Documentation: "Using Scopes to manage Resources"

Author: bschnitz

docs/scopes.rst

@@ -62,3 +62,105 @@ Scopes can be retrieved from the injector, as with any other instance. They are
     True
 
 For scopes with a transient lifetime, such as those tied to HTTP requests, the usual solution is to use a thread or greenlet-local cache inside the scope. The scope is "entered" in some low-level code by calling a method on the scope instance that creates this cache. Once the request is complete, the scope is "left" and the cache cleared.
+
+Using Scopes to manage Resources
+````````````````````````````````
+
+Sometimes You need to inject classes, which manage resources, like database
+connections. Imagine You have an :class:`App`, which depends on multiple other
+services and some of these services need to access the Database. The naive
+approach would be to open and close the connection everytime it is needed::
+
+    class App:
+       @inject
+        def __init__(self, service1: Service1, service2: Service2):
+            Service1()
+            Service2()
+
+    class Service1:
+        def __init__(self, cm: ConnectionManager):
+            cm.openConnection()
+            # do something with the opened connection
+            cm.closeConnection()
+
+    class Service2:
+        def __init__(self, cm: ConnectionManager):
+            cm.openConnection()
+            # do something with the opened connection
+            cm.closeConnection()
+
+Now You may figure, that this is inefficient. Instead of opening a new
+connection everytime a connection is requested, it may be useful to reuse
+already opened connections. But how and when should these connections be closed
+in the example above?
+
+This can be achieved with some small additions to the :class:`SingletonScope` and
+to :class:`Injector` we can create singletons, which will be
+cared for automatically, if they only implement a :meth:`cleanup` method and are
+associated with our custom scope. Let's reduce our example from above a bit for
+the sake of brevity to just one class, which needs cleanup. Remark the `@cleaned`
+decorator, which we will implement shortly afterwards and will associate the
+class with our custom scope::
+
+    @cleaned
+    class NeedsCleanup:
+        def __init__(self) -> None:
+            print("NeedsCleanup: I'm alive and claiming lot's of resources!")
+
+        def doSomething(self):
+            print("NeedsCleanup: Now I have plenty of time to work with these resources.")
+
+        def cleanup(self):
+            print("NeedsCleanup: Freeing my precious resources!")
+
+To achieve this, we first need to create a custom scope. This scope will just
+collect all singletons, which were accessed using the :meth:`Scope.get`-method::
+
+    T = TypeVar('T')
+
+    class CleanupScope(SingletonScope):
+        def __init__(self, injector: 'Injector') -> None:
+            super().__init__(injector)
+            # We have singletons here, so never cache them twice, since otherwise
+            # the cleanup method might be invoked twice.
+            self.cachedProviders = set()
+
+        def get(self, key: Type[T], provider: Provider[T]) -> Provider[T]:
+            obj = super().get(key, provider)
+            self.cachedProviders.add(obj)
+            return obj
+
+    cleaned = ScopeDecorator(CleanupScope)
+
+Next we will also create a custom :class:`Injector`, which will do the cleanup of all
+our objects belonging to :class:`CleanupScope` after a call to :meth:`get`::
+
+    ScopeType = Union[ScopeDecorator, Type[Scope], None]
+
+    class CleanupInjector:
+        def __init__(self, injector: Injector) -> None:
+            self.injector = injector
+
+        @contextmanager
+        def get(self, interface: Type[T], scope: ScopeType = None) -> Generator[T, None, None]:
+            yield self.injector.get(interface, scope)
+            self.cleanup()
+
+        def cleanup(self):
+            print("CleanupInjector: Invoking 'cleanup' for all who need it.")
+            cleanupScope = self.injector.get(CleanupScope)
+            for provider in cleanupScope.cachedProviders:
+                obj = provider.get(self.injector)
+                if hasattr(obj, 'cleanup') and callable(obj.cleanup):
+                    obj.cleanup()
+
+Now we can simply use our custom injector and freeing resources will be done for
+each object in :class:`CleanupScope` automatically::
+
+    injector = CleanupInjector(Injector())
+    with injector.get(NeedsCleanup) as obj:
+        obj.doSomething()
+
+This is of course a simple example. In a real world example `NeedsCleanup` could
+be nested deep and multiple times anywhere in a dependency structure. This
+pattern would work irrespectively of where `NeedsCleanup` would be injected.