ios - 自动布局约束不会因爱或金钱而更新

Question

我在自定义类中有一个方法可以调整和约束自定义 UIView:

-(void) sizeAndConstrainHeaderView:(HeaderView *)headerView inView:(UIView *)view {

    CGFloat height;
    // code to calculate height is omitted.

    headerView.heightConstraint = [NSLayoutConstraint constraintWithItem:headerView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:height];

    headerView.translatesAutoresizingMaskIntoConstraints = NO;

    if (!headerView.isConstrained) {
        [view addConstraint:[NSLayoutConstraint constraintWithItem:headerView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:view attribute:NSLayoutAttributeTop multiplier:1 constant:0]];
        [view addConstraint:[NSLayoutConstraint constraintWithItem:headerView attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:view attribute:NSLayoutAttributeTop multiplier:1 constant:0]];
        [view addConstraint:[NSLayoutConstraint constraintWithItem:headerView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:view.frame.size.width]];
        [headerView addConstraint:headerView.heightConstraint];
        headerView.isConstrained = YES;
    }

    headerView.heightConstraint.constant = height;
    // trying everything here to get the constraint to update
    [headerView setNeedsUpdateConstraints];
    [headerView layoutIfNeeded];
    [view setNeedsUpdateConstraints];
    [view layoutIfNeeded];

    NSLog(@"height constraint constant: %f", headerView.heightConstraint.constant);

}

我在 View Controller 中这样调用它:

[customClass sizeAndConstrainHeaderView:_headerView inView:self.view];

由于它被调用了多次(一次在检索数据之前，一次在检索数据之后)，所以有一个标志用于是否添加约束。然后，无论如何，将高度常数设置为计算出的高度。但是无论我怎么调用，在 View 上还是自定义 View 上， View 的高度都不会改变，我也想不通为什么。其他约束工作正常。日志显示高度已更改。但是 View 没有反射(reflect)它。我肯定知道，因为我调试了 View 层次结构并且可以看到它没有更新。

Answer 1

你说

Since this gets called several times (once before data is retrieved, once after), there is a flag for whether to add constraints.

但是您的方法会在每次 调用时执行此操作:

headerView.heightConstraint = [NSLayoutConstraint constraintWithItem:headerView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:height];

因此在每次调用时，您都会创建一个新约束，将其存储在 headerView.heightConstraint 中，并设置新约束的常量。

但是，您只能在第一次调用该方法时安装此新约束。

也许您应该只在第一次调用时创建约束:

-(void) sizeAndConstrainHeaderView:(HeaderView *)headerView inView:(UIView *)view {

    CGFloat height;
    // code to calculate height is omitted.

    if (!headerView.isConstrained) {
        headerView.translatesAutoresizingMaskIntoConstraints = NO;

        [view addConstraint:[NSLayoutConstraint constraintWithItem:headerView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:view attribute:NSLayoutAttributeTop multiplier:1 constant:0]];
        [view addConstraint:[NSLayoutConstraint constraintWithItem:headerView attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:view attribute:NSLayoutAttributeTop multiplier:1 constant:0]];
        [view addConstraint:[NSLayoutConstraint constraintWithItem:headerView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:view.frame.size.width]];

        headerView.heightConstraint = [NSLayoutConstraint constraintWithItem:headerView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:height];

        [headerView addConstraint:headerView.heightConstraint];
        headerView.isConstrained = YES;
    }

    headerView.heightConstraint.constant = height;
}