Function pushTarget

vue.js:1046–1053

(_target)

    var targetStack = [];

    function pushTarget(_target) {
        //target 是Watcher dep就是dep对象
        if (Dep.target) { //静态标志 Dep当前是否有添加了target
            //添加一个pushTarget
            targetStack.push(Dep.target);
        }
        Dep.target = _target;
    }

    //
    function popTarget() {