Var fun

gistfile1.txt

// Base Var
alex-boron@ val base = Var(1)
base: Var[Int] with Updatable[Int] with Extractable[Int] = Var(1)@848564270

// Derived Var using map (or flatMap, etc)
alex-boron@ val mapped = base.map { x => println("applying map"); x * 2 }
mapped: Var[Int] = com.twitter.util.Var$$anon$2@5d00e30e

// Observing the derived Var causes the map to be applied for each new observation
alex-boron@ val closable = mapped.changes.respond(println)
applying map
2
closable: Closable = com.twitter.util.Closable$$anon$3@24facb47
alex-boron@ closable.close()
res6: Future[Unit] = ConstFuture(Return(()))

// Create a reference counted Var that caches the value of a Var
alex-boron@ val stable = Var.async(mapped.sample()) { up => mapped.changes.respond(up.update) }
applying map
stable: Var[Int] = com.twitter.util.Var$$anon$4@184218af

// The first observation causes the map to be applied and the result cached
alex-boron@ val closable1 = stable.changes.respond(println)
applying map
2
closable1: Closable = com.twitter.util.Closable$$anon$3@6225c800

// Subsequent observations (as long as the Var remains observed) use the cached value
alex-boron@ val closable2 = stable.changes.respond(println)
2
closable2: Closable = com.twitter.util.Closable$$anon$3@3fc261b7
alex-boron@ val closable3 = stable.changes.respond(println)
2
closable3: Closable = com.twitter.util.Closable$$anon$3@7953699e