 to the completion block on finishing the background task successfully, as well as supplying an object for thread safe access inside the background task.</p>
<div class="aside aside-warning">
    <p class="aside-title">Warning</p>
    <p>Be careful not to access anything that is not thread safe inside background task.</p>

</div>

                      </div>
                      <div class="declaration">
                        <h4>Declaration</h4>
                        <div class="language">
          