ios - 在 cellForRowAtIndexPath 中执行繁重的计算

Question

我有一个加载产品的表格 View 。每个的价格都需要一些繁重的计算，所以我想在主 UI 线程之外进行此操作，因为否则滚动表格会非常非常跳跃，几乎不可能。无论我尝试什么，我都无法正确理解语法和概念。我当前的代码确实在后台加载了价格，但是当我滚动时，一些单元格给出了不属于它们的价格。我不知道如何调试这个。任何建议表示赞赏。

- (UITableViewCell *)tableView:(UITableView *)theTableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    if ([self.products count] == 0) {
        UITableViewCell *cell = [[UITableViewCell alloc] init];
        return cell;
    }

    AvailableCustomerProductTableViewCell *cell = [self.tableView dequeueReusableCellWithIdentifier:CellIdentifier forIndexPath:indexPath];
    cell.accessoryType = UITableViewCellAccessoryNone;

    cell.buttonAdd.tag = indexPath.row;
    [cell.buttonAdd addTarget: self action: @selector(addToSelectedProduct:) forControlEvents: UIControlEventTouchUpInside];

    Product *prod = nil;
    if (theTableView == self.searchDisplayController.searchResultsTableView) {
        prod = (Product *)[self.filteredProducts objectAtIndex:indexPath.row];
    } else {
        prod = (Product *)[self.products objectAtIndex:indexPath.row];
    }

    if (prod != nil) {
        cell.pNumber.text = prod.number;
        cell.description.text = prod.desc;

        if ([Common getProductPromotion:prod] != nil)  {
            cell.btnPromotionTag.hidden = NO;
            cell.btnPromotionTag.tag = indexPath.row;
            [cell.btnPromotionTag addTarget: self action: @selector(showPromotionDetails:) forControlEvents: UIControlEventTouchUpInside];
        }
        else{
            cell.btnPromotionTag.hidden = YES;
        }

        //Get the customer product price, first:
        //If if the product has a record in the productCustomerPrices list
        //if not get the price from the standard price.
        if (self.order.orderOrderCustomer != nil) {
            // Do the Customer price fetching in a background thread
            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                CustomerPrice *custPrice = [Common getPriceForCustomer:self.order.orderOrderCustomer.customerId forProduct:prod.productId inManagedObjectContext:self.backgroundContext];

                // Return to the main thread to update the UI
                dispatch_async(dispatch_get_main_queue(), ^{
                    if (custPrice) {
                        NSManagedObjectID *objId = custPrice.objectID;
                        CustomerPrice *custPrice = (CustomerPrice*)[self.managedObjectContext objectWithID:objId];

                        if (custPrice) {
                            // Check that the relevant data is still required and get the correct cell (it might have changed)
                            AvailableCustomerProductTableViewCell * correctCell = nil;
                            if ([[theTableView indexPathsForVisibleRows] containsObject:indexPath]) {
                                correctCell = (AvailableCustomerProductTableViewCell*)[self.tableView cellForRowAtIndexPath:indexPath];
                            }

                            [correctCell.btnPrice setTitle:[Common getCurrencyFormattedStringFromFloat:[custPrice.price floatValue]] forState:UIControlStateNormal];
                            [correctCell.btnPrice setTitleColor:[UIColor colorWithRed:0.01 green:0.65 blue:0.77 alpha:1] forState:UIControlStateNormal];
                            correctCell.btnPrice.enabled = NO;
                            [correctCell setNeedsLayout];
                        }                    }
                });
            });
        }
    }

    UISwipeGestureRecognizer* sgr = [[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(cellSwiped:)];
    [sgr setDirection:UISwipeGestureRecognizerDirectionRight];
    [cell addGestureRecognizer:sgr];

    return cell;
}

Answer 1

您是否尝试过分几个步骤执行此操作:

1. 在加载 View 时(在 viewWillAppear: 中)加载您的产品列表以快速显示 ListView，
2. 在后台调用服务器以在单独的 NSDictionary*prices 中检索价格列表，键为 productId。当计算结束后，你应该在主线程调用[self.tableView reloadData]!
3. 在 cellForRowAtIndexPath: 方法中，您应该只检查您的 prices 字典中是否有价格并显示您的按钮。如果您没有价格，请务必隐藏该按钮，以避免在回收电池时出现显示故障。

像这样，您可以避免一些复杂的按行和关联重新加载，这可能会导致您的问题。