@implementation MBTableDetailStyleFooterView

static void MBTableDetailStyleFooterViewCommonSetup(MBTableDetailStyleFooterView *_self) {
    UILabel *rightLabel = [[UILabel alloc] init];
    _self.rightLabel = rightLabel;
    rightLabel.translatesAutoresizingMaskIntoConstraints = NO;
    [_self.contentView addSubview:rightLabel];

    UILabel *leftLabel = [[UILabel alloc] init];
    _self.leftLabel = leftLabel;
    leftLabel.translatesAutoresizingMaskIntoConstraints = NO;
    [_self.contentView addSubview:leftLabel];

    NSDictionary *views = NSDictionaryOfVariableBindings(rightLabel,leftLabel);

    NSArray *horizontalConstraints = [NSLayoutConstraint constraintsWithVisualFormat:@"|-10-[leftLabel]-(>=10)-[rightLabel]-10-|" options:0 metrics:nil views:views];
    [_self.contentView addConstraints:horizontalConstraints];

    // center views vertically in super view
    NSLayoutConstraint *leftCenterYConstraint = [NSLayoutConstraint constraintWithItem:leftLabel attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:_self.contentView attribute:NSLayoutAttributeCenterY multiplier:1 constant:0];
    [_self.contentView addConstraint:leftCenterYConstraint];
    NSLayoutConstraint *rightCenterYConstraint = [NSLayoutConstraint constraintWithItem:rightLabel attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:_self.contentView attribute:NSLayoutAttributeCenterY multiplier:1 constant:0];
    [_self.contentView addConstraint:rightCenterYConstraint];

    // same height for both labels
    NSLayoutConstraint *sameHeightConstraint = [NSLayoutConstraint constraintWithItem:leftLabel attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:rightLabel attribute:NSLayoutAttributeHeight multiplier:1 constant:0];
    [_self.contentView addConstraint:sameHeightConstraint];
}

+ (BOOL)requiresConstraintBasedLayout {
    return YES;
}

- (id)initWithReuseIdentifier:(NSString *)reuseIdentifier {
    self = [super initWithReuseIdentifier:reuseIdentifier];
    MBTableDetailStyleFooterViewCommonSetup(self);
    return self;
}

@end

这个类用作tableView中的第一部分的页脚,具有2个部分.第一部分包含动态项目.第二部分只有一行,用于向第一部分添加新项目.

如果第一部分没有项目,我隐藏了脚注视图.所以当我添加第一个新项目时,我必须重新加载该部分,以便footerView出现.所有这些代码看起来像这样：

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
    [tableView deselectRowAtIndexPath:indexPath animated:YES];
    if (indexPath.section == 1) {
        BOOL sectionNeedsReload = ([self.data count] == 0); // reload section when no data (and therefor no footer) was present before the add
        [self.data addObject:[NSDate date]];
        NSIndexPath *newIndexPath = [NSIndexPath indexPathForRow:[self.data count]-1 inSection:0];
        if (sectionNeedsReload) {
            [self.tableView reloadSections:[NSIndexSet indexSetWithIndex:0] withRowAnimation:UITableViewRowAnimationAutomatic];
        }
        else {
            [self.tableView insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
        }
        [self configureFooter:(MBTableDetailStyleFooterView *)[tableView footerViewForSection:0] forSection:0];
    }
}

- (void)configureFooter:(MBTableDetailStyleFooterView *)footer forSection:(NSInteger)section {
    footer.leftLabel.text = @"Total";
    footer.rightLabel.text = [NSString stringWithFormat:@"%d",[self.data count]];
}

- (UIView *)tableView:(UITableView *)tableView viewForFooterInSection:(NSInteger)section {
    MBTableDetailStyleFooterView *footer = nil;
    if (section == 0 && [self.data count]) {
        footer = [tableView dequeueReusableHeaderFooterViewWithIdentifier:@"Footer"];
        [self configureFooter:footer forSection:section];
    }
    return footer;
}

- (CGFloat)tableView:(UITableView *)tableView heightForFooterInSection:(NSInteger)section {
    CGFloat height = 0;
    if (section == 0 && [self.data count]) {
        height = 20.0f;
    }
    return height;
}

没什么好想的但是,只要reloadSections：withRowAnimations：在我的tableView上被调用,它会抛出异常,因为它是“无法同时满足约束”.

在某处,tableView将翻译的自动调整大小的掩码约束添加到我的页脚.

(
    "<NSLayoutConstraint:0x718a1f0 H:[UILabel:0x7189130]-(10)-|   (Names: '|':_UITableViewHeaderFooterContentView:0x7188df0 )>","<NSLayoutConstraint:0x7189e30 H:[UILabel:0x71892c0]-(>=10)-[UILabel:0x7189130]>","<NSLayoutConstraint:0x718a0a0 H:|-(10)-[UILabel:0x71892c0]   (Names: '|':_UITableViewHeaderFooterContentView:0x7188df0 )>","<NSAutoresizingMaskLayoutConstraint:0x7591ab0 h=--& v=--& H:[_UITableViewHeaderFooterContentView:0x7188df0(0)]>"
)

当我替换reloadSections：withRowAnimations：调用reloadData不添加自动调整掩码约束,一切正常.

有趣的是,异常告诉我,它试图中断约束< NSLayoutConstraint：0x7189e30 H：[UILabel：0x71892c0] - (> = 10) – [UILabel：0x7189130]>

但是当我在对configureFooter的后续调用中记录约束：forSection：该约束仍然存在,但是自动调整大小掩码约束已经消失

这些约束正是我所设定的.

(
    "<NSLayoutConstraint:0x718a0a0 H:|-(10)-[UILabel:0x71892c0]   (Names: '|':_UITableViewHeaderFooterContentView:0x7188df0 )>","<NSLayoutConstraint:0x718a1f0 H:[UILabel:0x7189130]-(10)-|   (Names: '|':_UITableViewHeaderFooterContentView:0x7188df0 )>","<NSLayoutConstraint:0x718a3f0 UILabel:0x71892c0.centerY == _UITableViewHeaderFooterContentView:0x7188df0.centerY>","<NSLayoutConstraint:0x718a430 UILabel:0x7189130.centerY == _UITableViewHeaderFooterContentView:0x7188df0.centerY>","<NSLayoutConstraint:0x718a4b0 UILabel:0x71892c0.height == UILabel:0x7189130.height>"
)

这个自动调整大小面膜约束来自哪里？哪里去

我错过了什么吗？我第一次看汽车布局就像一个星期前,所以这是完全可能的.