延迟特性初始化 [英] Delaying trait initialization

问题描述

我需要一个组件组合的智能机制，允许混合在traits中之后初始化组合组件。以下引发 NullPointerException ：

 类Component {
 def addListener（pf：PartialFunction [Any，Unit]）{} 
} 
 
 trait DynamicComponent {
 protected def component：Component 
 
 component.addListener {
 case x => 
} 
} 
 
类Foo extends DynamicComponent {
 protected val component = new Component 
} 
 
 new Foo // - > NullPointerException 
  

以下是我的不是选项：

• 使用 protected lazy val组件;这将产生一个 avalange 几十个vals需要变得懒惰，我不想。


• 放置 addListener 在方法中，例如 initDynamic（）;因为我会混合许多特质，我不想记得调用一半的 initFoo（）方法。


• 使用 DelayedInit 。这不适用于traits，至少根据scaladocs。

我可以用一个 init（）调用，但仅在以下条件下：

• 全部混合在traits中可以很容易地声明为在此一次调用中调用


• 这是一个编译错误，忘记了 init（）语句。

解决方案

您可以使用早期定义延迟trait的初始化。 （请参见 scala语言规范的第5.1.6节）

class Foo extends {
protected val component = new Component
} with DynamicComponent
pre>

I need a smart mechanism for component composition which allows mixed in traits to initialize after the composed component. The following throws a NullPointerException:

class Component {
  def addListener(pf: PartialFunction[Any, Unit]) {}
}

trait DynamicComponent {
  protected def component: Component

  component.addListener {
    case x =>
  }
}

class Foo extends DynamicComponent {
  protected val component = new Component
}

new Foo  // -> NullPointerException

The following things are not options for me:

• Using protected lazy val component; that would produce an avalange of dozens of vals needing to become lazy, something I do not want.
• Putting addListener in a method, e.g. initDynamic(); because I will be mixing in many traits, and I don't want to remember to call half a dozen initFoo() methods.
• Using DelayedInit. This doesn't work with traits, at least according to the scaladocs.

I could live with a single init() call, but only under the following conditions:

• all mixed in traits can easily declare to be invoked in this one single call
• it is a compile error to forget the init() statement.

解决方案

You can delay the initialization of a trait by by using early definitions. (See section 5.1.6 of the scala language specification)

class Foo extends {
  protected val component = new Component
} with DynamicComponent

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