206 lines
6.6 KiB
Markdown
206 lines
6.6 KiB
Markdown
# Dynamic forms
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{@a top}
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Building handcrafted forms can be costly and time-consuming,
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especially if you need a great number of them, they're similar to each other, and they change frequently
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to meet rapidly changing business and regulatory requirements.
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It may be more economical to create the forms dynamically, based on
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metadata that describes the business object model.
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This cookbook shows you how to use `formGroup` to dynamically
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render a simple form with different control types and validation.
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It's a primitive start.
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It might evolve to support a much richer variety of questions, more graceful rendering, and superior user experience.
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All such greatness has humble beginnings.
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The example in this cookbook is a dynamic form to build an
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online application experience for heroes seeking employment.
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The agency is constantly tinkering with the application process.
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You can create the forms on the fly *without changing the application code*.
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{@a toc}
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See the <live-example name="dynamic-form"></live-example>.
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{@a bootstrap}
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## Bootstrap
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Start by creating an `NgModule` called `AppModule`.
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This cookbook uses [reactive forms](guide/reactive-forms).
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Reactive forms belongs to a different `NgModule` called `ReactiveFormsModule`,
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so in order to access any reactive forms directives, you have to import
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`ReactiveFormsModule` from the `@angular/forms` library.
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Bootstrap the `AppModule` in `main.ts`.
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<code-tabs>
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<code-pane header="app.module.ts" path="dynamic-form/src/app/app.module.ts">
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</code-pane>
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<code-pane header="main.ts" path="dynamic-form/src/main.ts">
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</code-pane>
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</code-tabs>
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{@a object-model}
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## Question model
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The next step is to define an object model that can describe all scenarios needed by the form functionality.
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The hero application process involves a form with a lot of questions.
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The _question_ is the most fundamental object in the model.
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The following `QuestionBase` is a fundamental question class.
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<code-example path="dynamic-form/src/app/question-base.ts" header="src/app/question-base.ts">
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</code-example>
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From this base you can derive two new classes in `TextboxQuestion` and `DropdownQuestion`
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that represent textbox and dropdown questions.
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The idea is that the form will be bound to specific question types and render the
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appropriate controls dynamically.
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`TextboxQuestion` supports multiple HTML5 types such as text, email, and url
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via the `type` property.
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<code-example path="dynamic-form/src/app/question-textbox.ts" header="src/app/question-textbox.ts"></code-example>
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`DropdownQuestion` presents a list of choices in a select box.
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<code-example path="dynamic-form/src/app/question-dropdown.ts" header="src/app/question-dropdown.ts"></code-example>
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Next is `QuestionControlService`, a simple service for transforming the questions to a `FormGroup`.
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In a nutshell, the form group consumes the metadata from the question model and
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allows you to specify default values and validation rules.
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<code-example path="dynamic-form/src/app/question-control.service.ts" header="src/app/question-control.service.ts"></code-example>
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{@a form-component}
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## Question form components
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Now that you have defined the complete model you are ready
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to create components to represent the dynamic form.
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`DynamicFormComponent` is the entry point and the main container for the form.
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<code-tabs>
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<code-pane header="dynamic-form.component.html" path="dynamic-form/src/app/dynamic-form.component.html">
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</code-pane>
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<code-pane header="dynamic-form.component.ts" path="dynamic-form/src/app/dynamic-form.component.ts">
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</code-pane>
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</code-tabs>
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It presents a list of questions, each bound to a `<app-question>` component element.
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The `<app-question>` tag matches the `DynamicFormQuestionComponent`,
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the component responsible for rendering the details of each _individual_
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question based on values in the data-bound question object.
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<code-tabs>
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<code-pane header="dynamic-form-question.component.html" path="dynamic-form/src/app/dynamic-form-question.component.html">
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</code-pane>
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<code-pane header="dynamic-form-question.component.ts" path="dynamic-form/src/app/dynamic-form-question.component.ts">
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</code-pane>
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</code-tabs>
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Notice this component can present any type of question in your model.
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You only have two types of questions at this point but you can imagine many more.
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The `ngSwitch` determines which type of question to display.
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In both components you're relying on Angular's **formGroup** to connect the template HTML to the
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underlying control objects, populated from the question model with display and validation rules.
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`formControlName` and `formGroup` are directives defined in
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`ReactiveFormsModule`. The templates can access these directives
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directly since you imported `ReactiveFormsModule` from `AppModule`.
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{@a questionnaire-data}
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## Questionnaire data
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`DynamicFormComponent` expects the list of questions in the form of an array bound to `@Input() questions`.
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The set of questions you've defined for the job application is returned from the `QuestionService`.
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In a real app you'd retrieve these questions from storage.
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The key point is that you control the hero job application questions
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entirely through the objects returned from `QuestionService`.
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Questionnaire maintenance is a simple matter of adding, updating,
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and removing objects from the `questions` array.
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<code-example path="dynamic-form/src/app/question.service.ts" header="src/app/question.service.ts">
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</code-example>
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Finally, display an instance of the form in the `AppComponent` shell.
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<code-example path="dynamic-form/src/app/app.component.ts" header="app.component.ts">
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</code-example>
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{@a dynamic-template}
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## Dynamic Template
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Although in this example you're modelling a job application for heroes, there are
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no references to any specific hero question
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outside the objects returned by `QuestionService`.
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This is very important since it allows you to repurpose the components for any type of survey
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as long as it's compatible with the *question* object model.
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The key is the dynamic data binding of metadata used to render the form
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without making any hardcoded assumptions about specific questions.
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In addition to control metadata, you are also adding validation dynamically.
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The *Save* button is disabled until the form is in a valid state.
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When the form is valid, you can click *Save* and the app renders the current form values as JSON.
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This proves that any user input is bound back to the data model.
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Saving and retrieving the data is an exercise for another time.
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The final form looks like this:
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<div class="lightbox">
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<img src="generated/images/guide/dynamic-form/dynamic-form.png" alt="Dynamic-Form">
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</div>
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[Back to top](guide/dynamic-form#top)
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