Knockout:限制可观察字段中的字符数

Question

我的淘汰视图模型中有一个可观察的名称字段.现在我想限制该字段中的字符数,如果它超过一定数量.

好像名字="约翰·史密斯",我有6个字符的限制,然后
显示"john s ..."

Answer 1

另一种可重用的解决方案是创建一个自定义绑定,显示文本的剪裁版本.

这允许基础值保持不受影响,但修剪文本以用于显示目的.这对于消息预览或将数据拟合到网格列中非常有用.

示例绑定:

ko.bindingHandlers.trimLengthText = {};
ko.bindingHandlers.trimText = {
    init: function (element, valueAccessor, allBindingsAccessor, viewModel) {
        var trimmedText = ko.computed(function () {
            var untrimmedText = ko.utils.unwrapObservable(valueAccessor());
            var defaultMaxLength = 20;
            var minLength = 5;
            var maxLength = ko.utils.unwrapObservable(allBindingsAccessor().trimTextLength) || defaultMaxLength;
            if (maxLength < minLength) maxLength = minLength;
            var text = untrimmedText.length > maxLength ? untrimmedText.substring(0, maxLength - 1) + '...' : untrimmedText;
            return text;
        });
        ko.applyBindingsToNode(element, {
            text: trimmedText
        }, viewModel);

        return {
            controlsDescendantBindings: true
        };
    }
};

像这样使用它:

<div data-bind="trimText: myText1"></div>

要么...

<div data-bind="trimText: myText1, trimTextLength: 10"></div>

见小提琴

Answer 2

<span data-bind="text: (name.length > 6 ? name().substring(0, 5) + '...' : name)"></span>

或者,您可以在ViewModel中创建计算的observable,例如:

var self = this;

this.shortName = ko.computed(function() {
   return (self.name.length > 6 ? self.name().substring(0, 5) + '...' : self.name);
});

然后:

<span data-bind="text: shortName"></span>

Answer 3

如果您有一个具有最大长度的字段,Chris Dixon的解决方案是完美的.但是如果你不得不多次重复这个操作就会变得很麻烦.那时你应该编写一个自定义的可观察扩展器,如下所示:

ko.extenders.maxLength = function(target, maxLength) {
    //create a writeable computed observable to intercept writes to our observable
    var result = ko.computed({
        read: target,  //always return the original observables value
        write: function(newValue) {
            var current = target(),
                valueToWrite = newValue ? newValue.substring(0, Math.min(newValue.length, maxLength)) : null;

            //only write if it changed
            if (valueToWrite !== current) {
                target(valueToWrite);
            } else {
                //if the rounded value is the same, but a different value was written, force a notification for the current field
                if (newValue !== current) {
                    target.notifySubscribers(valueToWrite);
                }
            }
        }
    });

    //initialize with current value to make sure it is rounded appropriately
    result(target());

    //return the new computed observable
    return result;
};

然后,您可以在任何可观察对象上使用它,并且可以为它们中的任何一个指定不同的最大长度.这消除了HTML(解决方案1)中的混乱,以及编写计算的可观察量(解决方案2)的必要性.您只需按以下方式定义您的observable:

this.shortName = ko.observable().extend({ maxLength: 25 });