如何定义使用先前值来确定Knockout中的类的自定义绑定？ [英] How to define a custom binding who use previous value to determine class in Knockout?

问题描述

我需要使用knockout绑定一个表，如果新值高于或低于之前的值，我希望表格单元获得不同的css类。

<我想到了不同的可能性，例如将之前的值存储在bindingContext中并且有一个返回正确类的函数，但是是否可以添加一个自定义绑定处理程序来接收先前的值和新值？

解决方案

虽然 Jeff 和Sławomir的答案会有效，我找到了一个不需要对其进行任何更改的替代方案。视图模型也不依赖于改变DOM元素对象。

  function subscribeToPreviousValue（observable，fn）{
 observable.subscribe （fn，this，'beforeChange'）; 
} 
 
 ko.bindingHandlers ['bindingWithPrevValue'] = {
 init：function（element，valueAccessor）{
 var observable = valueAccessor（）; 
 var current = observable（）; 
 
 console.log（'初始值为'，当前）; 
 
 subscribeToPreviousValue（observable，function（previous）{
 console.log（'value from'，previous，'to'，current）; 
}）; 
} 
}; 
  

当然，这只有在绑定属性是可观察的情况下才有效。

I need to bind a table with knockout, and I would like the table cell to get a different css class if the new value is higher or lower of the previous.

I have in mind different possibilities, such as storing the previous value in the bindingContext and have a function which returns the right class, but is it possible to add a custom binding handler which receives the previous value and the new value?

解决方案

Although Jeff's and Sławomir's answers would work, I found an alternative that doesn't need any change to the view model nor relies on altering the DOM element object.

function subscribeToPreviousValue(observable, fn) {
  observable.subscribe(fn, this, 'beforeChange');
}

ko.bindingHandlers['bindingWithPrevValue'] = {
  init: function (element, valueAccessor) {
    var observable = valueAccessor();
    var current = observable();

    console.log('initial value is', current);

    subscribeToPreviousValue(observable, function (previous) {
      console.log('value changed from', previous, 'to', current);
    });
  }
};

Naturally, that will only work if the bound property is an observable.

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