Add optional value injection when values cannot be injected by injectors

TUTORIAL.md

@@ -140,7 +140,7 @@ Besides injecting class instances, Poodinis can also inject values:
 ```
 class ExampleClass {
 	@Value("a.key.for.this.value")
-	private int someNumber;
+	private int someNumber = 9; // Assignment is kept when the injector cannot find the value associated with key
 }
 ```
 The value will automatically be injected during the autowiring process. In order for Poodinis to be able to inject values, ValueInjectors must be available and registered with the dependency container:

source/poodinis/autowire.d

@@ -134,7 +134,7 @@ private void autowireMember(string member, size_t memberIndex, Type)(shared(Depe
 			injectInstance!(member, memberIndex, UseMemberType)(container, instance);
 		} else static if (is(typeof(attribute) == Value)) {
 			enum key = attribute.key;
-			injectValue!(member, memberIndex, key)(container, instance);
+			injectValue!(member, memberIndex, key, false)(container, instance);
 		}
 	}
 }
@@ -204,7 +204,7 @@ private QualifierType createOrResolveInstance(MemberType, QualifierType, bool cr
 	}
 }
 
-private void injectValue(string member, size_t memberIndex, string key, Type)(shared(DependencyContainer) container, Type instance) {
+private void injectValue(string member, size_t memberIndex, string key, bool mandatory, Type)(shared(DependencyContainer) container, Type instance) {
 	alias MemberType = typeof(Type.tupleof[memberIndex]);
 	try {
 		auto injector = container.resolve!(ValueInjector!MemberType);
@@ -214,6 +214,10 @@ private void injectValue(string member, size_t memberIndex, string key, Type)(sh
 		}
 	} catch (ResolveException e) {
 		throw new ValueInjectionException(format("Could not inject value of type %s into %s.%s: value injector is missing for this type.", typeid(MemberType), typeid(Type), member));
+	} catch (ValueNotAvailableException e) {
+		static if (mandatory) {
+			throw new ValueInjectionException(format("Could not inject value of type %s into %s.%s", typeid(MemberType), typeid(Type), member), e);
+		}
 	}
 }
 

source/poodinis/valueinjection.d

@@ -12,6 +12,7 @@
 module poodinis.valueinjection;
 
 import std.exception;
+import std.string;
 
 /**
  * Thrown when something goes wrong during value injection.
@@ -20,6 +21,19 @@ class ValueInjectionException : Exception {
 	mixin basicExceptionCtors;
 }
 
+/**
+ * Thrown by injectors when the value with the given key cannot be found.
+ */
+class ValueNotAvailableException : Exception {
+	this(string key) {
+		super(format("Value for key %s is not available", key));
+	}
+
+	this(string key, Throwable cause) {
+		super(format("Value for key %s is not available", key), cause);
+	}
+}
+
 /**
  * UDA used for marking class members which should be value-injected.
  *
@@ -53,9 +67,8 @@ struct Value {
  * type or that of a struct. While class types are also supported, value injectors
  * are not intended for them.
  *
- * Note that value injectors are also autowired before being used. Value injectors should
- * not contain dependencies on classes which require value injection. Neither should a
- * value injector have members which are to be value-injected.
+ * Note that value injectors are also autowired before being used. Values within dependencies of
+ * a value injector are not injected. Neither are values within the value injector itself.
  *
  * Value injection is not supported for constructor injection.
  *
@@ -75,6 +88,8 @@ interface ValueInjector(Type) {
 	 *
 	 * The key can have any format. Generally you are encouraged
 	 * to accept a dot separated path, for example: server.http.port
+	 *
+	 * Throws: ValueNotAvailableException when the value for the given key is not available for any reason
 	 */
 	Type get(string key);
 }

test/poodinis/valueinjectiontest.d

@@ -25,6 +25,11 @@ version(unittest) {
 		Thing thing;
 	}
 
+	class ConfigWithDefaults {
+		@Value("conf.missing")
+		int noms = 9;
+	}
+
 	// Test injection of values
 	unittest {
 		auto container = new shared DependencyContainer();
@@ -69,4 +74,21 @@ version(unittest) {
 
 		assertThrown!ValueInjectionException(autowire(container, new MyConfig()));
 	}
+
+	// Test injection of values with defaults
+	unittest {
+		auto container = new shared DependencyContainer();
+		container.register!ConfigWithDefaults;
+
+		class IntInjector : ValueInjector!int {
+			public override int get(string key) {
+				throw new ValueNotAvailableException(key);
+			}
+		}
+
+		container.register!(ValueInjector!int, IntInjector);
+
+		auto instance = container.resolve!ConfigWithDefaults;
+		assert(instance.noms == 9);
+	}
 }