code: function (arg, classId, callback) {
    var view = inView('.' + classId)
    arg === 'on' ? view.on('enter', callback.enter).on('exit', callback.exit) :
    arg === 'once' ? view.once('enter', callback.enter).once('exit', callback.exit) :
    console.warn('[in-view] event handler not found')
  }
docs: (arg → string, classId → string, callback → function)
  - register inView handler