Add single instance scope, make it the default scope of new registrations

source/poodinis/container.d

@@ -44,7 +44,7 @@ class Container {
 			checkValidity!(InterfaceType)(registeredType, instantiatableType);
 		}
 		
-		Registration newRegistration = { registeredType, instantiatableType, new NullScope() };
+		Registration newRegistration = { registeredType, instantiatableType, new SingleInstanceScope(instantiatableType) };
 		registrations[newRegistration.registeredType] = newRegistration;
 		return newRegistration;
 	}

source/poodinis/registration.d

@@ -10,7 +10,7 @@ struct Registration {
 			throw new NoScopeDefinedException(registeredType);
 		}
 		
-		return instantiatableType.create();
+		return registationScope.getInstance();
 	}
 }
 
@@ -29,3 +29,20 @@ class NullScope : RegistrationScope {
 		return null;
 	}
 }
+
+class SingleInstanceScope : RegistrationScope {
+	TypeInfo_Class instantiatableType = null;
+	Object instance = null;
+	
+	this(TypeInfo_Class instantiatableType) {
+		this.instantiatableType = instantiatableType;
+	}
+	
+	public Object getInstance() {
+		if (instance is null) {
+			instance = instantiatableType.create();
+		}
+		
+		return instance;
+	}
+}

test/poodinis/registrationtest.d

@@ -12,4 +12,14 @@ version(unittest) {
 		registration.registeredType = typeid(TestType);
 		assertThrown!(NoScopeDefinedException)(registration.getInstance());
 	}
+	
+	// Test getting instance from single instance scope
+	unittest {
+		Registration registration = Registration();
+		registration.registeredType = typeid(TestType);
+		registration.registationScope = new SingleInstanceScope(typeid(TestType));
+		auto instance1 = registration.getInstance();
+		auto instance2 = registration.getInstance();
+		assert(instance1 is instance2, "Registration with single instance scope did not return the same instance");
+	}
 }