Javascript Knockout.js使用拦截器扩展值绑定

这似乎是一种常用的清除/验证/格式化数据的方法，当绑定到输入字段时，它会创建一个可重用的自定义绑定，该绑定使用计算的可观察值。它基本上扩展了默认值绑定，以包括一个拦截器，该拦截器将在写入/读取之前格式化/清理/验证输入

ko.bindingHandlers.amountValue = {
  init: function (element, valueAccessor, allBindingsAccessor) {
    var underlyingObservable = valueAccessor();

    var interceptor = ko.computed({
      read: function () {
        // this function does get called, but it's return value is not used as the value of the textbox.
        // the raw value from the underlyingObservable is still used, no dollar sign added. It seems like 
        // this read function is completely useless, and isn't used at all
        return "$" + underlyingObservable();
      },

      write: function (newValue) {
        var current = underlyingObservable(),
            valueToWrite = Math.round(parseFloat(newValue.replace("$", "")) * 100) / 100;

        if (valueToWrite !== current) {
          // for some reason, if a user enters 20.00000 for example, the value written to the observable
          // is 20, but the original value they entered (20.00000) is still shown in the text box.
          underlyingObservable(valueToWrite);
        } else {
          if (newValue !== current.toString())
            underlyingObservable.valueHasMutated();
        }
      }
    });

    ko.bindingHandlers.value.init(element, function () { return interceptor }, allBindingsAccessor);
  },

  update: ko.bindingHandlers.value.update
};

JSFIDLE示例：

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我错过什么了吗？我在任何地方都见过这种方法，但它似乎并不完全有效。read函数似乎完全没有用处，因为它根本没有被使用过。在write函数中，输入“23.0000”会将写入值更改为23，但文本框值不会更新。

问题来自自定义绑定的

更新部分。此部件将基于原始模型值更新字段。因此，
init
中附加的事件处理程序将通过可写计算程序发送新值，但字段的更新实际上发生在
update
中

一个选项是应用
init
函数中的值绑定，并跳过
update
函数，如：

ko.bindingHandlers.amountValue = {
  init: function (element, valueAccessor, allBindingsAccessor) {
    var underlyingObservable = valueAccessor();

    var interceptor = ko.computed({
      read: function () {
        // this function does get called, but it's return value is not used as the value of the textbox.
        // the raw value from the underlyingObservable, or the actual value the user entered is used instead, no   
        // dollar sign added. It seems like this read function is completely useless, and isn't used at all
        return "$" + underlyingObservable();
      },

      write: function (newValue) {
        var current = underlyingObservable(),
            valueToWrite = Math.round(parseFloat(newValue.replace("$", "")) * 100) / 100;

        if (valueToWrite !== current) {
          // for some reason, if a user enters 20.00000 for example, the value written to the observable
          // is 20, but the original value they entered (20.00000) is still shown in the text box.
          underlyingObservable(valueToWrite);
        } else {
          if (newValue !== current.toString())
            underlyingObservable.valueHasMutated();
        }
      }
    });

      ko.applyBindingsToNode(element, { value: interceptor });
  }
};

更新的小提琴：
谢谢，这让人沮丧；）