Form inputs

Aurelia provides a powerful and intuitive approach to handling form inputs and user interactions. At the core of Aurelia's form handling is two-way binding, which creates a seamless connection between the view (HTML) and the view model (JavaScript).

Key Characteristics of Aurelia Form Inputs

• Automatic Two-Way Binding: By default, form elements automatically sync changes between the view and view model

• Reactive Data Flow: Input changes are immediately reflected in the view model

• Minimal Boilerplate: Requires less code compared to traditional form handling approaches

Data Flow Mechanism

When a user interacts with a form input, the following process occurs:

1. User enters/modifies input value

2. Native form input events are triggered

3. Aurelia's binding system detects the change

4. View model is automatically updated

5. Any dependent properties or computations are instantly refreshed

Basic Form Input Binding

Text Inputs

<template>
  <input type="text" value.bind="firstName">
  <p>Hello, ${firstName}</p>
</template>

export class MyViewModel {
  firstName = '';
}

Input Binding Variations

Binding Mode	Syntax	Description
Two-Way Binding	value.bind	Default, syncs changes in both directions
One-Way Binding	value.one-way	Updates view model, but not vice versa
One-Time Binding	value.one-time	Initial value only

Textarea

<textarea value.bind="description"></textarea>

Checkbox Inputs

<template>
  <input type="checkbox" checked.bind="isSubscribed">
  <label>Subscribe to newsletter</label>
</template>

export class MyViewModel {
  isSubscribed = false;
}

Radio Buttons

<template>
  <input type="radio" value="option1" model.bind="selectedOption">
  <input type="radio" value="option2" model.bind="selectedOption">
</template>

export class MyViewModel {
  selectedOption = '';
}

Select Dropdowns

<template>
  <select value.bind="selectedCountry">
    <option repeat.for="country of countries" 
            model.bind="country.code">
      ${country.name}
    </option>
  </select>
</template>

export class MyViewModel {
  countries = [
    { name: 'United States', code: 'US' },
    { name: 'Canada', code: 'CA' }
  ];
  selectedCountry = '';
}

Input Binding Considerations

Note: When using model.bind, ensure you're binding to the entire object, not just a primitive value. This allows for more complex data binding scenarios.

Tip: Always initialize form-related properties in your view model to prevent undefined errors and provide default states.

Advanced Input Techniques

Custom Validation

Aurelia provides flexible validation through the aurelia-validation plugin. Here's a comprehensive approach to form validation:

import { inject } from 'aurelia-framework';
import { ValidationController, ValidationRules } from 'aurelia-validation';

@inject(ValidationController)
export class RegistrationViewModel {
  email = '';
  
  constructor(validationController) {
    this.validationController = validationController;

    ValidationRules
      .ensure('email')
        .required()
        .email()
      .on(this);
  }
}

<template>
  <form>
    <input 
      type="text" 
      value.bind="email" 
      error.bind="emailErrors">
    <div if.bind="emailErrors" class="error">
      ${emailErrors}
    </div>
  </form>
</template>

Input Formatting

Number Formatting

<template>
  <input 
    type="text" 
    value.bind="price" 
    matcher.bind="currencyFormatter">
</template>

export class ViewModel {
  price = 0;
  
  currencyFormatter = {
    toView(value) {
      return value 
        ? `$${value.toFixed(2)}` 
        : '$0.00';
    },
    fromView(value) {
      return parseFloat(
        value.replace('$', '').replace(',', '')
      );
    }
  };
}

Conditional Binding

<template>
  <input 
    type="text" 
    value.bind="username"
    disabled.bind="isUsernameLocked">
  
  <select 
    value.bind="accountType"
    show.bind="canSelectAccountType">
    <option>Basic</option>
    <option>Premium</option>
  </select>
</template>

import { computedFrom } from 'aurelia-framework';

export class UserViewModel {
  username = '';
  isUsernameLocked = false;
  accountType = 'Basic';
  
  @computedFrom('username')
  get canSelectAccountType() {
    return this.username.length > 5;
  }
}

Computed Properties with Inputs

import { computedFrom } from 'aurelia-framework';

export class CalculatorViewModel {
  firstNumber = 0;
  secondNumber = 0;
  
  // Computed property automatically updates
  @computedFrom('firstNumber', 'secondNumber')
  get sum() {
    return this.firstNumber + this.secondNumber;
  }
  
  @computedFrom('firstNumber', 'secondNumber')
  get average() {
    return (this.firstNumber + this.secondNumber) / 2;
  }
}

<template>
  <input type="number" value.bind="firstNumber">
  <input type="number" value.bind="secondNumber">
  
  <p>Sum: ${sum}</p>
  <p>Average: ${average}</p>
</template>

Validation and Error Handling

Technique	Description	Use Case
Required Fields	Ensure input is not empty	Form completeness
Email Validation	Check email format	User registration
Number Range	Validate numeric inputs	Age, quantity limits
Custom Validators	Complex validation logic	Specialized form rules

Always validate both client-side and server-side to ensure data integrity.

Use meaningful error messages that guide users to correct their input.

Form Submission

Form submission in Aurelia involves managing user input, validation, and processing data with a clean, reactive approach. The framework provides multiple strategies for handling form submissions efficiently.

Basic Form Submission

import {inject} from 'aurelia-framework';
import {HttpClient} from 'aurelia-fetch-client';

@inject(HttpClient)
export class RegistrationViewModel {
  username = '';
  email = '';
  password = '';

  constructor(http) {
    this.http = http;
  }

  submitForm() {
    this.http.fetch('/api/register', {
      method: 'post',
      body: JSON.stringify({
        username: this.username,
        email: this.email,
        password: this.password
      })
    })
    .then(response => response.json())
    .then(data => {
      // Handle successful registration
    })
    .catch(error => {
      // Handle errors
    });
  }
}

<template>
  <form submit.trigger="submitForm()">
    <input type="text" value.bind="username">
    <input type="email" value.bind="email">
    <input type="password" value.bind="password">
    <button type="submit">Register</button>
  </form>
</template>

Comprehensive Form Validation

import {inject} from 'aurelia-framework';
import {ValidationController, ValidationRules} from 'aurelia-validation';
import {computedFrom} from 'aurelia-framework';

@inject(ValidationController)
export class AdvancedFormViewModel {
  username = '';
  email = '';
  age = null;

  constructor(validationController) {
    this.validationController = validationController;

    // Define validation rules
    ValidationRules
      .ensure('username')
        .required()
        .minLength(3)
      .ensure('email')
        .required()
        .email()
      .ensure('age')
        .required()
        .between(18, 99)
      .on(this);
  }

  @computedFrom('username', 'email', 'age')
  get isFormValid() {
    return this.username && 
           this.email && 
           this.age !== null;
  }

  submitForm() {
    this.validationController.validate()
      .then(result => {
        if (result.valid) {
          // Process form submission
          this.processRegistration();
        }
      });
  }

  processRegistration() {
    // Submission logic
  }
}

Preventing Default Form Submission

<template>
  <form submit.trigger="handleSubmit($event)">
    <!-- form fields -->
  </form>
</template>

export class FormViewModel {
  handleSubmit(event) {
    // Prevent default form submission
    event.preventDefault();

    // Custom submission logic
    this.processForm();
  }

  processForm() {
    // Form processing
  }
}

Handling Form Errors

import {inject} from 'aurelia-framework';
import {HttpClient} from 'aurelia-fetch-client';

@inject(HttpClient)
export class ErrorHandlingViewModel {
  formErrors = [];

  constructor(http) {
    this.http = http;
  }

  submitForm() {
    // Reset previous errors
    this.formErrors = [];

    this.http.fetch('/api/submit', {
      method: 'post',
      body: JSON.stringify(this.formData)
    })
    .then(response => response.json())
    .then(result => {
      // Handle successful submission
    })
    .catch(error => {
      // Populate form errors
      if (error.details) {
        this.formErrors = error.details.map(err => err.message);
      }
    });
  }
}

Always prevent default form submission and handle it programmatically to ensure full control over the process.

Working with Complex Forms

Complex Object Binding

import {inject} from 'aurelia-framework';
import {computedFrom} from 'aurelia-framework';

export class ComplexFormViewModel {
  user = {
    personal: {
      firstName: '',
      lastName: '',
      age: null
    },
    contact: {
      email: '',
      phone: ''
    },
    preferences: {
      newsletter: false,
      interests: []
    }
  };

  interestOptions = [
    'Technology', 
    'Sports', 
    'Music', 
    'Travel'
  ];

  @computedFrom('user.personal.firstName', 'user.personal.lastName')
  get fullName() {
    return `${this.user.personal.firstName} ${this.user.personal.lastName}`;
  }

  addInterest(interest) {
    if (!this.user.preferences.interests.includes(interest)) {
      this.user.preferences.interests.push(interest);
    }
  }

  removeInterest(interest) {
    const index = this.user.preferences.interests.indexOf(interest);
    if (index > -1) {
      this.user.preferences.interests.splice(index, 1);
    }
  }
}

<template>
  <form>
    <!-- Personal Information -->
    <section>
      <input type="text" value.bind="user.personal.firstName" placeholder="First Name">
      <input type="text" value.bind="user.personal.lastName" placeholder="Last Name">
      <input type="number" value.bind="user.personal.age" placeholder="Age">
    </section>

    <!-- Contact Information -->
    <section>
      <input type="email" value.bind="user.contact.email" placeholder="Email">
      <input type="tel" value.bind="user.contact.phone" placeholder="Phone">
    </section>

    <!-- Preferences -->
    <section>
      <label>
        <input type="checkbox" checked.bind="user.preferences.newsletter">
        Subscribe to Newsletter
      </label>

      <h3>Interests</h3>
      <div>
        <select value.bind="selectedInterest">
          <option repeat.for="interest of interestOptions" model.bind="interest">
            ${interest}
          </option>
        </select>
        <button click.delegate="addInterest(selectedInterest)">Add Interest</button>
      </div>

      <ul>
        <li repeat.for="interest of user.preferences.interests">
          ${interest}
          <button click.delegate="removeInterest(interest)">Remove</button>
        </li>
      </ul>
    </section>
  </form>
</template>

Nested Form Structures

import {inject} from 'aurelia-framework';
import {ValidationController, ValidationRules} from 'aurelia-validation';

@inject(ValidationController)
export class NestedFormViewModel {
  addresses = [{
    type: 'home',
    street: '',
    city: '',
    country: ''
  }];

  constructor(validationController) {
    this.validationController = validationController;

    // Validation for nested structures
    ValidationRules
      .ensure('addresses[0].street').required()
      .ensure('addresses[0].city').required()
      .on(this);
  }

  addAddress() {
    this.addresses.push({
      type: 'additional',
      street: '',
      city: '',
      country: ''
    });
  }

  removeAddress(index) {
    if (this.addresses.length > 1) {
      this.addresses.splice(index, 1);
    }
  }
}

Handling Arrays of Form Data

export class OrderFormViewModel {
  orderItems = [];

  addOrderItem() {
    this.orderItems.push({
      product: '',
      quantity: 1,
      price: 0
    });
  }

  removeOrderItem(index) {
    this.orderItems.splice(index, 1);
  }

  @computedFrom('orderItems')
  get totalOrderValue() {
    return this.orderItems.reduce((total, item) => 
      total + (item.quantity * item.price), 0);
  }
}

Practical Examples

User Registration Form

import {inject} from 'aurelia-framework';
import {HttpClient} from 'aurelia-fetch-client';
import {ValidationController, ValidationRules} from 'aurelia-validation';

@inject(HttpClient, ValidationController)
export class RegistrationViewModel {
  registration = {
    username: '',
    email: '',
    password: '',
    confirmPassword: '',
    agreeTOS: false
  };

  constructor(http, validationController) {
    this.http = http;
    this.validationController = validationController;

    // Complex validation rules
    ValidationRules
      .ensure('username')
        .required()
        .minLength(3)
      .ensure('email')
        .required()
        .email()
      .ensure('password')
        .required()
        .minLength(8)
      .ensure('confirmPassword')
        .equals('password', 'Passwords must match')
      .ensure('agreeTOS')
        .equals(true, 'You must agree to Terms of Service')
      .on(this.registration);
  }

  register() {
    this.validationController.validate()
      .then(result => {
        if (result.valid) {
          this.http.fetch('/api/register', {
            method: 'POST',
            body: JSON.stringify(this.registration)
          });
        }
      });
  }
}

Break complex forms into logical sections for improved maintainability.

Always sanitize and validate user input, both client-side and server-side.