Using PrivateMethodTester
ScalaTest's PrivateMethodTester trait facilitates the testing of private methods.
To test a private method, mix in trait PrivateMethodTester and
create a PrivateMethod object, like this:
val decorateToStringValue = PrivateMethod[String]('decorateToStringValue)
The type parameter on PrivateMethod, in this case String, is the result type of the private method
you wish to invoke. The symbol passed to the PrivateMethod.apply factory method, in this
case 'decorateToStringValue, is the name of the private method to invoke. To test
the private method, use the invokePrivate operator, like this:
targetObject invokePrivate decorateToStringValue(1)
Here, targetObject is a variable or singleton object name referring to the object whose
private method you want to test. You pass the arguments to the private method in the parentheses after
the PrivateMethod object.
The result type of an invokePrivate operation will be the type parameter of the PrivateMethod
object, thus you need not cast the result to use it. In other words, after creating a PrivateMethod object, the
syntax to invoke the private method
looks like a regular method invocation, but with the dot (.) replaced by invokePrivate.
The private method is invoked dynamically via reflection, so if you have a typo in the method name symbol, specify the wrong result type,
or pass invalid parameters, the invokePrivate operation will compile, but throw an exception at runtime.
One limitation to be aware of is that you can't use PrivateMethodTester to test a private method
declared in a trait because the class the trait gets mixed into will not declare that private method. Only the
class generated to hold method implementations for the trait will have that private method. If you want to
test a private method declared in a trait, and that method does not use any state of that trait, you can move
the private method to a companion object for the trait and test it using PrivateMethodTester that
way. If the private trait method you want to test uses the trait's state, your best options are to test it
indirectly via a non-private trait method that calls the private method, or make the private method package access
and test it directly via regular static method invocations.
Also, if you want to use PrivateMethodTester to invoke a parameterless private method, you'll need to use empty parens. Instead of:
targetObject invokePrivate privateParameterlessMethod
You'll need to write:
targetObject invokePrivate privateParameterlessMethod()
Next we'll look at one last goodie, using WrapWith.
