docs: update with forms overview review feedback (#25663)
PR Close #25663
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Brandon Roberts
Kara Erickson
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# Introduction to forms in Angular
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Handling user input with forms is the cornerstone of many common applications. Applications use forms to log in, to update a profile, to enter sensitive information, and to perform many other data-entry tasks.
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Handling user input with forms is the cornerstone of many common applications. Applications use forms to enable users log in, to update a profile, to enter sensitive information, and to perform many other data-entry tasks.
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Angular provides two different approaches to handling user input through forms: reactive and template-driven. Both capture user input events from the view, validate the user input, create a form model and data model to update, and provide a way to track changes. Each approach uses a distinct way of processing forms and offers different advantages. There are many different factors such as how data flows from the view to the data model and data model to the view, predictability of access to the form model, validation and testing, mutability, and scalability that influence your decision on which approach works best for your situation. The sections below provide a description of the common building blocks with each approach and a comparison of those key factors.
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Angular provides two different approaches to handling user input through forms: reactive and template-driven. Both capture user input events from the view, validate the user input, create a form model and data model to update, and provide a way to track changes.
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Reactive and template-driven forms differ, however, in how they do the work of processing and managing forms and form data. Each offers different advantages.
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**In general:**
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* **Reactive forms** are more robust: they are more scalable, reusable, and testable. If forms are a key part of your application, or you're already using reactive patterns for building your application, use reactive forms.
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* **Template-driven forms** are useful for adding a simple form to an app, such as an email list signup form. They are easy to add to an app, but they do not scale as well as reactive forms. If you have very basic form requirements and logic that can be managed solely in the template, use template-driven forms.
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This guide provides information to help you decide which approach works best for your situation. It introduces the common building blocks used by both approaches. It also summarizes the key differences between the two approaches, and demonstrates those differences in the context of setup, data flow, and testing.
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<div class="alert is-important">
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*Note:* For complete information about each kind of form, see the [Reactive Forms](guide/reactive-forms) and [Template-driven Forms](guide/forms) guides.
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</div>
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## Key differences
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The table below summarizes the key differences in reactive and template driven forms.
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The table below summarizes the key differences between reactive and template-driven forms.
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<style>
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table {width: 100%};
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@ -20,33 +30,49 @@ The table below summarizes the key differences in reactive and template driven f
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||Reactive|Template-driven|
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|--- |--- |--- |
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|Setup/Form Model|More explicit/Created in the component class.|Less explicit/Created by the directives.|
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|Setup (form model)|More explicit created in the component class.|Less explicit, created by the directives.|
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|Data model|Structured|Unstructured|
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|Predictability|Synchronous|Asynchronous|
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|Form validation|Functions|Directives|
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|Mutability|Immutable|Mutable|
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|Scalability|Low-level API access|Abstraction on top of APIs|
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## A common foundation
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## Common foundation
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Both reactive and template-driven forms share underlying building blocks.
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- A `FormControl` instance that tracks the value and validation status of an individual form control.
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- A `FormGroup` instance that tracks the same values and statuses for a collection of form controls.
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- A `FormArray` instance that tracks the same values and statues for an array of form controls.
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- A `FormGroup` instance that tracks the same values and status for a collection of form controls.
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- A `FormArray` instance that tracks the same values and status for an array of form controls.
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- A `ControlValueAccessor` that creates a bridge between Angular `FormControl` instances and native DOM elements.
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How these control instances are created and managed with reactive and template-driven forms is introduced in the [form model](#the-form-model) section below and detailed further in the [data flow section](#data-flow-in-forms) of this guide.
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How these control instances are created and managed with reactive and template-driven forms is introduced in the [form model setup](#setup-the-form-model) section below and detailed further in the [data flow section](#data-flow-in-forms) of this guide.
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## The form model
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## Setup: The form model
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In reactive forms, the source of truth is the form model (the `FormControl` instance)
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Reactive and template-driven forms both use a form model to track value changes between Angular forms and form input elements. The examples below show how the form model is defined and created.
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### Setup in reactive forms
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Here is a component with an input field for a single control implemented using reactive forms.
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<code-example path="forms-overview/src/app/reactive/favorite-color/favorite-color.component.ts">
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</code-example>
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The source of truth provides the value and status of the form element at a given point in time. In reactive forms, the form model is source of truth. The form model in the above example is the `FormControl` instance.
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<figure>
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<img src="generated/images/guide/forms-overview/key-diff-reactive-forms.png" alt="Reactive forms key differences">
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</figure>
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With reactive forms, the form model is explicitly defined in the component class. The reactive form directive (in this case, `FormControlDirective`) then links the existing form control instance to a specific form element in the view using a value accessor. Updates from the view-to-model and model-to-view are synchronous and not dependent on the UI rendered.
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With reactive forms, the form model is explicitly defined in the component class. The reactive form directive (in this case, `FormControlDirective`) then links the existing form control instance to a specific form element in the view using a value accessor (instance of `ControlValueAccessor`).
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### Setup in template-driven forms
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Here is the same component with an input field for a single control implemented using template-driven forms.
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<code-example path="forms-overview/src/app/template/favorite-color/favorite-color.component.ts">
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</code-example>
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In template-driven forms, the source of truth is the template.
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@ -54,58 +80,54 @@ In template-driven forms, the source of truth is the template.
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<img src="generated/images/guide/forms-overview/key-diff-td-forms.png" alt="Template-driven forms key differences">
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</figure>
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The abstraction of the form model promotes simplicity over structure. It is less explicit, but you no longer have direct control over the form model. Updates from the view-to-model and model-to-view must pass through directives (in this case, the `NgModel` directive), and be asynchronous to work around the change detection cycle. Value changes are delayed until the next tick, which allows change detection to complete and a new change detection process to be triggered with the updated value.
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The abstraction of the form model promotes simplicity over structure. The template-driven form directive `NgModel` is responsible for creating and managing the form control instance for a given form element. It is less explicit, but you no longer have direct control over the form model.
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## Data flow in forms
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When building forms in Angular, it's important to understand how the the framework handles data flowing from the user or from programmatic changes. Reactive and template-driven forms follow two different strategies when handling form input. The components below use an input field to manage the *favorite color* value and to show how changes are handled.
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When building forms in Angular, it's important to understand how the the framework handles data flowing from the user or from programmatic changes. Reactive and template-driven forms follow two different strategies when handling form input. The data flow examples below begin with the favorite color input field example from above, and they show how changes to favorite color are handled in reactive forms compared to template-driven forms.
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### Data flow in reactive forms
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Here is a component with an input field for a single control implemented using reactive forms.
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<code-example path="forms-overview/src/app/reactive/favorite-color/favorite-color.component.ts">
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</code-example>
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The diagrams below show how the data flows for an input with reactive forms.
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As described above, in reactive forms each form element in the view is directly linked to a form model (FormControl instance). Updates from the view to model and model to view are synchronous and not dependent on the UI rendered. The diagrams below uses the same favorite color example to demonstrate how data flows when an input field's value is changed from the view and then from the model.
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<figure>
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<img src="generated/images/guide/forms-overview/dataflow-reactive-forms-vtm.png" alt="Reactive forms view to model data flow" width="100%">
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<img src="generated/images/guide/forms-overview/dataflow-reactive-forms-vtm.png" alt="Reactive forms data flow - view to model" width="100%">
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</figure>
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From the view-to-model:
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The steps below outline the view to model data flow.
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1. The end user types a value into the input element.
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1. The end user types a value into the input element, in this case the favorite color "Blue".
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1. The form input element emits an input event with the latest value.
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1. The control value accessor on the form input element immediately relays the new value to the `FormControl` instance, which then emits the value through the `valueChanges` observable.
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1. The control value accessor listening for events on the form input element immediately relays the new value to the `FormControl` instance.
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1. The `FormControl` instance emits the new value through the `valueChanges` observable.
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1. Any subscribers to the `valueChanges` observable receive the new value.
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<figure>
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<img src="generated/images/guide/forms-overview/dataflow-reactive-forms-mtv.png" alt="Reactive forms model to view data flow" width="100%">
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<img src="generated/images/guide/forms-overview/dataflow-reactive-forms-mtv.png" alt="Reactive forms data flow - model to view" width="100%">
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</figure>
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From the model-to-view:
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The steps below outline the model to view data flow.
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1. The `favoriteColorControl.setValue()` method is called, which updates the `FormControl` value and emits the latest value through the `valueChanges` observable.
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1. The control value accessor on the form input element updates the element with the latest value.
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1. The `favoriteColorControl.setValue()` method is called, which updates the `FormControl` value.
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1. The `FormControl` instance emits the new value through the `valueChanges` observable.
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1. Any subscribers to the `valueChanges` observable receive the new value.
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1. The control value accessor on the form input element updates the element with the new value.
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### Data flow in template-driven forms
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Here is the same component with an input field for a single control implemented using template-driven forms.
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<code-example path="forms-overview/src/app/template/favorite-color/favorite-color.component.ts">
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</code-example>
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The diagrams below show how the data flows for an input with template-driven forms.
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In template-driven forms each form element in the linked to a directive that manages the form model internally. The diagrams below uses the same favorite color example to demonstrate how data flows when an input field's value is changed from the view and then from the model.
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<figure>
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<img src="generated/images/guide/forms-overview/dataflow-td-forms-vtm.png" alt="Template-driven forms view to model data flow" width="100%">
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</figure>
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From the view-to-model:
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The steps below outline the view to model data flow.
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1. The end user types "Blue" into the input element.
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1. The input element emits an "input" event with the value "Blue".
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1. The control value accessor attached to the input triggers the `setValue()` method on the `FormControl` instance, which emits that value through the `valueChanges` observable.
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1. The control value accessor attached to the input triggers the `setValue()` method on the `FormControl` instance.
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1. The `FormControl` instance emits the new value through the `valueChanges` observable.
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1. Any subscribers to the `valueChanges` observable receive the new value.
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1. The control value accessor also calls the `NgModel.viewToModel()` method which emits an `ngModelChange` event.
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1. Because the component template uses two-way data binding for the `favoriteColor`, the `favoriteColor` property in the component
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is updated to the value emitted by the `ngModelChange` event ("Blue").
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@ -114,16 +136,21 @@ is updated to the value emitted by the `ngModelChange` event ("Blue").
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<img src="generated/images/guide/forms-overview/dataflow-td-forms-mtv.png" alt="Template-driven forms model to view data flow" width="100%">
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</figure>
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From the model-to-view:
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The steps below outline the model to view data flow.
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1. The `favoriteColor` value is updated in the component.
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1. Change detection calls the `ngOnChanges` method on the `NgModel` directive instance, updates the `NgModel` instance model value, and queues an async task to set the value for the internal `FormControl` instance.
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1. On the next tick, the task to set the `FormControl` instance value is executed, when then emits the latest value through the `valueChanges` observable.
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1. On the next tick, the task to set the `FormControl` instance value is executed.
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1. The `FormControl` instance emits the latest value through the `valueChanges` observable.
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1. Any subscribers to the `valueChanges` observable receive the new value.
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1. The control value accessor updates the form input element in the view with the latest `favoriteColor` value.
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## Form validation
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Validation is an integral part of managing any set of forms. Whether you’re checking for required fields or querying an external API for an existing username, Angular provides a set of built-in validators as well as the ability to create custom validators. With reactive forms, custom validators are functions that receive a control to validate. Because template-driven forms are tied to directives, custom validator directives wrap validation functions so that the template can use them.
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Validation is an integral part of managing any set of forms. Whether you’re checking for required fields or querying an external API for an existing username, Angular provides a set of built-in validators as well as the ability to create custom validators.
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* **Reactive forms** define custom validators as **functions** that receive a control to validate.
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* **Template-driven forms** are tied to template **directives**, and must provide custom validator directives that wrap validation functions.
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For more on form validation, see the [Form Validation](guide/form-validation) guide.
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@ -131,47 +158,59 @@ For more on form validation, see the [Form Validation](guide/form-validation) gu
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Testing also plays a large part in complex applications and an easier testing strategy is always welcomed. One difference in testing reactive forms and template-driven forms is their reliance on rendering the UI in order to perform assertions based on form control and form field changes. The following examples demonstrate the process of testing forms with reactive and template-driven forms.
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### Testing a reactive form
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### Testing reactive forms
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Reactive forms provide a relatively easy testing strategy to due to synchronous access to the form and data models, and being independent of the UI. In these set of tests, controls and data are queried and manipulated through the control without interacting with the change detection cycle. The following tests below use the favorite color components mentioned earlier to validate the view-to-model and model-to-view data flows for a reactive form.
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Reactive forms provide a relatively easy testing strategy because they provide synchronous access to the form and data models, and they can be tested without rendering the UI. In these set of tests, controls and data are queried and manipulated through the control without interacting with the change detection cycle.
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The following test validates the view-to-model data flow:
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The following tests use the favorite color components mentioned earlier to verify the view to model and model to view data flows for a reactive form.
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<code-example path="forms-overview/src/app/reactive/favorite-color/favorite-color.component.spec.ts" region="view-to-model" title="Favorite color test (view-to-model)">
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The following test verifies the view to model data flow:
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<code-example path="forms-overview/src/app/reactive/favorite-color/favorite-color.component.spec.ts" region="view-to-model" title="Favorite color test - view to model">
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</code-example>
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The steps performed in the view to model test.
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1. Query the view for the form input element, and create a custom input event for the test.
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1. Set the new value for the input is set to *Red*, and dispatch the input event on the form input element.
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1. Assert that the `favoriteColor` form control instance value matches in value from the input.
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The following test validates the model-to-view data flow:
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The following test verifies the model to view data flow:
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<code-example path="forms-overview/src/app/reactive/favorite-color/favorite-color.component.spec.ts" region="model-to-view" title="Favorite color test (model-to-view)">
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<code-example path="forms-overview/src/app/reactive/favorite-color/favorite-color.component.spec.ts" region="model-to-view" title="Favorite color test - model to view">
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</code-example>
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The steps performed in the model to view test.
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1. Use the `favoriteColor` form control instance to set the new value.
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1. Query the view for the form input element.
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1. Assert that the new value set on the control matches the value in the input.
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### Testing a template-driven form
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### Testing template-driven forms
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Writing tests with template-driven forms is more involved and requires more detailed knowledge of the change detection process and how directives run on each cycle to ensure elements are queried, tested, or changed at the correct time. The following tests below use the favorite color components mentioned earlier to validate the view-to-model and model-to-view data flows for a template-driven form.
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Writing tests with template-driven forms requires more detailed knowledge of the change detection process and how directives run on each cycle to ensure elements are queried, tested, or changed at the correct time.
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The following test validates the view-to-model data flow:
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The following tests use the favorite color components mentioned earlier to verify the view to model and model to view data flows for a template-driven form.
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<code-example path="forms-overview/src/app/template/favorite-color/favorite-color.component.spec.ts" region="view-to-model" title="Favorite color test (view-to-model)">
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The following test verifies the view to model data flow:
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<code-example path="forms-overview/src/app/template/favorite-color/favorite-color.component.spec.ts" region="view-to-model" title="Favorite color test - view to model">
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</code-example>
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The steps performed in the view to model test.
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1. Query the view for the form input element, and create a custom input event for the test.
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1. Set the new value for the input is set to *Red*, and dispatch the input event on the form input element.
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1. Run change detection through the test fixture.
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1. Assert that the component `favoriteColor` property value matches the value from the input.
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The following test validates the model-to-view data flow:
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The following test verifies the model to view data flow:
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<code-example path="forms-overview/src/app/template/favorite-color/favorite-color.component.spec.ts" region="model-to-view" title="Favorite color test (model-to-view)">
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<code-example path="forms-overview/src/app/template/favorite-color/favorite-color.component.spec.ts" region="model-to-view" title="Favorite color test - model to view">
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</code-example>
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The steps performed in the model to view test.
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1. Use the component instance to set the value of `favoriteColor` property.
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1. Run change detection through the test fixture.
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1. Use the `tick()` method to simulate passage of time within the `fakeAsync()` task.
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How changes are tracked plays a role in the efficiency of your application.
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- Reactive forms keep the data model pure by providing it as an immutable data structure. Each time a change is triggered on the data model, the form control instance returns a new data model rather than updating the data model directly. This gives you the ability track unique changes to the data model through the control's observable. This allows change detection to be more efficient because it only needs to update on unique changes. It also follows reactive patterns that integrate with observable operators to transform data.
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- Template-driven forms rely on mutability with two-way data binding to update the data model in the component as changes are made in the template. Because there are no unique changes to track on the data model when using two-way data binding, change detection becomes less efficient at determining when updates are required.
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- **Reactive forms** keep the data model pure by providing it as an immutable data structure. Each time a change is triggered on the data model, the form control instance returns a new data model rather than updating the data model directly. This gives you the ability track unique changes to the data model through the control's observable. This allows change detection to be more efficient because it only needs to update on unique changes. It also follows reactive patterns that integrate with observable operators to transform data.
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- **Template-driven** forms rely on mutability with two-way data binding to update the data model in the component as changes are made in the template. Because there are no unique changes to track on the data model when using two-way data binding, change detection is less efficient at determining when updates are required.
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The difference is demonstrated in the examples above using the **favorite color** input element. With reactive forms, the **form control instance** always returns a new value when the control's value is updated. With template-driven forms, the **favorite color property** is always modified to its new value.
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The difference is demonstrated in the examples above using the **favorite color** input element.
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- With reactive forms, the **form control instance** always returns a new value when the control's value is updated.
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- With template-driven forms, the **favorite color property** is always modified to its new value.
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## Scalability
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If forms are a central part of your application, scalability is very important. Being able to reuse form models across components is critical. Reactive forms make creating large scale forms easier by providing access to low-level APIs and synchronous access to the form model. Because template-driven forms focus on simple scenarios, they are not as reusable, abstract away the low-level APIs and access to the form model is provided asynchronously. This surfaces in testing also, where testing reactive forms requires less setup and no dependance on the change detection cycle when updating and validating the form and data models during testing.
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If forms are a central part of your application, scalability is very important. Being able to reuse form models across components is critical.
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- **Reactive forms** make creating large scale forms easier by providing access to low-level APIs and synchronous access to the form model.
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- **Template-driven** forms focus on simple scenarios, are not as reusable, abstract away the low-level APIs and access to the form model is provided asynchronously. The abstraction with template-driven forms surfaces in testing also, where testing reactive forms requires less setup and no dependance on the change detection cycle when updating and validating the form and data models during testing.
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## Final Thoughts
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Choosing a strategy begins with understanding the strengths and weaknesses of the options presented. Low-level API and form model access, predictability, mutability, straightforward validation and testing strategies, and scalability are all important consideration in choosing the infrastructure you use when building your forms in Angular. Template-driven forms come from a familiar place in AngularJS, but fall short given the criteria in the modern world of Angular apps. Reactive forms integrate with reactive patterns already present in other areas of the Angular architecture, and complement those requirements well.
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Choosing a strategy begins with understanding the strengths and weaknesses of the options presented. Low-level API and form model access, predictability, mutability, straightforward validation and testing strategies, and scalability are all important consideration in choosing the infrastructure you use when building your forms in Angular. Template-driven forms are similar to patterns in AngularJS, but they have limitations given the criteria of many modern, large-scale Angular apps. Reactive forms integrate with reactive patterns already present in other areas of the Angular architecture, and complement those requirements well. Those limitations are alleviated with reactive forms.
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## Next Steps
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You can learn more with examples and best practices using reactive forms or template-driven forms. The following guides are the next steps in the learning process.
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The following guides are the next steps in the learning process.
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To learn more about reactive forms, see the following guides:
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