QML

1.ApplicationWindow  top-level window, 有menubar, header, footer item等组件，通过QQmlApplicationEngine来使用它（控制widow属性，出现以及layout）。

2.Component  用来定义resuable 组件， 可以作为views得图形组件，如ListView::delegate 提供一个component去显示每个listview. 
 MyItem.qml
 Item {
     property Component mycomponent: comp1

     QtObject {
         id: internalSettings
         property color color: "green"
     }

     Component {
         id: comp1
         Rectangle { color: internalSettings.color; width: 400; height: 50 }
     }
 }
 
 main.qml
  ListView {
     width: 400; height: 400
     model: 5
     delegate: myItem.mycomponent    //will create green Rectangles

     MyItem { id: myItem }
 }