iOS: programmatically changing constraints set from Interface Builder

Question

Good day, friends!

I failed trying to animate views that were set in IB from code. App crashes with following reason:

The view hierarchy is not prepared for the constraint...

I saw some similar questions here and the reason was always that the view which was created programmatically, is not yet added to superview. But I created all views in IB!

The console also says:

View not found in container hierarchy: (here goes it's superview)

It doesn't make any sense to me because in fact it is a subview of appropriate superview, and xcode knows it - it prints view hierarchy right away and it fits.

What could the reason? Thank you!

Edit: code I use:

    - (void)setEditingConstraintsForView:(UIView *)view
{
    // Pin given view to top, fix it height
    NSDictionary *givenView = @{@"view":view};
    view.translatesAutoresizingMaskIntoConstraints = NO;

    NSArray *horizontalConstraints = [NSLayoutConstraint constraintsWithVisualFormat:@"H:|[view]|" options:0 metrics:nil views:givenView];
    NSArray *verticalConstraints = [NSLayoutConstraint constraintsWithVisualFormat:@"V:|[view(height)]" options:0 metrics:@{@"height":@(viewHeight)} views:givenView];

    [self.animatedVIew addSubview:view];

    for (NSArray *constraints in @[horizontalConstraints, verticalConstraints]) {
        [view addConstraints:constraints];
    }
}

Also I delete all constraints that I set in IB before installing new:

    - (NSDictionary *)constraintsFromIB
{
    if (!_constraintsFromIB) {
        _constraintsFromIB = @{@"view1":self.view1.referencingConstraintsInSuperviews,
                               @"view2":self.view2.referencingConstraintsInSuperviews,
                               @"view3":self.view3.referencingConstraintsInSuperviews };
    }

    return _constraintsFromIB;
}

And then:

- (void)updateViewConstraints
{
    [super updateViewConstraints];

    // clear all constraints
    for (NSString *viewName in self.constraintsFromIB.allKeys) {
        for (NSLayoutConstraint *constraint in self.constraintsFromIB[viewName]) {
            [constraint remove];
           }

} }

UPDATE 2: Method I use invoke change: when user touches the view, this one is called:

- (void)animateConstraintsForState:(LDYEditingLabel)state
{
    self.editingLabel = state;        
    [UIView animateWithDuration:0.3 animations:^{
        [self updateViewConstraints];
        [self.view layoutIfNeeded];
    }];
}

Later in updateViewConstraints: there is a code that triggers my method setEditingConstraintsForView:(UIView *)view

Which method do you invoke this code from? viewDidAppear? viewDidAppear? or some other method? — Paulw11
– Paulw11, Commented Jan 16, 2015 at 3:18
Just added another edit. It triggers from the tap on the screen using animation block. — Aleksandr Shcherbakov
– Aleksandr Shcherbakov, Commented Jan 16, 2015 at 3:41
I can't see the problem from what you have shown here. You may need to set a breakpoint, but I was going to suggest what Charlie Wu just put in his answer - modify the existing constraints if you can - much easier — Paulw11
– Paulw11, Commented Jan 16, 2015 at 3:56
Thank you anyway for the advice. It looks like it's no good to mix up IB and hardcode approach. — Aleksandr Shcherbakov
– Aleksandr Shcherbakov, Commented Jan 16, 2015 at 15:57

Charlie Wu · Accepted Answer · 2015-01-16 03:55:42Z

23

if you use IB to create constrains you can add constrains as outlet.

You then update the constrain and call [self updateViewConstraints]; in the animation block

enter image description here

answered Jan 16, 2015 at 3:55

Charlie Wu

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2 Comments

Aleksandr Shcherbakov Over a year ago

Thank you for the answer, that definitely would work. But what if I had a lot of constraints to adit? I need views to appear and disappear, change their places on screen and so on. I think if I create outlet for each constraint - view controller implementation would be a horrible mess

Charlie Wu Over a year ago

if you are using code, wouldn't you have tone of code loop through view constrains and look for the correct one? it's much harder to maintain when view layout is changed. This one it might not be the cleanest code, but it should be fairly easy to maintain and understand

skwashua · Accepted Answer · 2015-01-16 03:59:10Z

2

If it were me, I'd wire the constraints to IBOutlets and just modify the existing constraints. It would probably end up being easier than adding and removing them programmatically.

You may also need to call setNeedsLayout after you've modified the constraints.

answered Jan 16, 2015 at 3:59

skwashua

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2 Comments

Aleksandr Shcherbakov Over a year ago

Thank you for the advice! It works perfectly fine. But the reason why I tried to do it programmatically is that I have a lot of views to change constraints. At some point I need all views to disappear from the screen and display new ones - datePicker, in particular and several custom controls. And than it should return to previous state. What would you do in that kind of situation? Thanks again!

Nick Turner Over a year ago

I keep an array of changing constraints when setting the view and since you can only change the constant anyways. I subclass the UIVIew and works fine for me.

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