Question

我想在cellForRowAtIndexPath中添加一些视图到我的单元格内容视图，对于它们的约束，但没有任何作用。我有这样的事情：

NSLayoutConstraint *constraint = [NSLayoutConstraint constraintWithItem:imageView
                                  attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:cell.contentView attribute:NSLayoutAttributeLeft multiplier:1.0f constant:10.0f];

[cell.contentView addConstraint:constraint];

我该怎么做？

Answer 1

有几点意见：

您创建此特定约束是正确的。显然，您不能只设置左约束，但需要指定将明确定义单元格子视图的frame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，但关键是你必须添加所有的约束，明确定义所有子视图的frame。

例如，您可能会遇到以下内容：

- (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)提示符下输入以下内容：
```
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。

如果您想查看frame对所有观看的内容，可以在(lldb)提示符下输入以下内容：
```
po [[UIWindow keyWindow] recursiveDescription]
```
请务必为所有子视图指定translatesAutoresizingMaskIntoConstraints到NO，就像我在上面的代码示例中所做的那样。

虽然您可以使用constraintWithItem定义约束，但人们通常会使用constraintsWithVisualFormat，因为您通常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;
}

iOS以编程方式为表视图单元格内容创建NSLayoutConstraint

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