Vue.js watch用法及代碼示例

interface ComponentOptions {
  watch?: {
    [key: string]: WatchOptionItem | WatchOptionItem[]
  }
}

type WatchOptionItem = string | WatchCallback | ObjectWatchOptionItem

type WatchCallback<T> = (
  value: T,
  oldValue: T,
  onCleanup: (cleanupFn: () => void) => void
) => void

type ObjectWatchOptionItem = {
  handler: WatchCallback | string
  immediate?: boolean // default: false
  deep?: boolean // default: false
  flush?: 'pre' | 'post' | 'sync' // default: 'pre'
  onTrack?: (event: DebuggerEvent) => void
  onTrigger?: (event: DebuggerEvent) => void
}

為了便於閱讀，類型被簡化了。

export default {
  data() {
    return {
      a: 1,
      b: 2,
      c: {
        d: 4
      },
      e: 5,
      f: 6
    }
  },
  watch: {
    // watching top-level property
    a(val, oldVal) {
      console.log(`new: ${val}, old: ${oldVal}`)
    },
    // string method name
    b: 'someMethod',
    // the callback will be called whenever any of the watched object properties change regardless of their nested depth
    c: {
      handler(val, oldVal) {
        console.log('c changed')
      },
      deep: true
    },
    // watching a single nested property:
    'c.d': function (val, oldVal) {
      // do something
    },
    // the callback will be called immediately after the start of the observation
    e: {
      handler(val, oldVal) {
        console.log('e changed')
      },
      immediate: true
    },
    // you can pass array of callbacks, they will be called one-by-one
    f: [
      'handle1',
      function handle2(val, oldVal) {
        console.log('handle2 triggered')
      },
      {
        handler: function handle3(val, oldVal) {
          console.log('handle3 triggered')
        }
        /* ... */
      }
    ]
  },
  methods: {
    someMethod() {
      console.log('b changed')
    },
    handle1() {
      console.log('handle 1 triggered')
    }
  },
  created() {
    this.a = 3 // => new: 3, old: 1
  }
}