Angular表达式

【原文】https://code.angularjs.org/1.2.23/docs/guide/expression

【翻译者】kowen@live.cn

https://code.angularjs.org/1.2.23/docs/guide/expression 写道

Angular expressions are JavaScript-like code snippets that are usually placed in bindings such as {{ expression }}.
For example,these are valid expressions in Angular:

Angular表达式是看起来和javascript类似的代码片段，通常是被放到双层大括号里：{{表达式}}。

例如，下面是一些有效的Angular表达式：

Angular代码

1. 1+2

2. a+b

3. user.name

4. items[index]

Angular Expressions vs. JavaScript Expressions

Angular 表达式和 js表达式的对比

写道

Angular expressions are like JavaScript expressions with the following differences:

Angular表达式和Javascript表达式非常相似，但有以下几点不同：

Context: JavaScript expressions are evaluated against the global window. In Angular,expressions are evaluated against a scope object.

上下文：Javascript表达式在全局窗口中起作用，Angular的表达式是在scope范围内中起作用。（不太恰当？）

Forgiving: In JavaScript,trying to evaluate undefined properties generates ReferenceError or TypeError. In Angular,expression evaluation is forgiving to undefined and null.

对出错宽大处理：Javascript计算含有未定义属性的表达式会产生ReferenceError或者TypeError，Angular 表达式允许计算含有未命名或者null的表达式。

No Control Flow Statements: you cannot use the following in an Angular expression: conditionals,loops,or exceptions.

禁止流程控制语句：Angular表达式中不能包含条件、循环或者异常语句。

Filters: You can use filters within expressions to format data before displaying it.

过滤器：你可以对Angular表达式进行格式化得到要显示的结果。

写道

If you want to run more complex JavaScript code,you should make it a controller method and call the method from your view. If you want to eval() an Angular expression yourself,use the $eval() method.

如果需要运行比较复杂的Javascript语句，可以在控制器（controller）中创建一个方法，然后从视图（view）中去调用它。如果你想运行eval（）计算angular表达式，你应当使用$eval()方法。

You can try evaluating different expressions here:

下面的例子可以计算各种表达式：

index.html

Html代码

1. <spanstyle="font-size:14px;">divng-controller="ExampleController"class="expressions">

2. Expression:

3. inputtype='text'ng-model="expr"size="80"/>

4. buttonng-click="addExp(expr)">Evaluate</button>

5. ulling-repeat="exprinexprstrackby$index">

6. [ahref=""ng-click="removeExp($index)">Xa>]

7. tt>{{expr}}>=spanng-bind="$parent.$eval(expr)"spanlidiv>

script.js

Js代码

1. <spanstyle="font-size:14px;">angular.module('expressionExample',[])

2. .controller('ExampleController',['$scope',function($scope){

3. varexprs=$scope.exprs=[];

4. $scope.expr='3*10|currency';

5. $scope.addExp=function(expr){

6. exprs.push(expr);

7. };

9. $scope.removeExp=function(index){

10. exprs.splice(index,1);

11. };

12. }]);

13. </span>

Context

上下文

Angular does not use JavaScript's eval() to evaluate expressions. Instead Angular's $parse service processes these expressions.

Angular不使用Javascript的eval()计算表达式的值，Angular有自己的$parse服务来计算表达式。

Angular expressions do not have access to global variables like window,document or location. This restriction is intentional. It prevents accidental access to the global state ? a common source of subtle bugs.

Angular表达式没有访问window、doucument或者location这些全局变量的权限。这样做的目的是：防止不小心访问了相同名称的全局的变量，很多难以排除的bug都是这个原因引起的（应该是这个意思吧）。

Instead use services like $window and $location in functions called from expressions. Such services provide mockable access to globals.

那怎么访问global资源呢？有一个变通的方式：Angular表达式调用的function中，可以使用$window和$location等服务来访问global资源。例子：

divclass="example2"ng-controller="ExampleController">

Forgiving

宽大处理

Expression evaluation is forgiving to undefined and null. In JavaScript,evaluating a.b.c throws an exception if a is not an object. While this makes sense for a general purpose language,the expression evaluations are primarily used for data binding,which often look like this:

Angular表达式求值对undefined和null都采取了宽大处理的态度。在Javascript中，如果a不是一个对象，那么a.b.c将会抛出一个异常。但是这个表达式本身是有意义的，它主要是用来作数据绑定的，例如

{{a.b.c}}

It makes more sense to show nothing than to throw an exception if a is undefined (perhaps we are waiting for the server response,and it will become defined soon). If expression evaluation wasn't forgiving we'd have to write bindings that clutter the code,for example: {{((a||{}).b||{}).c}}

其实，这种情况与其抛出异常，不如直接啥也不显示（也许是正在等待服务器的相应，等一会就会变成defined了）。如果表达式不进行宽大处理的话，我们写的代码就乱套成这个样子了：{{((a||{}).b||{}).c}}

Similarly,invoking a function a.b.c() on undefined or null simply returns undefined.

类似的，调用一个包含undefined或这null的function a.b.c()，返回值是undefined。

No Control Flow Statements

禁用流程控制语句

Apart from the ternary operator (a ? b : c),you cannot write a control flow statement in an expression. The reason behind this is core to the Angular philosophy that application logic should be in controllers,not the views. If you need a real conditional,loop,or to throw from a view expression,delegate to a JavaScript method instead.

除了三元运算符（a ? b : c），angular表达式中不允许使用流程控制语句。这么做的原因是Angular秉承着这样的思想：程序的逻辑部分应该在控制器中实现，不应该在视图中出现。如果在视图中必须使用条件、循环或者抛出异常，直接使用Javascript方法吧。

$event

事件对象

Directives like ngClick and ngFocus expose a $event object within the scope of that expression.

ngClick、ngFocus类似的指令会在表达式的有效范围内产生一个$event对象。

index.html

Html代码divng-controller="EventController"buttonng-click="clickMe($event)">Eventpcode>$event>:pre>{{$event|json}}>clickEvent>{{clickEvent|json}}script.js

Js代码

Note in the example above how we can pass in $event to clickMe,but how it does not show up in {{$event}}. This is because $event is outside the scope of that binding.

注意：这个例子中，$event对象可以传递到clickMe中显示，但是它却不能使用{{$event}}显示，因为$event是在这个绑定的scope之外（也就是$event只在ngClick指定的clickMe中存在）。

（编辑：李大同）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!