原文出处:http://www.artima.com/scalazine/articles/selfless_trait_pattern.html
This article describes a simple Scala design pattern that allows library
designers to provide services that their clients can access either
through mixins or imports. Giving users a choice between mixin
composition and importing makes a library easier to use.
When designing a Scala library, you can partition the services offered
by the library into traits. This gives your users more flexibility:
they can mix into each class only the services they need from your
library.
For example, in ScalaTest
you create a basic suite of tests
by mixing in any of several core Suite
traits:
class MySuite extends Suite // a basic suite of test methods
class MySuite extends FunSuite // a basic suite of test functions
class MySpec extends Spec // a BDD-style suite of tests
class MyeSpec extends FeatureSpec // a higher level BDD-style suite
class MySuite extends FixtureSuite // a suite of tests that take a fixture parameter
You can also mix in any of several other traits that add additional, or modify existing,
behavior of the core trait:
class MySuite extends FunSuite with ShouldMatchers with EasyMockSugar with PrivateMethodTester
Although splitting a library's behavior into composable parts with traits gives
users flexibility, one downside is that
users may end up repeatedly mixing together the same traits, resulting in code
duplication. User can easily eliminate this duplication, however, by creating a convenience
trait that mixes together the behavior they prefer, and then mixing in that convenience
trait into their classes instead. For example:
trait ProjectSpec extends WordSpec with ShouldMatchers with EasyMockSugar
class OneSpec extends ProjectSpec
class TwoSpec extends ProjectSpec
class RedSpec extends ProjectSpec with PrivateMethodTester
class BlueSpec extends ProjectSpec
Besides just mixing together the things they need, users can add value inside their convenience traits.
For example, projects often require many tests that share a common fixture, such as a connection to
a database containing a clean set of test data. This could be addressed simply by creating yet another trait that mixes in
ProjectSpec
and adds the needed pre- and post-test behavior:
trait DBProjectSpec extends ProjectSpec with BeforeAndAfterEach {
override def beforeEach() {
// initialize the database and open the connection
}
override def afterEach() {
// clean up the database and close the connection
}
}
Now test classes that need a database can simply mix in DBProjectSpec
:
class MySpec extends DBProjectSpec
Although the ease of behavior composition afforded by traits is very useful, it has some downsides.
One downside is that name conflicts are difficult to resolve. A user can't, for example, mix together two
traits that contain methods with signatures that cause an overload conflict. Still another downside is
that it is slightly awkward to experiment with the services offered by a trait in the Scala interpreter,
because before the trait's services can be accessed, it must be mixed into some class or object.
Both of these downsides can be addressed by making it easy to import the members of a trait as
an alternative to mixing them in.
Implementing the selfless trait pattern
Scala has two features that make it easy to offer users this choice between mixins and imports.
First, Scala allows users to import the members of any object. Second, Scala allows traits
to have a companion object
---a singleton object that has the same name as its
companion trait
. You implement the selfless trait pattern simply by providing a companion object
for a trait that itself mixes in the trait. Here's a simple example of the selfless trait pattern:
trait Friendly {
def greet() { println("hi there") }
}
object Friendly extends Friendly
Trait Friendly
in this example has one method, greet
. It
also has a companion object, named Friendly
, which mixes in trait
Friendly
. Given this friendly design, client programmers of this
library can access the services of Friendly
either via mix in composition, like this (imports and uses of Friendly
are in bold):
object MixinExample extends Application with Friendly
{
greet()
}
Or by importing the members of the Friendly
companion object, like this:
import Friendly._
object ImportExample extends Application {
greet()
}
Although the external behavior of MixinExample
is the same as ImportExample
,
when MixinExample
invokes greet
it is calling greet
on itself (i.e.
, on
this
), but when ImportExample
invokes greet
, it is calling
greet
on the Friendly
singleton object. This is why being able to fall back on an import
allows users resolve name conflicts. For example, a user would not be able to mix the following
Functional
trait into the same class as Friendly
:
trait Functional {
def greet: String = "hi there"
}
Because Friendly
and Functional
's greet
methods
have the same signature but different return types, they will not overload if mixed into the same class:
object Problem extends Application with Friendly with Functional // Won't compile
By contrast, the offending method can be renamed on import, like this:
import Friendly.{greet => sayHi}
object Solved extends Application with Functional {
sayHi()
println(greet)
}
A good real-world example is trait ShouldMatchers
from ScalaTest, which follows the selfless trait pattern.
I expect the most common way ShouldMatchers
will be used is by mixing it into test classes, often by
means of a convenience trait. Here's an example:
import org.scalatest.WordSpec
import scala.collection.mutable.Stack
import org.scalatest.matchers.ShouldMatchers
class StackSpec extends WordSpec with ShouldMatchers
{
"A Stack" should {
"pop values in last-in-first-out order" in {
val stack = new Stack[Int]
stack.push(1)
stack.push(2)
stack.pop() should be === 2
stack.pop() should be === 1
}
"throw NoSuchElementException if an empty stack is popped" in {
val emptyStack = new Stack[String]
evaluating { emptyStack.pop() } should produce [NoSuchElementException]
}
}
}
Occasionally, however, users may want to experiment with the ShouldMatchers
syntax in the
Scala interpreter, which they can do with an import:
scala> import org.scalatest.matchers.ShouldMatchers._
import org.scalatest.matchers.ShouldMatchers._
scala> Map("hi" -> "there") should (contain key ("hi") and not contain value ("dude"))
scala> List(1, 2, 3) should have length 2
org.scalatest.TestFailedException: List(1, 2, 3) did not have length 2
at org.scalatest.matchers.Matchers$class.newTestFailedException(Matchers.scala:148)
at org.scalatest.matchers.ShouldMatchers$.newTestFailedException(ShouldMatchers.scala:2318)
at org.scalatest.matchers.Matchers$ResultOfHaveWordForSeq.length(Matchers.scala:2891)
at .(:7)
at .()
...
The reason this is called the selfless trait pattern is that it generally only makes sense to do this
with traits that don't declare a self type. (A self type
is a more specific type for this
that restricts what the trait can be mixed into.) One other way in
which traits that follow this
pattern are "selfless" is that rather than forcing users to mix in
their services they instead give users
a choice: either mixing in the trait or importing the
members of its companion object. If you are designing a Scala library
that has a trait
that doesn't declare a self type, consider implementing the Selfless
Trait pattern by creating a companion object that mixes in the trait.
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