Context: add isInjectable(Class) method

Now you can ask the context itself whether a particular field type
would have its value assigned by an injection operation, or not.
Previously, you simply had to know (assume, really) that Service
and Context fields would be injected, and others would not.

Author: ctrueden

src/main/java/org/scijava/Context.java

@@ -388,6 +388,25 @@ public void inject(final Object o) {
 		subscribeToEvents(o);
 	}
 
+	/**
+	 * Reports whether a parameter of the given type would be assigned a value as
+	 * a consequence of calling {@link #inject(Object)}.
+	 * <p>
+	 * This method is notably useful for downstream code to discern between
+	 * {@link Parameter} fields whose values would be injected, versus those whose
+	 * values would not, without needing to hardcode type comparison checks
+	 * against the {@link Service} and {@link Context} types.
+	 * </p>
+	 * 
+	 * @param type The type of the @{@link Parameter}-annotated field.
+	 * @return True iff a member field of the given type would have its value
+	 *         assigned.
+	 */
+	public boolean isInjectable(final Class<?> type) {
+		if (Service.class.isAssignableFrom(type)) return true;
+		return Context.class.isAssignableFrom(type) && type.isInstance(this);
+	}
+
 	// -- Disposable methods --
 
 	@Override