WIP: Zooming into a Var should not require an Owner (#119)

src/main/scala/com/raquo/airstream/state/PureDerivedVar.scala

@@ -0,0 +1,58 @@
+package com.raquo.airstream.state
+
+import com.raquo.airstream.core.AirstreamError.VarError
+import com.raquo.airstream.core.{AirstreamError, Transaction}
+import com.raquo.airstream.ownership.Owner
+
+import scala.util.{Failure, Success, Try}
+
+// #nc Update comments
+
+/** DerivedVar has the same Var contract as SourceVar, but instead of maintaining its own state
+  * it is essentially a lens on the underlying SourceVar.
+  *
+  * This Var is active for as long as its signal has listeners.
+  * Being a StrictSignal, it already starts out with a subscription owned by `owner`,
+  * but even if owner kills its subscriptions, this Var's signal might have other listeners.
+  */
+class PureDerivedVar[A, B](
+  parent: Var[A],
+  zoomIn: A => B,
+  zoomOut: (A, B) => A,
+  displayNameSuffix: String
+) extends Var[B] {
+
+  override private[state] def underlyingVar: SourceVar[_] = parent.underlyingVar
+
+  private[this] val _varSignal = new PureDerivedVarSignal(parent, zoomIn, displayName)
+
+  // #Note this getCurrentValue implementation is different from SourceVar
+  //  - SourceVar's getCurrentValue looks at an internal currentValue variable
+  //  - That currentValue gets updated immediately before the signal (in an already existing transaction)
+  //  - I hope this doesn't introduce weird transaction related timing glitches
+  //  - But even if it does, I think keeping derived var's current value consistent with its signal value
+  //    is more important, otherwise it would be madness if the derived var was accessed after its owner
+  //    was killed
+  override private[state] def getCurrentValue: Try[B] = signal.tryNow()
+
+  override private[state] def setCurrentValue(value: Try[B], transaction: Transaction): Unit = {
+    // #nc Unlike the old DerivedVar, we don't check `_varSignal.isStarted` before updating the parent.
+    //  - Is that "natural" because we don't have an explicit "owner" here, or is that a change in semantics?
+    parent.tryNow() match {
+      case Success(parentValue) =>
+        // This can update the parent without causing an infinite loop because
+        // the parent updates this derived var's signal, it does not call
+        // setCurrentValue on this var directly.
+        val nextValue = value.map(zoomOut(parentValue, _))
+        // println(s">> parent.setCurrentValue($nextValue)")
+        parent.setCurrentValue(nextValue, transaction)
+
+      case Failure(err) =>
+        AirstreamError.sendUnhandledError(VarError(s"Unable to zoom out of derived var when the parent var is failed.", cause = Some(err)))
+    }
+  }
+
+  override val signal: StrictSignal[B] = _varSignal
+
+  override protected def defaultDisplayName: String = parent.displayName + displayNameSuffix
+}

src/main/scala/com/raquo/airstream/state/PureDerivedVarSignal.scala

@@ -0,0 +1,33 @@
+package com.raquo.airstream.state
+
+import com.raquo.airstream.core.{Protected, Signal}
+import com.raquo.airstream.misc.MapSignal
+
+import scala.util.Try
+
+class PureDerivedVarSignal[I, O](
+  parent: Var[I],
+  zoomIn: I => O,
+  parentDisplayName: => String
+) extends MapSignal[I, O](parent.signal, project = zoomIn, recover = None) with StrictSignal[O] { self =>
+
+  // Note that even if owner kills subscription, this signal might remain due to other listeners
+  // override protected[state] def isStarted: Boolean = super.isStarted
+
+  override protected def defaultDisplayName: String = parentDisplayName + ".signal"
+
+  override def tryNow(): Try[O] = {
+    val newParentLastUpdateId = Protected.lastUpdateId(parent.signal)
+    if (newParentLastUpdateId != _parentLastUpdateId) {
+      // This branch can only run if !isStarted
+      val nextValue = currentValueFromParent()
+      updateCurrentValueFromParent(nextValue, newParentLastUpdateId)
+      nextValue
+    } else {
+      super.tryNow()
+    }
+  }
+
+  override protected[state] def updateCurrentValueFromParent(nextValue: Try[O], nextParentLastUpdateId: Int): Unit =
+    super.updateCurrentValueFromParent(nextValue, nextParentLastUpdateId)
+}

src/main/scala/com/raquo/airstream/state/Var.scala

@@ -85,6 +85,10 @@ trait Var[A] extends SignalSource[A] with Sink[A] with Named {
     new DerivedVar[A, B](this, in, out, owner, displayNameSuffix = ".zoom")
   }
 
+  def zoomPure[B](in: A => B)(out: (A, B) => A): Var[B] = {
+    new PureDerivedVar[A, B](this, in, out, displayNameSuffix = ".zoomPure")
+  }
+
   def setTry(tryValue: Try[A]): Unit = writer.onTry(tryValue)
 
   final def set(value: A): Unit = setTry(Success(value))