vant-weapp/dist/mixins/observer/behavior.js

export var behavior = Behavior({
  created: function created() {
    var _this = this;

    if (!this.$options) {
      return;
    }

    var cache = {};

    var _this$$options = this.$options(),
        computed = _this$$options.computed;

    var keys = Object.keys(computed);

    this.calcComputed = function () {
      var needUpdate = {};
      keys.forEach(function (key) {
        var value = computed[key].call(_this);

        if (cache[key] !== value) {
          cache[key] = needUpdate[key] = value;
        }
      });
      return needUpdate;
    };
  },
  attached: function attached() {
    this.set();
  },
  methods: {
    // set data and set computed data
    set: function set(data, callback) {
      if (data) {
        this.setData(data, callback);
      }

      if (this.calcComputed) {
        this.setData(this.calcComputed());
      }
    }
  }
});