为什么在ng-show使用ng-hide可以适用于这两种情况

Question

I have gone through so many questions like this but got this only solutions that ng-show by default hides the element and show it if condition is true and on the other hand ng-hide by default show the element and hide it when condition is true.

But my concern is the condition can be taken care of with ng-show or ng-hide only then why we use different things.

For example

I saw this somewhere in this code user is using ng-show and ng-hide both

<div ng-init="isShow = 'one'">
    <a href="#" ng-click="isShow == 'one' ? isShow = 'two' : isShow = 'one'">
    <div ng-show="isShow=='one'">
        If One show this
    </div>
    <div ng-hide="isShow=='one'">
        If Two show this
    </div>
</div>

But according to me this can be achieved also with this code

<div ng-init="isShow = 'one'">
    <a href="#" ng-click="isShow == 'one' ? isShow = 'two' : isShow = 'one'">
    <div ng-show="isShow=='one'">
        If One show this
    </div>
    <div ng-show="isShow=='two'">
        If Two show this
    </div>
</div>

So what exactly is the difference between both the codes. There must be some specific difference if ng-show and ng-hide both exists. Anyone know it?

Thanks in advance!

Answer 1

I don't see any reason for this question to be downvoted - it's a valid thought. The reason is simple, though. AngularJS has "declarative" as one of its core philosophies. If 90% of the time you want an element shown, but occasionally it should be hidden, ng-hide="thatcondition" clearly indicates when it should be hdiden. If most of the time it should be HIDDEN, then ng-show="thatrarecondition" is more readable.

Clear, readable code is an important principle in any framework, and especially in AngularJS. The ! operator is narrow and easily missed, far more than any of the other comparisons like >, <, >=, <= , etc. Providing positive- and negative-visibility operators makes it much more readable what's going on here.

An important detail to note is that both directives look for "truthy" values, not exactly-equal ones. JS is pretty vague about this, and sometimes that's an advantage. For example, suppose you have an object that may have a sub-object (a detail element). You might have the detail-display DIV written as follows:

<div ng-show="{{ object.details }}">
    <!-- Render object.details here -->
</div>

This "truthy" comparison is also handy for the negative case. Suppose you want to HIDE an order-cancellation block in a sales system if the order has been shipped. Consider:

<div ng-hide="{{ order.shipped }}">
   Want to cancel this order? <a href="...">click here</a>
</div>

Why is this important? Well, it means ANY non-undefined/null value for order.shipped will hide this block! That means if today, you set it as a true/false, it will work. But tomorrow you change it to a DATE that the order was shipped? The rule will still work! This makes it easy to code (and maintain) displays like this.

Answer 2

This is actually pretty good explained in the docs. AngularJS ngShow

Here's a quote from the docs: "The ngShow directive shows or hides the given HTML element based on the expression provided to the ngShow attribute. The element is shown or hidden by removing or adding the .ng-hide CSS class onto the element. The .ng-hide CSS class is predefined in AngularJS and sets the display style to none (using an !important flag)."

Regarding ngHide: AngularJS ngHide

Quote: "The ngHide directive shows or hides the given HTML element based on the expression provided to the ngHide attribute. The element is shown or hidden by removing or adding the ng-hide CSS class onto the element. The .ng-hide CSS class is predefined in AngularJS and sets the display style to none (using an !important flag)."

Also check out this for short but accurate explanation about different Angular DOM handling: http://www.w3schools.com/angular/angular_htmldom.asp

So in the end they actually do the same thing. And as far as I know, you should not use them in combination. If you would like to create multiple boolean values as parameters to either one of them you could do it like this: <div ng-show="value1 && !value2">Something</div> . Still I suggest that if you need more paramter values you should go with a function.

<div ng-show="ShowMe()">Content</div>

$scope.ShowMe = function(){
    return $scope.value && !$scope.value2;
}