为什么指令 ng-href 需要 {{}} 而其他指令不需要? [英] Why does the directive ng-href need {{}} while other directives don't?

问题描述

我只是想知道为什么我需要为 ng-href 添加双花括号而其他一些指令不需要它们?

I am just wondering why I need to add double curly braces for ng-href while some of the other directives don't need them?

<a ng-href="{{myScopeVar}}" ng-if="myScopeVar">link</a>

注意 ng-href 需要大括号而 ng-if 不需要.

Notice that ng-href needs braces while ng-if doesn't.

推荐答案

我不太确定你的问题.但我认为您想知道为什么 angular 指令中有不同的语法样式.首先，请看看这篇文章:angular JS中双花括号和单花括号的区别? 答案解释了{{}}、{}和不带花括号的区别.

I am not quite sure about your question. But i think your are wondering why there are different syntax styles in angular directives. First off all, please have a look at this post: Difference between double and single curly brace in angular JS? The answer explains the difference between {{}}, {} and no braces.

对于您的具体示例，如文档所述:ng-href 需要一个模板(任何可以包含 {{}} 标记的字符串)，而 ng-if 需要一个表达式 - 例如您可能不会编写 {{}}，因为 angular 会对其进行评估.

For your concrete examples, as in the documentation stated: ng-href requires a template (any string which can contain {{}} markup), while ng-if requires an expression - e.g. you may not write {{}}, because angular evaluates it.

如果您查看 Angular 源，您会看到 ng-href 使用 attr.$observe 而 ng-if 使用 $scope.$watch 函数.$observe 会在属性值的每次更改时调用.$watch 被调用，当表达式结果为一个新值时.

If you have a look at the angular sources you will see, that ng-href uses attr.$observe while ng-if uses the $scope.$watch function. $observe is caled on every change of the attribute value. $watch is called, when the expression results to a new value.

但是为什么会有这两种不同的方式呢?我认为有一点是更容易使用和代码可读性.现在你可以写:

But why these two different ways? I think one point is easier usage and code readability. Right now you can write:

<a ng-href="http://yourdomain.com/users/{{userId}}/post/{{postId}}">title</a>

如您所见，您只需为动态插入的 userId 和 postId 值编写表达式.如果 ng-href 也将使用我们必须编写的 $watch 函数(不要这样做，因为它不起作用 - 它只演示了区别):

As you can see, you only write an expression for dynamically inserted userId and postId values. If ng-href would also use the $watch function we had to write (don't do this because it wil not work - it only demonstrates the difference):

<a ng-href="'http://yourdomain.com/users/'+userId+'/post'+postId">title</a>

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