gistfile1.js

Controller:
Todos.popupController = SC.Object.create(
   popupShowing: false,
   
   showPopup: function(){ this.set('popupShowing', true) },
   hidePopup: function(){ this.set('popupShowing', false) }
  // ... //
}) ;


View:

popup: SC.PanelPane.create({
        layout: { width: 400, height: 200, centerX: 0, centerY: 0 },
        visibileBinding: 'Todos.popupController.popupShowing', // doesn't work.
        contentView: SC.View.extend({
          layout: { top: 0, left: 0, bottom: 0, right: 0 },
          childViews: 'labelView inputView cancelButton addButton'.w(),

          labelView: SC.LabelView.extend({
            layout: { centerY: -30, height: 24, left: 0, right: 0 },
            textAlign: SC.ALIGN_CENTER,
            controlSize: SC.LARGE_CONTROL_SIZE,
            value: "Please enter a description"
          }),
          
          inputView: SC.TextFieldView.extend({
            layout: { centerY: 0, height: 24, left: 20, right: 20 },
            valueBinding: 'Todos.popupController.description'
          }),

          cancelButton: SC.ButtonView.extend({
            layout: { width: 80, bottom: 20, height: 21, right: 110 },
            title: "Cancel",
            action: "remove",
            target: "Todos.popupController.popup"
          }),
          
          addButton: SC.ButtonView.extend({
            layout: { width: 80, bottom: 20, height: 21, right: 20 },
            title: "Add",
            action: "hideAndAdd",
            target: "Todos.popupController"
          })
        })