Durandal TodoMVC - 无法为ko.computed写一个值 [英] Durandal TodoMVC - Cannot write a value to a ko.computed

问题描述

我正在尝试使用：

  function dependentObservable（）{
 if（arguments.length> ; 0）{
 if（typeof writeFunction ===function）{
 //写一个值
 writeFunction.apply（evaluateatorFunctionTarget，arguments）; 
} else {
 throw new Error（除非你指定一个'write'选项，否则不能写一个值到ko.computed，如果你想读取当前值，不要传递任何参数。 ）; 
} 
返回此; //允许链接赋值
} else {
 //读取值
 if（！_hasBeenEvaluated）
 evaluateImmediate（）; 
 ko.dependencyDetection.registerDependency（dependentObservable）; 
 return _latestValue; 
} 
} 
  

您在控制台中看到的是精简

如果你想看到属性返回的值，你必须调用它。

I'm trying to build a version of the todo app using Durandal (including Knockout + RequireJS) from a TodoMVC template. I realize that a todo app doesn't really show off the features of Durandal, but I'm on the learning path and figured it would be a good first project.

Anyway, in the process I've stumbled upon an error that I'm unable to solve (see below).

Error("Cannot write a value to a ko.computed unless you specify a 'write' option. If you wish to read the current value, don't pass any parameters.")

I've also attached an image that shows these in the console.

You can find the source code at https://github.com/robksawyer/durandal-todo. The todo viewmodel is located at https://github.com/robksawyer/durandal-todo/blob/master/viewmodels/todos.js.

Update: Most of the Knockout code is borrowed from the Knockout+Require TodoMVC project at https://github.com/tastejs/todomvc/tree/gh-pages/labs/dependency-examples/knockoutjs_require/

Thanks for your time.

解决方案

I think you're misreading the console.

For example, "allCompleted" is a property on your view model, which is declared as a dependent observable (i.e. a "computed"):

// writeable computed observable to handle marking all complete/incomplete
self.allCompleted = ko.computed({
    // -- trimmed --
});

What you're seeing in the console isn't the Cannot write a value error; it's the debug output for a computed property - i.e. its function definition. For reference, here's the function definition of a dependent observable straight from the knockout (2.2.1) source:

function dependentObservable() {
    if (arguments.length > 0) {
        if (typeof writeFunction === "function") {
            // Writing a value
            writeFunction.apply(evaluatorFunctionTarget, arguments);
        } else {
            throw new Error("Cannot write a value to a ko.computed unless you specify a 'write' option. If you wish to read the current value, don't pass any parameters.");
        }
        return this; // Permits chained assignments
    } else {
        // Reading the value
        if (!_hasBeenEvaluated)
            evaluateImmediate();
        ko.dependencyDetection.registerDependency(dependentObservable);
        return _latestValue;
    }
}

What you're seeing in your console is the minified version of this code.

If you want to see the value that's returned by the property you'd have to invoke it.

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