ios - 如何正确使用OperationQueue？

Question

此问题与此线程有关:how-to-set-the-height-of-a-cell-depending-on-a-uilabel-with-a-typewriter-effect

在我的 tableViewController 中，我的单元格包含 UILabel，它具有使用 setTextWithTypeAnimation 管理的打字机效果；

func configureCell(tableView: UITableView, cell: ParagraphTableViewCell, atIndexPath indexPath: IndexPath) {

    let paragraph = paragraphArray[indexPath.row] as! Paragraph
    cell.paragraph = paragraph

        self.queue = OperationQueue()

        let operation1 = BlockOperation(block: {

            cell.dialogueLabel.setTextWithTypeAnimation(typedText: paragraph.dialogueLabel.text!, queue:self.queue, callBackAfterCharacterInsertion: {

                self.tableView.beginUpdates()
                self.tableView.endUpdates()
            })

        })


    operation1.completionBlock = {

    cell.buttonsStackViewHeightConstraint.constant = CGFloat(HEIGHT_CONSTRAINT)

    UIView.animate(withDuration: 0.3, animations: {
        cell.contentView.layoutIfNeeded()
    }, completion: nil)

    }

    queue.addOperation(operation1)

   }

我的打字机在 UILabel 扩展中:

extension UILabel {

func setTextWithTypeAnimation(typedText: String, queue: OperationQueue, characterInterval: TimeInterval = 0.05, callBackAfterCharacterInsertion:(()->())?) {

    text = ""

    for (_, character) in typedText.characters.enumerated() {

        if queue.isSuspended {

            OperationQueue.main.isSuspended = true
            OperationQueue.main.cancelAllOperations()
            break;
        }

        OperationQueue.main.addOperation {

            self.text = self.text! + String(character)
            callBackAfterCharacterInsertion?()

        }

        Thread.sleep(forTimeInterval: characterInterval)
    }
}
}

首先，我使用 DispatchQueue 来管理单元格内的动画(参见 how-to-set-the-height-of-a-cell-depending-on-a-uilabel-with-a-typewriter-effect )，但我需要在用户关闭 View Controller 时停止线程。这就是我使用 OperationQueue 的原因(DispatchQueue 无法停止)

@IBAction func closeViewController(sender: AnyObject) {

    dismiss(animated: true, completion: nil)

        if self.queue != nil {

            self.queue.isSuspended = true
            self.queue.cancelAllOperations()
            self.queue = nil
    }
}

问题是调用 completionBlock 时，应用程序在我尝试更新布局约束时崩溃。

This application is modifying the autolayout engine from a background thread after the engine was accessed from the main thread.

如何避免这种崩溃？

Answer 1

您必须调用mainQueue 上的UI 内容。

例子:

operation1.completionBlock = {
    DispatchQueue.main.async {
        cell.buttonsStackViewHeightConstraint.constant = CGFloat(HEIGHT_CONSTRAINT)

        UIView.animate(withDuration: 0.3, animations: {
            cell.contentView.layoutIfNeeded()
        }, completion: nil)
    }
}