队列执行报错 Undefined property: App\Jobs\TranslateSlug::$topic

如题

代码如下：

TopicObserver.php

<?php

namespace App\Observers;

use App\Models\Topic;
use App\Handlers\SlugTranslateHandler;
use App\Jobs\TranslateSlug;

// creating, created, updating, updated, saving,
// saved,  deleting, deleted, restoring, restored

class TopicObserver
{
    public function saving(Topic $topic)
    {
        $topic->body = clean($topic->body, 'user_topic_body');

        $topic->excerpt = make_excerpt($topic->body);
    }

    public function saved(Topic $topic)
    {
        // 如 slug 字段无内容，即使用翻译器对 title 进行翻译
        if ( ! $topic->slug) {
            // 推送任务到队列
            dispatch(new TranslateSlug($topic));
        }
    }

    public function creating(Topic $topic)
    {
        //
    }

    public function updating(Topic $topic)
    {
        //
    }
}

app\Jobs\TranslateSlug.php

<?php

namespace App\Jobs;

use Illuminate\Bus\Queueable;
use Illuminate\Queue\SerializesModels;
use Illuminate\Queue\InteractsWithQueue;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Foundation\Bus\Dispatchable;

use App\Models\Topic;
use App\Handlers\SlugTranslateHandler;

class TranslateSlug implements ShouldQueue
{
    use Dispatchable, InteractsWithQueue, Queueable, SerializesModels;

    protected $topic;

    /**
     * Create a new job instance.
     *
     * @return void
     */
    public function __construct(Topic $topic)
    {
        // 队列任务构造器中接收了 Eloquent 模型，将会只序列化模型的 ID
        $this->topic = $topic;
    }

    /**
     * Execute the job.
     *
     * @return void
     */
    public function handle()
    {
        // 请求百度 API 接口进行翻译
        $slug = app(SlugTranslateHandler::class)->translate($this->topic->title);

        // 为了避免模型监控器死循环调用，我们使用 DB 类直接对数据库进行操作
        \DB::table('topics')->where('id', $this->topic->id)->update(['slug' => $slug]);
    }
}