Question

嗨有没有办法在实例化ko.computed

时触发该函数

示例是

我有这个ko.computed

ko.computed(function(){ alert(this.Test); } , this);

所以基本上如果我实例化这个计算，这将触发那里定义的函数有没有办法在实例化时解雇它？并且只有在依赖性改变时才激活它？

Answer 1

您需要设置deferEvaluation option：

deferEvaluation - 可选。如果此选项为true，则在实际尝试访问其值或手动订阅它之前，不会计算计算的observable的值。默认情况下，计算的observable在创建过程中会立即确定其值。

ko.computed(function(){ alert(this.Test); } , this, { deferEvaluation: true });

Answer 2

你也可以使用淘汰赛pureComputed。纯计算仅在存在依赖时评估。

示例：

var person, fullName,
    getFullName = function() { 
       return person().firstName() + " " + person().lastName();
    };

person = ko.observable(null);

// This won't work, because the computed evaluates and getFullName doesn't 
// check for `null`
try {
  fullName = ko.computed(getFullName);
} catch (err) {
  console.log("Regular computed:");
  console.log(err.message);
}

// This will work because there's no dependency to `fullName` yet
// At `applyBindings`, `fullName` needs to be evaluated
fullName = ko.pureComputed(getFullName);

person({
  firstName: ko.observable("Jane"),
  lastName: ko.observable("Doe")
});

ko.applyBindings({ heading: fullName })

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.2/knockout-min.js"></script>

<h3 data-bind="text: heading"></h3>

ko.computed在实例化时不触发函数

2 个答案: