Controls (input, select, textarea) are ways for a user to enter data. A Form is a collection of controls for the purpose of grouping related controls together.

Form and controls provide validation services, so that the user can be notified of invalid input before submitting a form. This provides a better user experience than server-side validation alone because the user gets instant feedback on how to correct the error. Keep in mind that while client-side validation plays an important role in providing good user experience, it can easily be circumvented and thus can not be trusted. Server-side validation is still necessary for a secure application.

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The key directive in understanding two-way data-binding is ngModel. The ngModel directive provides the two-way data-binding by synchronizing the model to the view, as well as view to the model. In addition it provides an API for other directives to augment its behavior.

Note that novalidate is used to disable browser's native form validation.

The value of ngModel won't be set unless it passes validation for the input field. For example: inputs of type email must have a value in the form of user@domain.

Using CSS classes

To allow styling of form as well as controls, ngModel adds these CSS classes:

• ng-valid: the model is valid
• ng-invalid: the model is invalid
• ng-valid-[key]: for each valid key added by $setValidity
• ng-invalid-[key]: for each invalid key added by $setValidity
• ng-pristine: the control hasn't been interacted with yet
• ng-dirty: the control has been interacted with
• ng-touched: the control has been blurred
• ng-untouched: the control hasn't been blurred
• ng-pending: any $asyncValidators are unfulfilled

The following example uses the CSS to display validity of each form control. In the example both user.name and user.email are required, but are rendered with red background only after the input is blurred (loses focus). This ensures that the user is not distracted with an error until after interacting with the control, and failing to satisfy its validity.

Binding to form and control state

A form is an instance of FormController. The form instance can optionally be published into the scope using the name attribute.

Similarly, an input control that has the ngModel directive holds an instance of NgModelController. Such a control instance can be published as a property of the form instance using the name attribute on the input control. The name attribute specifies the name of the property on the form instance.

This implies that the internal state of both the form and the control is available for binding in the view using the standard binding primitives.

This allows us to extend the above example with these features:

• Custom error message displayed after the user interacted with a control (i.e. when $touched is set)
• Custom error message displayed upon submitting the form ($submitted is set), even if the user didn't interact with a control

Custom model update triggers

By default, any change to the content will trigger a model update and form validation. You can override this behavior using the ngModelOptions directive to bind only to specified list of events. I.e. ng-model-options="{ updateOn: 'blur' }" will update and validate only after the control loses focus. You can set several events using a space delimited list. I.e. ng-model-options="{ updateOn: 'mousedown blur' }"

If you want to keep the default behavior and just add new events that may trigger the model update and validation, add "default" as one of the specified events.

I.e. ng-model-options="{ updateOn: 'default blur' }"

The following example shows how to override immediate updates. Changes on the inputs within the form will update the model only when the control loses focus (blur event).

Non-immediate (debounced) model updates

You can delay the model update/validation by using the debounce key with the ngModelOptions directive. This delay will also apply to parsers, validators and model flags like $dirty or $pristine.

I.e. ng-model-options="{ debounce: 500 }" will wait for half a second since the last content change before triggering the model update and form validation.

If custom triggers are used, custom debouncing timeouts can be set for each event using an object in debounce. This can be useful to force immediate updates on some specific circumstances (like blur events).

I.e. ng-model-options="{ updateOn: 'default blur', debounce: { default: 500, blur: 0 } }"

If those attributes are added to an element, they will be applied to all the child elements and controls that inherit from it unless they are overridden.

This example shows how to debounce model changes. Model will be updated only 250 milliseconds after last change.

Custom Validation

Angular provides basic implementation for most common HTML5 input types: (text, number, url, email, date, radio, checkbox), as well as some directives for validation (required, pattern, minlength, maxlength, min, max).

With a custom directive, you can add your own validation functions to the $validators object on the ngModelController. To get a hold of the controller, you require it in the directive as shown in the example below.

Each function in the $validators object receives the modelValue and the viewValue as parameters. Angular will then call $setValidity internally with the function's return value (true: valid, false: invalid). The validation functions are executed every time an input is changed ($setViewValue is called) or whenever the bound model changes. Validation happens after successfully running $parsers and $formatters, respectively. Failed validators are stored by key in ngModelController.$error.

Additionally, there is the $asyncValidators object which handles asynchronous validation, such as making an $http request to the backend. Functions added to the object must return a promise that must be resolved when valid or rejected when invalid. In-progress async validations are stored by key in ngModelController.$pending.

In the following example we create two directives:

• An integer directive that validates whether the input is a valid integer. For example, 1.23 is an invalid value, since it contains a fraction. Note that we validate the viewValue (the string value of the control), and not the modelValue. This is because input[number] converts the viewValue to a number when running the $parsers.

• A username directive that asynchronously checks if a user-entered value is already taken. We mock the server request with a $q deferred.

Modifying built-in validators

Since Angular itself uses $validators, you can easily replace or remove built-in validators, should you find it necessary. The following example shows you how to overwrite the email validator in input[email] from a custom directive so that it requires a specific top-level domain, example.com to be present. Note that you can alternatively use ng-pattern to further restrict the validation.

Implementing custom form controls (using ngModel)

Angular implements all of the basic HTML form controls (input, select, textarea), which should be sufficient for most cases. However, if you need more flexibility, you can write your own form control as a directive.

In order for custom control to work with ngModel and to achieve two-way data-binding it needs to:

• implement $render method, which is responsible for rendering the data after it passed the NgModelController.$formatters,
• call $setViewValue method, whenever the user interacts with the control and model needs to be updated. This is usually done inside a DOM Event listener.

See $compileProvider.directive for more info.

The following example shows how to add two-way data-binding to contentEditable elements.