a simplified code example of how Discord's Android app maintains panel states through device rotations while applying view updates before onLayout

PanelsPreLayoutViewUpdates.kt

class OverlappingPanelsLayout: FrameLayout {
  private lateinit var startPanel: View
  private lateinit var centerPanel: View
  private lateinit var endPanel: View
  private var pendingUpdate: (() -> Unit)? = null
  
  ...
  
  private fun openStartPanel() {
    
    // This can get called before onLayout() where centerPanel
    // gets initialized. If that happens, save the pendingUpdate
    // for after centerPanel gets initialized.
    if (!::centerPanel.isInitialized) {
      pendingUpdate = { openStartPanel(isFling) }
      return
    }
    updateCenterPanelX(x = startPanelOpenedCenterPanelX)
  }

  override fun onLayout(
    changed: Boolean,
    left: Int,
    top: Int,
    right: Int,
    bottom: Int
  ) {
    super.onLayout(changed, left, top, right, bottom)
    // OverlappingPanelsLayout expects exactly three child
    // views where each child view is a panel. If there are
    // not exactly three child views, OverlappingPanelsLayout will
    // throw an exception.
    if (!::centerPanel.isInitialized) {
      startPanel = getChildAt(0)
      centerPanel = getChildAt(1)
      endPanel = getChildAt(2)
      pendingUpdate?.invoke()
      pendingUpdate = null
      
      ...
    }
  }
}

epic