How to set topAnchor constraint relative to the superview height

Question

I have two views: toastView and view. toastView is subview of view. I want to position toastView on the y axis by 80% of view height. How can I do this using constants in the code?

I assumed that there is a method like:

[toastView.topAnchor constraintEqualToAnchor:view.heightAnchor multiplier:0.8].active = YES;

but i can't mixing NSLayoutDimension (width and height) and NSLayoutYAxisAnchor (X and Y)

This is how it looks in the design:

scheme of how it looks on the design

80% of view height ? means height of toastView = views 80 percent — Jawad Ali, Feb 01 '20 at 08:31

vacawama · Accepted Answer · 2020-02-01T15:10:02.830

5

The trick here is to set the top of toastView equal to the bottom of self.view with a multiplier of 0.8:

Objective-C:

[NSLayoutConstraint constraintWithItem: toastView attribute: NSLayoutAttributeTop
    relatedBy: NSLayoutRelationEqual toItem: self.view
    attribute: NSLayoutAttributeBottom multiplier: 0.8 constant: 0].active = YES;

Swift:

NSLayoutConstraint(item: toastView, attribute: .top, relatedBy: .equal,
  toItem: self.view, attribute: .bottom, multiplier: 0.8, constant: 0).isActive = true

edited Feb 01 '20 at 15:10

answered Feb 01 '20 at 14:48

vacawama

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Thanks, 1 question, is this method an old way to do NSLayout constraint? It takes more code but could be used for relative topAnchor. I found there is no `xxx.topAnchor.constraint(equalTo: holder.bottomAnchor, multiplier: 0.8)` such method when dealing with topAnchor. – Zhou Haibo May 30 '21 at 06:02
@ChuckZHB, using NSLayoutContraint is an older and more complete syntax. In this case, using the anchor style syntax doesn't give you access to the ability to set the .top of one item equal to the .bottom of another with a multiplier. – vacawama May 31 '21 at 15:33
Ok, so it is the same to what I think, though I didn't confirm it before. Also I google it, that says NSLayout constraint is the raw API. Thanks, I learn a new thing. – Zhou Haibo May 31 '21 at 16:02

score 0 · Answer 2 · answered Feb 01 '20 at 08:55

0

You can do it like this in swift

override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

        let yConstraint = NSLayoutConstraint(item: toastView, attribute: .Top, relatedBy: .Equal, toItem: view, attribute: .Top, multiplier: 1, constant: view.bounds.height * 0.8 )
        NSLayoutConstraint.activateConstraints([yConstraint])
    }

Objective c

- (void)viewDidLayoutSubviews {
    [super viewDidLayoutSubviews];

    NSLayoutConstraint *topAnchorConstraint = [NSLayoutConstraint constraintWithItem:toastView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:view attribute:NSLayoutAttributeTop multiplier:1 constant:(view.bounds.size.height * 0.8)];

    [self.view addConstraint:topAnchorConstraint];
}

answered Feb 01 '20 at 08:55

Jawad Ali

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It works, but I would not want to use method overrides – Vasiliy Korchagin Feb 02 '20 at 19:54
if you donnt want to use method overrides then simple add this line in `viewWillAppear()` and call `layoutifneeded()` – Jawad Ali Feb 03 '20 at 05:39
But this will be a viewWillAppear method override. – Vasiliy Korchagin Feb 03 '20 at 08:28
where you want to set them instead ? :) – Jawad Ali Feb 03 '20 at 08:45
its the same sir :) you can add that piece of code anywhere – Jawad Ali Feb 04 '20 at 16:41
1

In some cases, the view may have a zero frame. For example, when toast view is added to a view that is not yet on the screen (its view controller is not in the stack). In this case, view.bounds.size.height will be zero. The accepted answer uses view constraints instead of referring to its sizes (like view.bounds.height). – Vasiliy Korchagin Feb 05 '20 at 22:00

How to set topAnchor constraint relative to the superview height

2 Answers2