Widget Compiler Plugin v4

Versions & Milestones

• v1

  • The initial version. Didn't consider the return value.

• v2

  • All widgets are skippable:

    • Widget without return value

      @Widget
      fun Hi(a: String) {
          MyWidget(123, a)
      }

      into

      @Widget
      fun Hi(a: String, `$buildScope`: BuildScope, `$idState`: Long) {
          val changed = `$buildScope`.start(`$idState`, arrayOf(a))
          if(changed != 0) {
              MyWidget(
                  123, a, `$buildScope`,
                  137 /* id */ or ((
                      /* 1st */(0b10 /* unchanged */ shl 0) or
                      /* 2nd */(0b10 or (changed and 0b1 shr 0 /* delegate state from a */) shl 1)
                  ) shl 32)
              )
          }
          end()
      }

    • Widget with return value

      @Widget
      fun HelloWorld() = 3

      into

      @Widget
      fun HelloWorld(`$buildScope`: BuildScope, `$idState`: Long) {
          val changed = `$buildScope`.start(`$idState`, arrayOf())
          return if(changed != 0) run {
              3
          }.also { `$buildScope`.end(it) } else `$buildScope`.endSkip()
      }

    This compilation behavior may leak the return value and cause unexpected side effects. (fixed in v3)

  • Added special library functions: currentScope(), bridgeCall(widget).

  • Added @InlineWidget

    Compilation of this was not complete, though.

• v3

  • Added special library function nextId(): treated specially

  • divide widgets into skippable ones and else:

    Widgets with return value(not Unit) are not skippable

    • Widget without return value

         @Widget
         fun Hi(a: String) {
             MyWidget(123, a)
         }

      into

         @Widget
         fun Hi(a: String, `$buildScope`: BuildScope, `$idState`: Long) {
             // the same: skippable
         }

    • Widget with return value

         @Widget
         fun HelloWorld() = 3

      into

         @Widget
         fun HelloWorld(`$buildScope`: BuildScope, `$id`: Long /* just for compatibility reason: 32 bits are discarded */) {
             `$buildScope`.startExpr(`$id`.toInt())
             val returnValue = run { // not skippable
                 3
             }
             `$buildScope`.endExpr()
             return returnValue
         }

  • The output of @Widget inline fun ... is more simplified: inline widget fun signifies that it has @InlineWidget (actually there is no actual annotation: just meaning behavior)

• v4 (not implemented yet)

  • flat slot table: Currently building depends on children of Element.

  • make Element more private api: Implementation of flat slot table will require this.

  • optimize branches: for example, in if(...) Widget1() else Widget2(), if the condition changes, locating the element may be more expensive.

    Solution: group them as 'replaceable group'

  • support referencing widget function like: ::MyWidget

  • do not create Element instance for normal functional widgets

  • remove $id parameter: put in $buildScope.startExpr(..)

plus: Yah! Backend IR became beta stage!
https://blog.jetbrains.com/kotlin/2021/02/the-jvm-backend-is-in-beta-let-s-make-it-stable-together/