我在这里做错了计算？

Question

此demo工作正常，但当我将其更改为此demo时：

ko.bindingHandlers.limitCharacters = {
    update: function(element, valueAccessor, allBindingsAccessor, viewModel)
    {
        var allowedNumberOfCharacters = valueAccessor();
        var currentValue = allBindingsAccessor.get('value');
        var cutText = ko.unwrap(currentValue).substr(0, allowedNumberOfCharacters);
        currentValue(cutText);
    }
};

var viewModel = {
    comment : ko.observable(""),
    count : ko.computed(function(){
        var countNum = 20 - viewModel.comment().length;
        return countNum;
    })
};
ko.applyBindings(viewModel);

DEMO not working

Answer 1

如果您坚持使用an object literal作为视图模型，则需要设置deferEvaluation to true on your computed以访问计算内的viewModel：

var viewModel = {
    comment : ko.observable(""),
    count : ko.computed(function(){
        var countNum = 20 - viewModel.comment().length;
        return countNum;
    }, null,  { deferEvaluation: true })
};

演示JSFiddle。

或者在创建viewModel对象后声明计算：

var viewModel = {
    comment: ko.observable(""),
};
viewModel.count = ko.computed(function () {
    var countNum = 20 - viewModel.comment().length;
    return countNum;
});

演示JSFiddle。