元素生命周期 
元素从创建、添加、挂载,到移除、解除挂载、销毁的全过程,可在初始化事件中监听。
示例 
生命周期中的不同状态会通过 ChildEvent、 PropertyEvent 进行通知。
监听元素生命周期事件 
初始化方式监听
ts
// #监听元素生命周期事件 [通过 event 对象监听]
import { Leafer, Rect } from 'leafer-ui'
const leafer = new Leafer({ view: window })
const rect = new Rect({
    x: 100,
    y: 100,
    width: 200,
    height: 200,
    fill: '#32cd79',
    cornerRadius: [50, 80, 0, 80],
    draggable: true,
    event: {
        created() {
            console.log('rect created')
        },
        mounted: () => {
            console.log('mounted, leafer', rect.leafer) // 元素已挂载到 leafer 的树结构上,可以被渲染
        },
        unmounted() {
            console.log('unmounted')
        },
    }
})
leafer.add(rect)通过 on 监听
ts
// #监听元素生命周期事件 [通过 on 监听]
import { Leafer, Rect } from 'leafer-ui'
const leafer = new Leafer({ view: window })
const rect = new Rect({
    x: 100,
    y: 100,
    width: 200,
    height: 200,
    fill: '#32cd79',
    cornerRadius: [50, 80, 0, 80],
    draggable: true
})
leafer.add(rect)
rect.on('created', () => {
    console.log('rect created')
})