuredText
Singleton providers
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-------------------
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``Singleton`` provider creates new instance of specified class on first call
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and returns same instance on every next call.
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Example:
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.. image:: /images/singleton.png
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:width: 80%
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:align: center
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.. code-block:: python
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"""`Singleton` providers example."""
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from objects.providers import Singleton
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class UserService(object):
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"""Example class UserService."""
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# Singleton provider creates new instance of specified class on first call and
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# returns same instance on every next call.
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users_service_provider = Singleton(UserService)
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# Retrieving several UserService objects:
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user_service1 = users_service_provider()
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user_service2 = users_service_provider()
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# Making some asserts:
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assert user_service1 is user_service2
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assert isinstance(user_service1, UserService)
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assert isinstance(user_service2, UserService)
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Singleton providers and injections
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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``Singleton`` providers use ``Factory`` providers for first creation of
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specified class instance, so, all of the rules about injections are the same,
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as for ``Factory`` providers.
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.. image:: /images/singleton_internals.png
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:width: 80%
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:align: center
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.. note::
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Due that ``Singleton`` provider creates specified class instance only on
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the first call, all injections are done once, during the first call, also.
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Every next call, while instance has been already created and memorized, no
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injections are done, ``Singleton`` provider just returns memorized earlier
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instance.
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This may cause some problems, for example, in case of trying to bind
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``Factory`` provider with ``Singleton`` provider (provided by dependent
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``Factory`` instance will be injected only once, during the first call).
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Be aware that such behaviour was made with opened eyes and is not a bug.
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By the way, in such case, ``Delegate`` provider can be useful. It makes
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possible to inject providers *as is*. Please check out full example in
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*Providers delegation* section.
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Singleton providers resetting
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Created and memorized by ``Singleton`` instance can be reset. Reset of
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``Singleton``'s memorized instance is done by clearing reference to it. Further
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lifecycle of memorized instance is out of ``Singleton`` provider's control.
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Example:
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.. code-block:: python
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"""`Singleton` providers resetting example."""
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from objects.providers import Singleton
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class UserService(object):
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"""Example class UserService."""
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# Users service singleton provider:
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users_service_provider = Singleton(UserService)
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# Retrieving several UserService objects:
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user_service1 = users_service_provider()
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user_service2 = users_service_provider()
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# Making some asserts:
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assert user_service1 is user_service2
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assert isinstance(user_service1, UserService)
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assert isinstance(user_service2, UserService)
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# Resetting of memorized instance:
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users_service_provider.reset()
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# Retrieving one more UserService object:
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user_service3 = users_service_provider()
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# Making some asserts:
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assert user_service3 is not user_service1
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Singleton providers delegation
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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``Singleton`` provider could be delegated to any other provider via any kind of
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injection. Delegation of ``Singleton`` providers is the same as ``Factory``
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providers delegation, please follow *Factory providers delegation* section for
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example.
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``Singleton`` delegate could be created obviously using
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``Delegate(Singleton())`` or by calling ``Singleton.delegate()`` method.
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Example:
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.. code-block:: python
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"""`Singleton` providers delegation example."""
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from objects.providers import Singleton
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from objects.providers import Delegate
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# Some singleton provider and few delegates of it:
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singleton_provider = Singleton(object)
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singleton_provider_delegate1 = singleton_provider.delegate()
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singleton_provider_delegate2 = Delegate(singleton_provider)
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# Making some asserts:
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assert singleton_provider_delegate1() is singleton_provider
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assert singleton_provider_delegate2() is singleton_provider
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