val platformTypes = Seq("ANDROID", "IOS", "WEB")

type Seq[+A] = scala.collection.Seq[A]
val Seq = scala.collection.Seq

trait Seq[+A] extends PartialFunction[Int, A] //支持偏函数
                      with Iterable[A]//支持迭代器
                      with GenSeq[A]//可并行操作的所有序列的共同特质
                      with GenericTraversableTemplate[A, Seq]//常规集合类的伴随对象的模板特质，代表一种无约束的更高等级的类型
                      with SeqLike[A, Seq[A]] {//Seq[A]的模板特质，Seq主要操作的实现
   
   //构建该类的实例的工厂伴生对象
   //指定伴生对象，实际Seq()是调用的GenericCompanion的apply()方法
  override def companion: GenericCompanion[Seq] = Seq

  //类型
  override def seq: Seq[A] = this
}

/** 
 *  类似工厂方法，Coll的当前默认实现是List
 *  @define coll sequence
 *  @define Coll `Seq`
 */
object Seq extends SeqFactory[Seq] {//SeqFactory是Seq及其子类的伴生对象模板，仅有unapplySeq方法在模式匹配{case Seq(…)=>}中调用
  /**GenTraversableFactory是Traversable及其子类的伴随对象的模板。此类提供了一组用于创建$Coll对象的操作。它通常由Traversable子类的伴随对象继承。当然这里是间接的*/
  //每个集合的伴生对象都定义了隐式方法canBuildFrom
  implicit def canBuildFrom[A]: CanBuildFrom[Coll, A, Seq[A]] = ReusableCBF.asInstanceOf[GenericCanBuildFrom[A]]
  //Seq的构造器，默认使用的是不可变的Seq(使用构造器生成一个Seq)
  def newBuilder[A]: Builder[A, Seq[A]] = immutable.Seq.newBuilder[A]//然而最底层实现其实是一个可变的ListBuffer(继承了Builder特质)
}

/** Explicit instantiation of the `Seq` trait to reduce class file size in subclasses. */
abstract class AbstractSeq[+A] extends AbstractIterable[A] with Seq[A]

def to[Col[_]](implicit cbf: CanBuildFrom[Nothing, A, Col[A @uV]]): Col[A @uV] = {
  val b = cbf()
  b ++= seq
  b.result()
}

abstract class GenericCompanion[+CC[X] <: GenTraversable[X]] {
  /** The underlying collection type with unknown element type */
  protected[this] type Coll = CC[_]

  def newBuilder[A]: Builder[A, CC[A]]

  def empty[A]: CC[A] = newBuilder[A].result()

  //使用Seq中的定义def newBuilder[A]: Builder[A, Seq[A]] = immutable.Seq.newBuilder[A]实现来生成Seq对象实例
  def apply[A](elems: A*): CC[A] = {
    if (elems.isEmpty) empty[A]
    else {
      val b = newBuilder[A]
      //使用builder存储元素，并返回结果
      b ++= elems
      b.result()
    }
  }
}