The scenario in which this wiki can be useful is when you have to send an (huge) array of model ids and perform a time consuming computation with it like linking every model to other models. The idea is to split the array into smaller arrays and perform sequential ajax requests, showing the calculation progress using a Bootstrap Progress bar.

I have created a Country model and generated the CRUD operations using Gii. The model class look like this:

namespace app\models;
use Yii;
use yii\helpers\ArrayHelper;

/**
 * This is the model class for table "country".
 *
 * @property string $code
 * @property string $name
 * @property int $population
 *
 */
class Country extends \yii\db\ActiveRecord {

    /**
     * {@inheritdoc}
     */
    public static function tableName() {
        return 'country';
    }

    /**
     * {@inheritdoc}
     */
    public function rules() {
        return [
            [['code', 'name'], 'required'],
            [['population'], 'integer'],
            [['code'], 'string', 'max' => 3],
            [['name'], 'string', 'max' => 52],
            [['code'], 'unique'],
        ];
    }

    /**
     * {@inheritdoc}
     */
    public function attributeLabels() {
        return [
            'code' => 'Code',
            'name' => 'Name',
            'population' => 'Population',
        ];
    }
    /**
     * 
     * @return array
     */
    public static function getCodes(){
        return array_keys(ArrayHelper::map(Country::find()->select('code')->asArray()->all(), 'code', 'code'));
    }
}

I have also created another model that will take care to validate the ids and perform the time consuming calculation through the process() function. (in this example I just use a sleep(1) that will wait for one second for each element of the $codes array). I also want to keep track of the processed models, incrementing the $processed class attribute.

<?php

namespace app\models;

use app\models\Country;

/**
 * Description of BatchProcessForm
 *
 * @author toaster
 */
class BatchProcessForm extends \yii\base\Model {

    /**
     * The codes to process
     * @var string
     */
    public $codes;

    /**
     * The number of codes processed
     * @var integer
     */
    public $processed = 0;

    /**
     * @return array the validation rules.
     */
    public function rules() {
        return [
            ['codes', 'required'],
            ['codes', 'validateCodes'],
        ];
    }

    /**
     * Check whether codes exists in the database
     * @param type $attribute
     * @param type $params
     * @param type $validator
     */
    public function validateCodes($attribute, $params, $validator) {
        $input_codes = json_decode($this->$attribute);
        if (null == $input_codes || !is_array($input_codes)) {
            $this->addError($attribute, 'Wrong data format');
            return;
        }
        $codes = Country::getCodes();
        if (!array_intersect($input_codes, $codes) == $input_codes) {
            $this->addError($attribute, 'Some codes selected are not recognized');
        }
    }

    /**
     * Process the codes
     * @return boolean true if everything goes well
     */
    public function process() {
        if ($this->validate()) {
            $input_codes = json_decode($this->codes);
            foreach ($input_codes as $code) {
                sleep(1);
                $this->processed++;
            }
            return true;
        }
        return false;
    }

}

The code for the controller action is the following:

/**
 * Process an array of codes
 * @return mixes
 * @throws BadRequestHttpException if the request is not ajax
 */
public function actionProcess(){
    if(Yii::$app->request->isAjax){
        Yii::$app->response->format = Response::FORMAT_JSON;
        $model = new BatchProcessForm();
        if($model->load(Yii::$app->request->post()) && $model->process()){
            return ['result' => true, 'processed' => $model->processed];
        }
        return ['result' => false, 'error' => $model->getFirstError('codes')];
    }
    throw new \yii\web\BadRequestHttpException;
}

In my index.php view I have added a CheckboxColumn as first column of the Gridview that allows, out of the box, to collect the ids of the models selected via Javascript (in this case the values of the code attribute). I have also added a button-like hyperlink tag to submit the collected data (with id batch_process) and a Bootstrap Progress bar inside a Modal Window. The code of my view is the following:

<?php

use yii\helpers\Html;
use yii\grid\GridView;
use yii\bootstrap\Modal;

/* @var $this yii\web\View */
/* @var $searchModel app\models\CountrySearch */
/* @var $dataProvider yii\data\ActiveDataProvider */

$this->title = 'Countries';
$this->params['breadcrumbs'][] = $this->title;
?>
<div class="country-index">

    <h1><?= Html::encode($this->title) ?></h1>
    <?php // echo $this->render('_search', ['model' => $searchModel]); ?>

    <p>
        <?= Html::a('Create Country', ['create'], ['class' => 'btn btn-success']) ?>
        <?= Html::a('Batch Process', ['process'], ['class' => 'btn btn-info', 'id' => 'batch_process']) ?>
    </p>

    <?=
    GridView::widget([
        'dataProvider' => $dataProvider,
        'filterModel' => $searchModel,
        'options' => [
            'id' => 'country-index'
        ],
        'columns' => [
            ['class' => 'yii\grid\CheckboxColumn'],
            'code',
            'name',
            'population',
            ['class' => 'yii\grid\ActionColumn'],
        ],
    ]);
    ?>
</div>

<?php Modal::begin(['header' => '<h5 id="progress">0% Complete</h5>', 'id' => 'progress-modal', 'closeButton' => false]); ?>

<?=
yii\bootstrap\Progress::widget([
    'percent' => 0,
     'options' => ['class' => 'progress-success active progress-striped']
]);
?>

<?php Modal::end(); ?>

<?php $this->registerJsFile('@web/js/main.js', ['depends' => [\app\assets\AppAsset::class]]); ?>

The Javascript file that split in chunks the array and send each chunk sequentially is main.js. Although I have registered the Javascript file using registerJsFile() method I highly recommend to better handle and organize js files using Asset Bundles. Here is the code:

function updateProgressBar(percentage) {
    var perc_string = percentage + '% Complete';
    $('.progress-bar').attr('aria-valuenow', percentage);
    $('.progress-bar').css('width', percentage + '%');
    $('#pb-small').text(perc_string);
    $('#progress').text(perc_string);
}

function disposeProgressBar(message) {
    alert(message);
    $('#progress-modal').modal('hide');
}

function showProgressBar() {
    var perc = 0;
    var perc_string = perc + '% Complete';
    $('.progress-bar').attr('aria-valuenow', perc);
    $('.progress-bar').css('width', perc + '%');
    $('#pb-small').text(perc_string);
    $('#progress').text(perc_string);
    $('#progress-modal').modal('show');
}

function batchSend(set, iter, take, processed) {
    var group = iter + take < set.length ? iter + take : set.length;
    var progress = Math.round((group / set.length) * 100);
    var dataObj = [];
    for (var i = iter; i < group; i++) {
        dataObj.push(set[i]);
    }
    iter += take;
    $.ajax({
        url: '/country/process', ///?r=country/process
        type: 'post',
        data: {'BatchProcessForm[codes]': JSON.stringify(dataObj)},
        success: function (data) {
            if (data.result) {
                updateProgressBar(progress);
                processed += data.processed;
                if (progress < 100) {
                    batchSend(set, iter, take, processed);
                } else {
                    var plural = processed == 1 ? 'country' : 'countries';
                    disposeProgressBar(processed + ' ' + plural + ' correctly processed');
                }
                return true;
            }
            disposeProgressBar(data.error);
            return false;
        },
        error: function () {
            disposeProgressBar('Server error, please try again later');
            return false;
        }
    });
}

$('#batch_process').on('click', function (e) {
    e.preventDefault();
    var keys = $('#country-index').yiiGridView('getSelectedRows');
    if (keys.length <= 0) {
        alert('Select at least one country');
        return false;
    }
    var plural = keys.length == 1 ? 'country' : 'countries';
    if (confirm(keys.length + ' ' + plural + ' selected.. continue?')) {
        showProgressBar();
        batchSend(keys, 0, 5, 0);
    }
});

The first three functions take care to show, update and hide the progress bar inside the modal window, while the batchSend function perform the array split and, using recursion, send the data through post ajax request, encoding the array as JSON string and calling itself until there is no more chunks to send. The last lines of code bind the click event of the #batch_process button checking if at least one row of Gridview has been selected. The number of elements on each array chunk can be set as the third argument of the batchSend function (in my case 5), you can adjust it accordingly. The first parameter is the whole array obtained by the yiiGridView('getSelectedRows') function. The final result is something like this: ...looks cool, isn't it?