This tutorial explains in detail how to handle form validation failure scenarios gracefully in Laravel applications to ensure that the data previously entered by the user will not be lost. By using the withInput() method in the controller to flash the request data to the Session, and using the old() auxiliary function in the Blade template, automatic backfilling of form fields is achieved, significantly improving the user experience.

In web application development, after a user submits a form, if the data fails server-side verification, it is usually necessary to redirect the user back to the form page and display an error message. However, a common user experience pain point is that when a redirect occurs, the data the user has entered in the form is lost and the user has to fill in all fields again. This not only increases the user's operational burden, but may also lead to user loss. Laravel provides a simple and powerful mechanism to solve this problem, namely through the withInput() method and old() helper function.

withInput() method in controller

Laravel's Validator instance catches all validation errors when form validation fails. Typically, we redirect the user back to the form page with these error messages. In order to retain the form data submitted by the user at the same time, we need to call the withInput() method when redirecting.

The function of the withInput() method is to flash all the input data of the current request into the Session. This means that the data will only be available in the next request, and will be automatically cleared from the Session afterwards, avoiding unnecessary accumulation of Session data.

Original question code example (input not preserved):

The following code shows a scenario where after verification fails, it only redirects with an error message without retaining user input:

 // ...other code...

$validator = Validator::make($request->all(), [
    'PageLanguage.title.*' => 'required',
],[
    "required" => "Please check and refill all required form fields." // Optimize error message]);

if ($validator->fails()) {
    return redirect('admin/page/create')
                ->withErrors($validator); // User input data will not be retained at this time}

//...Logic after verification...

In the above code, if the verification fails, the user will be redirected to the admin/page/create page, but the previously filled form data will be lost.

Solution after adding withInput():

To solve the data loss issue, just add the ->withInput() method in the redirection chain:

 // ...other code...

$validator = Validator::make($request->all(), [
    'PageLanguage.title.*' => 'required',
],[
    "required" => "Please check and refill all required form fields."
]);

if ($validator->fails()) {
    return redirect('admin/page/create')
                ->withErrors($validator)
                ->withInput(); // Add this method to retain user input data}

//...Logic after verification...

Now, when validation fails and redirects, the previously submitted form data (including all $_POST and $_GET parameters) is stored in the Session for the next request.

old() helper function in Blade template

After flashing the data via withInput() in the controller, we need to retrieve the data correctly in the Blade template and populate it into the appropriate form fields. Laravel provides the old() helper function to do this.

The old() function accepts one parameter, the name attribute of the input field you want to retrieve. If the field exists in the input data in flash memory, old() returns its value; otherwise, it returns null.

Usage example:

Assuming your form field is named name, you can use old() in the Blade template like this:

 <input type="text" name="name" value="{{ old('name') }}" class="form-control" placeholder="Please enter a name">

If your application uses a form auxiliary package such as LaravelCollective/html, its usage will be simpler. For example, for a text input box:

 {!! Form::text('name', old('name'), ['class' => 'form-control', 'placeholder' => '']) !!}

Here, the second parameter of Form::text is the default value of the field. When old('name') returns a value, it is used as the default value; otherwise, the field will be empty.

Handle input in array form:

In the original question, the validation rule is 'PageLanguage.title.*' => 'required', which indicates that the form may receive input in the form of an array, such as PageLanguage[title][0], PageLanguage[title][1], etc. The old() function also supports this form of access:

 <!-- For input like PageLanguage[title][0] -->
<input type="text" name="PageLanguage[title][0]" value="{{ old('PageLanguage.title.0') }}" class="form-control">

<!-- Or if your form is dynamically generated, you can access it through a loop -->
@foreach($languages ??as $index => $language)
    <label for="title_{{ $index }}">{{ $language->name }} title:</label>
    <input type="text" name="PageLanguage[title][{{ $index }}]" id="title_{{ $index }}" value="{{ old('PageLanguage.title.' . $index) }}" class="form-control">
@endforeach

Note that for nested arrays, the old() function uses the dot . to access child elements, such as PageLanguage.title.0.

Things to note and best practices

1. Default value of old() function : The old() function can accept the second parameter as the default value. When the corresponding field is not found in the flash data, this default value will be used. This is particularly useful when editing existing records.

    <input type="text" name="name" value="{{ old('name', $user->name ?? '') }}" class="form-control">

   Here, if old('name') is empty, $user->name will be tried.

2. Form helper functions : Although packages like LaravelCollective/html provide convenient form generation methods, even without using them, it is perfectly feasible to use value="{{ old('field_name') }}" directly in the HTML tag.
3. User experience : Combined with validation error messages and data backfilling, the user experience can be greatly improved. Make sure the error message is clear, specific, and indicates which field needs to be corrected.
4. Security : The value returned by the old() function has been automatically encoded by Laravel into HTML entities to prevent XSS attacks. Therefore, there is usually no need to manually code again.

Summarize

By cleverly using the withInput() method in the Laravel controller and combining the old() helper function in the Blade template, we can easily implement automatic backfilling of user-entered data after form validation fails. This mechanism not only simplifies the development process, but more importantly, it provides users with a smooth and uninterrupted form submission experience, effectively avoiding the frustration caused by data loss. Mastering this skill is a crucial step in building user-friendly Laravel applications.

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