Vue watch原理源码层深入讲解

<template>
  <div>
    {{a}}
    <button @click="addModule">新增</button>
  </div>
</template>
<script>
export default {
  name: "TestWebpackTest",
  mounted() {
    console.log(this);
  },
  data() {
    return {
      num: 1,
      a:2
    };
  },
  watch:{
    a: function (val, oldVal) {
      console.log(val, oldVal)
    },
  },  
  methods: {
    addModule() {
      this.a++;
    }
  }
};
</script>
<style lang="scss">
div {
  .test {
    width: 10px;
    height: 15px;
    background-color: blue;
  }
}
</style>

function initState(vm) {
  var opts = vm.$options;
  ...
  if (opts.watch && opts.watch !== nativeWatch) {
    initWatch(vm, opts.watch);
  }
}
...
function initWatch(vm, watch) {
  for (var key in watch) {
    var handler = watch[key];
    if (isArray(handler)) {
      for (var i = 0; i < handler.length; i++) {
        createWatcher(vm, key, handler[i]);
      }
    }
    else {
      createWatcher(vm, key, handler);
    }
  }
}
...
function createWatcher(vm, expOrFn, handler, options) {
   if (isPlainObject(handler)) {
     options = handler;
     handler = handler.handler;
   }
   if (typeof handler === 'string') {
     handler = vm[handler];
   }
   return vm.$watch(expOrFn, handler, options);
 }

Vue.prototype.$watch = function (expOrFn, cb, options) {
  var vm = this;
  ...
  options = options || {};
  options.user = true;
  var watcher = new Watcher(vm, expOrFn, cb, options);
  ...
  return function unwatchFn() {
    watcher.teardown();
  };
};

function Watcher(vm, expOrFn, cb, options, isRenderWatcher) {
      ...
      this.dirty = this.lazy; // for lazy watchers
      this.deps = [];
      this.newDeps = [];
      ...
      // expOrFn = 'a'
      if (isFunction(expOrFn)) {
        this.getter = expOrFn;
      }
      else {
        this.getter = parsePath(expOrFn);
        ...
      }
      this.value = this.lazy ? undefined : this.get();
    }

function parsePath(path) {
  ...
  // ['a']
  var segments = path.split('.');
  return function (obj) {
    for (var i = 0; i < segments.length; i++) {
      if (!obj)
        return;
      obj = obj[segments[i]];
    }
    return obj;
  };
}

Watcher.prototype.get = function () {
  pushTarget(this);
  var value;
  var vm = this.vm;
  try {
    value = this.getter.call(vm, vm);
  }
  catch (e) {
   ...
  }
  finally {
    ...
    popTarget();
    this.cleanupDeps();
  }
  return value;
};

// obj = 'vm' segments = ['a']
function (obj) {
  for (var i = 0; i < segments.length; i++) {
     if (!obj)
       return;
     obj = obj[segments[i]];
   }
   return obj;
 }

Dep.prototype.notify = function (info) {
  // stabilize the subscriber list first
  var subs = this.subs.slice();
  ...
  for (var i = 0, l = subs.length; i < l; i++) {
    if (info) {
      var sub = subs[i];
      sub.onTrigger &&
        sub.onTrigger(__assign({ effect: subs[i] }, info));
    }
    subs[i].update();
  }
};
Watcher.prototype.update = function () {
 // this.lazy = false
  if (this.lazy) {
    this.dirty = true;
  }
  ...
  else {
    queueWatcher(this);
  }
};

Watcher.prototype.run = function () {
  if (this.active) {
    var value = this.get();
    if (value !== this.value ||
      // Deep watchers and watchers on Object/Arrays should fire even
      // when the value is the same, because the value may
      // have mutated.
      isObject(value) ||
      this.deep) {
      // set new value
      var oldValue = this.value;
      this.value = value;
      if (this.user) {
        var info = "callback for watcher \"".concat(this.expression, "\"");
        // this.cb是watch方法a的函数
        invokeWithErrorHandling(this.cb, this.vm, [value, oldValue], this.vm, info);
      }
      else {
        this.cb.call(this.vm, value, oldValue);
      }
    }
  }
};

function invokeWithErrorHandling(handler, context, args, vm, info) {
  var res;
  try {
    res = args ? handler.apply(context, args) : handler.call(context);
    if (res && !res._isVue && isPromise(res) && !res._handled) {
      res.catch(function (e) { return handleError(e, vm, info + " (Promise/async)"); });
      res._handled = true;
    }
  }
  catch (e) {
    handleError(e, vm, info);
  }
  return res;
}