iOS以编程方式为表视图单元格内容创建NSLayoutConstraint

几点意见:

您创建此特定约束是正确的.显然,您不能只设置左约束,而是需要指定将明确定义frame单元格子视图的所有约束.例如,不仅要定义左(或前导)约束,还要定义顶部,底部和宽度约束.或者定义左约束加上宽度和高度约束,并指定垂直y约束.有很多不同的方法可以做到这一点,但关键是你必须添加所有明确定义frame所有子视图的约束.

例如,您可能具有以下内容:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *cellIdentifier = @"Cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];

    UIImageView *customImageView;
    UILabel *customLabel;

    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier];

        customImageView = [[UIImageView alloc] init];
        customImageView.translatesAutoresizingMaskIntoConstraints = NO;
        customImageView.tag = IMAGEVIEWTAG;
        [cell.contentView addSubview:customImageView];

        [cell.contentView addConstraint:[NSLayoutConstraint constraintWithItem:customImageView
                                                                     attribute:NSLayoutAttributeLeading
                                                                     relatedBy:NSLayoutRelationEqual
                                                                        toItem:cell.contentView
                                                                     attribute:NSLayoutAttributeLeft
                                                                    multiplier:1.0
                                                                      constant:25.0]];

        [cell.contentView addConstraint:[NSLayoutConstraint constraintWithItem:customImageView
                                                                     attribute:NSLayoutAttributeWidth
                                                                     relatedBy:NSLayoutRelationEqual
                                                                        toItem:nil
                                                                     attribute:NSLayoutAttributeNotAnAttribute
                                                                    multiplier:1.0
                                                                      constant:30.0]];

        [cell.contentView addConstraint:[NSLayoutConstraint constraintWithItem:customImageView
                                                                     attribute:NSLayoutAttributeTop
                                                                     relatedBy:NSLayoutRelationEqual
                                                                        toItem:cell.contentView
                                                                     attribute:NSLayoutAttributeTop
                                                                    multiplier:1.0
                                                                      constant:3.0]];

        [cell.contentView addConstraint:[NSLayoutConstraint constraintWithItem:customImageView
                                                                     attribute:NSLayoutAttributeBottom
                                                                     relatedBy:NSLayoutRelationEqual
                                                                        toItem:cell.contentView
                                                                     attribute:NSLayoutAttributeBottom
                                                                    multiplier:1.0
                                                                      constant:-3.0]];

        customLabel = [[UILabel alloc] init];
        customLabel.translatesAutoresizingMaskIntoConstraints = NO;
        customLabel.tag = LABELTAG;
        [cell.contentView addSubview:customLabel];

        [cell.contentView addConstraint:[NSLayoutConstraint constraintWithItem:customLabel
                                                                     attribute:NSLayoutAttributeLeading
                                                                     relatedBy:NSLayoutRelationEqual
                                                                        toItem:customImageView
                                                                     attribute:NSLayoutAttributeTrailing
                                                                    multiplier:1.0
                                                                      constant:10.0]];

        [cell.contentView addConstraint:[NSLayoutConstraint constraintWithItem:customLabel
                                                                     attribute:NSLayoutAttributeTrailing
                                                                     relatedBy:NSLayoutRelationEqual
                                                                        toItem:cell.contentView
                                                                     attribute:NSLayoutAttributeTrailing
                                                                    multiplier:1.0
                                                                      constant:-10.0]];

        [cell.contentView addConstraint:[NSLayoutConstraint constraintWithItem:customLabel
                                                                     attribute:NSLayoutAttributeTop
                                                                     relatedBy:NSLayoutRelationEqual
                                                                        toItem:cell.contentView
                                                                     attribute:NSLayoutAttributeTop
                                                                    multiplier:1.0
                                                                      constant:3.0]];

        [cell.contentView addConstraint:[NSLayoutConstraint constraintWithItem:customLabel
                                                                     attribute:NSLayoutAttributeBottom
                                                                     relatedBy:NSLayoutRelationEqual
                                                                        toItem:cell.contentView
                                                                     attribute:NSLayoutAttributeBottom
                                                                    multiplier:1.0
                                                                      constant:-3.0]];

    }
    else {
        customImageView = (id)[cell.contentView viewWithTag:IMAGEVIEWTAG];
        customLabel     = (id)[cell.contentView viewWithTag:LABELTAG];
    }

    customImageView.image = ...;
    customLabel.text      = ...;

    return cell;
}

显然,您经常使用UITableViewCell子类来促进跟踪自定义控件的过程,但我希望保持示例简单.

如果您不确定是否已明确定义约束,请运行应用程序,并在呈现UI后,暂停应用程序并在(lldb)提示符处输入以下内容:

po [[UIWindow keyWindow] _autolayoutTrace]

如果任何视图被明确定义(即是否缺少任何约束),这将通知您.

如果要查看frame所有视图的内容,可以在(lldb)提示符处输入以下内容:

po [[UIWindow keyWindow] recursiveDescription]

确保指定translatesAutoresizingMaskIntoConstraints到NO所有子视图,如我上面的代码示例中一样.

虽然您可以使用constraintWithItem常用的人来定义约束,但通常constraintsWithVisualFormat可以更简洁地定义约束.将上面的代码示例与此代码示例进行对比:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *cellIdentifier = @"Cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];

    UIImageView *customImageView;
    UILabel *customLabel;

    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier];

        customImageView = [[UIImageView alloc] init];
        customImageView.translatesAutoresizingMaskIntoConstraints = NO;
        customImageView.tag = IMAGEVIEWTAG;
        [cell.contentView addSubview:customImageView];

        customLabel = [[UILabel alloc] init];
        customLabel.translatesAutoresizingMaskIntoConstraints = NO;
        customLabel.tag = LABELTAG;
        [cell.contentView addSubview:customLabel];

        NSDictionary *views = NSDictionaryOfVariableBindings(customImageView, customLabel);
        [cell.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-25-[customImageView(30)]-[customLabel]|" options:0 metrics:nil views:views]];
        [cell.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-3-[customImageView]-3-|"                 options:0 metrics:nil views:views]];
        [cell.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-3-[customLabel]-3-|"                     options:0 metrics:nil views:views]];
    }
    else {
        customImageView = (id)[cell.contentView viewWithTag:IMAGEVIEWTAG];
        customLabel     = (id)[cell.contentView viewWithTag:LABELTAG];
    }

    customImageView.image = ...;
    customLabel.text      = ...;

    return cell;
}