How to use angularjs form validation in a non-angular SPA app

Question

I wonder if anyone can help. On the surface of it, my question title may sound kind of stupid! I'm trying to use angularjs form validation in a non-angularjs application! Let me try to explain ...

We have a 'traditional' web app. Its not an SPA. The backend is java, and the java app server manages the session and the data within. Each page is a full http request/response. (There are a small number of ajax request/responses, but these are to add some bling to the page, rather than it's core functionality). In this respect, the architecture of the app is very traditional/old-skool, in that the server-side java code is responsible for generating the markup and populating form field values from it's version of the model data held in it's session store. (I think this is the crux of the problem)

The app is predominantly a HTML form based application, and to enhance the UX we have written some javascript field validators based around jQuery. For a number of reasons these have started to get a bit out of control, and we are exploring alternative options.

A simple google search finds countless jQuery plug ins for form validation. We are looking at these, but random jQuery plug ins are not our favoured approach (we tend to steer clear of 'somebloke.com' plug ins because we can't guarantee how well they're written, browser compatibility, future maintenance, how well they work with other plug ins etc - we've had our fingers burnt with this kind of thing before)

So we are looking at other approaches, and are currently exploring the use of angularjs.
Being able to 'gently introduce' angularjs into our architecture has some advantages. It's from a stable best-of-breed organisation (ie. it's not 'somebloke.com') so is well supported and maintained. It encourages us to write our js in a very modular & testable manner (our current rats nest of jQuery plugins, callbacks etc is far from that!). angularjs form validation is based around html5 standards and is declarative and semantic. It gives us a way forward for migrating our other jQuery based code to something better (angularjs directives). And overall, if we can layer angularjs into our current app, it gives us a good foundation for converting the app to a modern SPA at some point in the future.

Rewriting the entire app as an angularjs (or any other mv* framework) SPA at this point in time is not an option, so as mentioned above, we are looking at introducing small bits of functionality at a time; and today's challenge is form validation.

So, that's the background.

I've stripped out our current js client side validation, and our server-side java code is generating markup like this:

<form method="POST" action="/renew">
    <input name="firstname" type="text" value="alf" />
    <input name="surname" type="text" value="garnet" />
    <input name="age" type="number" value="88" />
    <input type="submit" />
</form>

(where the values for the input fields have been populated server-side from the model held by the server)

I've added the angularjs library to the page, and have got form validation working as follows:

<form method="POST" action="/renew" novalidate name="renewForm" 
      ng-controller="yourDetails" ng-submit="submitForm(renewForm, $event)">
    <input name="firstname" type="text" value="alf" required ng-model="firstname"/>
    <input name="surname" type="text" value="garnet" required ng-model="surname"/>
    <input name="age" type="number" value="88" required ng-model="age"/>
    <input type="submit" />
</form>

app.controller('yourDetails', function($scope) {
    $scope.submitForm = function(form, $event) {
        if (!form.$valid) {
            $event.preventDefault();
            return false;
        }
    };
});

This is a reasonable starting point. Broadly speaking it works in that angularjs is handling the form validation and submission. The submitForm method is executed, and if the form is not valid then the if block is entered and the form submission is cancelled. From here I can see it would be easy to add in the field error messages etc using ng-show etc.

The problem however is the use of ng-model on each html field. As I understand it I need to use this so that angularjs binds the field to the form, and can therefore track each fields valid status.
However, ng-model also appears to setup the 2-way data binding and sets the value of the field to it's version of the model data ... which is empty. For example:

Our server-side template might contain this:
<input th:field="*{firstname}" type="text" required ng-model="firstname"/>

Which might generate this markup:
<input name="firstname" value="alf" type="text" required ng-model="firstname"/>

The markup that gets served to the client includes value="alf"
But then angularjs steps in and sets up 2-way binding for the field. Because we don't have a firstname property in the angularjs scope, it initialises one with a blank value, and sets that blank value in the DOM of the field.
This results in the page being rendered by the browser with blank values in the fields, even though server-side we have values in the model, and the server has correctly generated the markup etc.

So, I think I understand the core problem and why it's happening. My question is, can I do angularjs form validation without the ng-model attribute on each field, or is there a version of the ng-model directive that only does 1-way binding - specifically DOM -> model

Any help would be very much appreciated;

Thanks
Nathan

Alexandre TRINDADE · Accepted Answer · 2014-04-24 09:52:14Z

1

When generating your form at server side, you can initialize your model with ng-init:

<input ng-init="firstname='alf'" th:field="*{firstname}" type="text" required ng-model="firstname" />

answered Apr 24, 2014 at 9:52

Alexandre TRINDADE

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Nathan Russell Over a year ago

Thanks for your answer, though I can't get ng-init to work as suggested. I've tried it on the input element as above, and also on the form element where the controller is defined (therefore where the scope is defined I think ?), but neither work. The angular docs suggest its only valid for ng-repeat ?? docs.angularjs.org/api/ng/directive/ngInit

Nathan Russell · Accepted Answer · 2014-04-25 08:18:36Z

OK, @Alexandre's answer was almost right, and it was his answer that pointed me in the direction of my final solution (so he should get the credit for this really :) )

ng-init does work as @Alexandre has suggested. The reason I couldn't get it to work was that I was trying to use it on a number field.

The following works because the value being set on the model with ngInit and the html input type are both text/string:

<input ng-init="firstname='alf'" th:field="*{firstname}" type="text" required ng-model="firstname" />

I was trying it on the age field as follows:

<input ng-init="age='88'" th:field="*{age}" type="number" required ng-model="age" />

This does not work because the age property was being set on the model as a string, but the html input type is a number. The following does work:

<input ng-init="age=88" th:field="*{age}" type="number" required ng-model="age" />

This led me to realise that the value being set on the angular model needs to match the data type of the html input type (certainly true of chrome, not sure about other browsers) (ie. string properties in the model - even if they are parse-able as numbers - cannot be used in a html number field with ngModel)

With this in mind, I decided there were 2 options. I could either do it server-side in the code that generates the markup:

    <input ng-init="age=(some-potentially-complex-logic-to-workout-whether-its-a-string-or-number)" th:field="*{age}" type="number" required ng-model="age" />

Or I could do it client-side with a custom directive. In the end I went with a custom directive because a) it meant I could have a go at writing a directive (all part of the learning :)) and b) I realised there might be other cases that need special consideration which might make doing it server-side even more complex (ie. select fields don't have a value, they have a selected index of which you need to get it's value; radio buttons all have a value but you only want to set the value of the checked radio)

Here's what I came up with:

angularApp.directive('lvInitializeValueOnScope', function() {
    return {
        restrict: 'A',
        link: function(scope, element, attrs, controller) {
            var propertyName = attrs.name,
                propertyValue = attrs.value,
                elementName = element.get(0).tagName.toLowerCase(),
                fieldType = ( elementName === 'input' ? attrs.type.toLowerCase() : elementName ),
                // set expression to assume propertyValue is a string value
                expression = propertyName + '=\'' + ( !!propertyValue ? propertyValue : '' ) + '\'';

            // if the input field type is number and propertyValue is parse-able as a number
            if (fieldType === 'number' && !isNaN(parseFloat(propertyValue))) {
                // set expression without quotes surrounding propertyValue
                expression = propertyName + '=' + propertyValue;
            }
            // if the field is a html select element
            if (fieldType === 'select') {
                // propertyValue will be blank because select elements don't have a value attribute
                // instead, we need to use the value of the child option element that is selected
                propertyValue = $(element.html()).filter(":selected").val();
                // set expression to assume propertyValue is a string value
                expression = propertyName + '=\'' + ( !!propertyValue ? propertyValue : '' ) + '\'';
            }
            // if the input field type is a radio button but its not checked (selected)
            if (fieldType === 'radio' && !element.is(':checked')) {
                // we need to reset the expression so a blank value is used
                // doing this means that only the checked/selected radio button values get set on the model
                expression = propertyName + '=\'\'';
            }

            // evaluate the expression, just as angular's ngInit does
            scope.$eval(expression);
        }
    };
});

(it looks more complex than it actual is because I've left the comments in - strip those out and there's really nothing to it)

To use it, each field that I am using ng-model on, I also need to use the attribute data-lv-initialize-value-on-scope. You don't need to pass any value on the attribute, the directive gets everything it needs from the element and attr parameters

So far it caters with text, number, radio and select fields. The pages I'm working on don't have any checkboxes, but as and when I come across those I dare say I'll need to add some code for those.

It works, and I think I prefer this to using ng-init with a load of server-side to determine whether to set a value, what its data type should be, etc

The only downside I can see at the moment is that processing a radio button set might be inefficient. IE. if you have 10 radio buttons, each marked up with ng-model and data-lv-initialize-value-on-scope; all 10 will run the data-lv-initialize-value-on-scope directive and set a value on the scope. At least 9 of the iterations will set a blank value on the scope, and at most only 1 will set the actual value on the scope.

Hey-ho, it seems to work for now :). Hope this helps someone in the future

Nathan

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