问题描述
我可以用两种不同的方式编写一个敲除js数据绑定表达式:
I can write a knockoutjs data binding expression in two different ways:
1. <div data-bind="text: FirstName"></div>
2. <div data-bind="text: FistName()"></div>
在第二个示例中，请注意FirstName之后的两个括号.他们俩似乎都工作.有区别吗?
Note the two parens after FirstName in the second example. They both seem to work. Is there a difference?
推荐答案
有区别.
仅当FirstName是ko.observable时，它们都将同时起作用.如果它是一个普通值，则只有第一个变体将起作用.
They will only work both if FirstName is a ko.observable. If it is a plain value, only the first variant will work.
可观察对象不直接保留其值.它是提供对值的访问的功能.因此必须调用以获取值(即严格正确的是text: FistName()).
An observable does not hold its value directly. It is a function that provides access to the value. Therefore it must be called to get the value (i.e., strictly correct is text: FistName()).
但是淘汰赛是有帮助的，并且可以预见.因此，如果您自己未执行此操作，它将为您调用可观察对象.
But knockout is helpful and anticipates that. Therefore it will call the observable for you if you did not do it yourself.
对此负责的是辅助函数ko.utils.unwrapObservable()，该函数带有一个参数，确定它是可观察值还是纯值，并在两种情况下均返回其值.
Responsible for this is the helper function ko.utils.unwrapObservable(), which takes a parameter, determines whether it is an observable or a plain value and returns its value in both cases.
淘汰赛在您定义的每个绑定上都使用此功能，因此，您自己调用可观察对象(text: FistName())还是让淘汰赛在后台进行(text: FirstName)都没关系.
Knockout uses this function on every binding you define, so it does not matter if you call the observable yourself (text: FistName()) or let knockout do it behind the scenes (text: FirstName).
古怪:
敲除解析绑定表达式<binding>: <expr>，并确定<expr>是简单标识符还是更复杂的标识符.
Knockout parses the binding expression <binding>: <expr> and determines whether <expr> is a simple identifier or something more complex.
只要<expr>是一个简单的标识符(如FirstName)，由于情况是明确的，敲除将自动应用unwrapObservable().
Whenever <expr> is a simple identifier—like FirstName—knockout will apply unwrapObservable() automatically, because the situation is unambiguous.
但是，每当<expr>更复杂时(例如'Dear ' + FirstName)，knockout都会根据表达式构造自定义函数.现在事情变得模棱两可了，您需要自己调用可观察对象，例如:'Dear ' + FirstName().
But whenever <expr> is more complex—like 'Dear ' + FirstName—knockout constructs a custom function from the expression. Now things become ambiguous and you are required to call the observable yourself, like this: 'Dear ' + FirstName().
比较: http://jsfiddle.net/Tomalak/cU4qw/
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