//定义按需加载JS方法，注意加载顺序在最后 
public static function addScript($view, $jsfile) { 
$view->registerJsFile($jsfile, [AppAsset::className(), 'depends' => 'backend\assets\AppAsset']); 
} 
//定义按需加载css方法，注意加载顺序在最后 
public static function addCss($view, $cssfile) { 
$view->registerCssFile($cssfile, [AppAsset::className(), 'depends' => 'backend\assets\AppAsset']); 
}

<?= $form->field($model, 'content')->textarea(['style' => 'width:760px;height:500px;']) ?> 

use backend\assets\AppAsset;
AppAsset::register($this);
AppAsset::addCss($this,'/css/umeditor/themes/default/css/umeditor.css');
AppAsset::addScript($this,'/css/umeditor/umeditor.config.js');
AppAsset::addScript($this,'/css/umeditor/umeditor.min.js');
AppAsset::addScript($this,'/css/umeditor/lang/zh-cn/zh-cn.js');

<?php $this->beginBlock('js-block') ?>
$(function () {
var um = UM.getEditor('article-content', {
});
});
<?php $this->endBlock() ?>
<?php $this->registerJs($this->blocks['js-block'], \yii\web\View::POS_END); ?> 

use backend\models\Upload;
use yii\web\UploadedFile;
/**
* 百度umeditor上传
*/
public function actionUmUpload ()
{
$model = new Upload();

if (Yii::$app->request->isPost) {

$model->file = UploadedFile::getInstance($model, 'file');

$dir = ‘文件保存目录';
if (!is_dir($dir))
mkdir($dir);

if ($model->validate()) {
$fileName = $model->file->baseName . '.' . $model->file->extension;
$dir = $dir.'/'. $fileName;
$model->file->saveAs($dir);
$info = [
"originalName" => $model->file->baseName,
"name" => $model->file->baseName,
"url" => $dir,
"size" => $model->file->size,
"type" => $model->file->type,
"state" => 'SUCCESS',
];
exit(json_encode($info));
} 
}
}