javascript - Knockout.js 使用拦截器扩展值绑定(bind)

Question

这似乎是一种在绑定(bind)到输入字段时使用 knockout 来清理/验证/格式化数据的常用方法，它创建了一个使用计算可观察值的可重用自定义绑定(bind)。它基本上扩展了默认值绑定(bind)以包含一个拦截器，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
};

jsFiddle 示例:http://jsfiddle.net/6wxb5/1/

我错过了什么吗？我看到到处都在使用这种方法，但它似乎并不完全有效。读取函数似乎完全没用，因为它根本没有被使用过。在写入函数中，输入“23.0000”会将写入值更改为 23，但文本框值不会更新。

Answer 1

问题来自自定义绑定(bind)的 update 部分。这部分将根据原始模型值更新字段。因此，附加在 init 中的事件处理程序将通过您的可写计算发送新值，但字段的更新实际上发生在 update 中。

一种选择是从您的 init 函数应用值绑定(bind)并跳过 update 函数，例如:

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 fiddle :http://jsfiddle.net/rniemeyer/Sr8Ev/