Centering a view in its superview using Visual For

问题:

I just started learning AutoLayout for iOS and had a look at Visual Format Language.

It all works fine except for one thing: I just can't get a view to center within its superview.
Is this possible with VFL or do I need to manually create a constraint myself?

回答1:

Currently, no, it doesn't look like it is possible to center a view in the superview using only VFL. It is, however, not that difficult to do it using a single VFL string and a single extra constraint (per axis):

VFL: "|-(>=20)-[view]-(>=20)-|"

[NSLayoutConstraint constraintWithItem:view
                             attribute:NSLayoutAttributeCenterX
                             relatedBy:NSLayoutRelationEqual
                                toItem:view.superview
                             attribute:NSLayoutAttributeCenterX
                            multiplier:1.f constant:0.f];

One would think that you would simply be able to do this (which is what I initially thought and tried when I saw this question):

[NSLayoutConstraint constraintsWithVisualFormat:@"|-(>=20)-[view(==200)]-(>=20)-|"
                                 options: NSLayoutFormatAlignAllCenterX | NSLayoutFormatAlignAllCenterY
                                 metrics:nil
                                   views:@{@"view" : view}];

I tried many different variations of the above trying to bend it to my will, but this does not appear to apply to the superview even when explicitly having two separate VFL strings for both axes (H:|V:). I then started to try and isolate exactly when the options do get applied to the VFL. They appear to not apply to the superview in the VFL and will only apply to any explicit views that are mentioned in the VFL string (which is disappointing in certain cases).

I hope in the future Apple adds some kind of new option to have the VFL options take into account the superview, even if doing it only when there is only a single explicit view besides the superview in the VFL. Another solution could be another option passed into the VFL that says something like: NSLayoutFormatOptionIncludeSuperview.

Needless to say, I learned a lot about VFL trying to answer this question.

回答2:

Yes, it is possible to center a view in its superview with Visual Format Language. Both vertically and horizontally. Here is the demo:

https://github.com/evgenyneu/center-vfl

回答3:

I think it is better to manually create constraints.

[superview addConstraint:
 [NSLayoutConstraint constraintWithItem:view
                              attribute:NSLayoutAttributeCenterX
                              relatedBy:NSLayoutRelationEqual
                                 toItem:superview
                              attribute:NSLayoutAttributeCenterX
                             multiplier:1
                               constant:0]];
[superview addConstraint:
 [NSLayoutConstraint constraintWithItem:view
                              attribute:NSLayoutAttributeCenterY
                              relatedBy:NSLayoutRelationEqual
                                 toItem:superview
                              attribute:NSLayoutAttributeCenterY
                             multiplier:1
                               constant:0]];

---

Now we can use NSLayoutAnchor to programatically define AutoLayout:

(Available in iOS 9.0 and later)

view.centerXAnchor.constraint(equalTo: superview.centerXAnchor).isActive = true
view.centerYAnchor.constraint(equalTo: superview.centerYAnchor).isActive = true

---

I recommend using SnapKit, which is a DSL to make Auto Layout easy on both iOS and macOS.

view.snp.makeConstraints({ $0.center.equalToSuperview() })

回答4:

Absolutely possible was able to do it like this:

[NSLayoutConstraint constraintsWithVisualFormat:@"H:[view]-(<=1)-[subview]"
                                        options:NSLayoutFormatAlignAllCenterY
                                        metrics:nil
                                          views:NSDictionaryOfVariableBindings(view, subview)];

Learned this from here. Code above centers subview in view by Y obviously.

Swift3:

        NSLayoutConstraint.constraints(withVisualFormat: "H:[view]-(<=1)-[subview]",
                                       options: .alignAllCenterY,
                                                       metrics: nil,
                                                       views: ["view":self, "subview":_spinnerView])

回答5:

Add below code and have your button at the center of the screen. Absolutely possible.

[self.view addConstraints:[NSLayoutConstraint
                           constraintsWithVisualFormat:@"H:|-[button]-|"
                           options:NSLayoutFormatAlignAllCenterY
                           metrics:0
                           views:views]];

[self.view addConstraints:[NSLayoutConstraint
                           constraintsWithVisualFormat:@"V:|-[button]-|"
                           options:NSLayoutFormatAlignAllCenterX
                           metrics:0
                           views:views]];

回答6:

I know it's not want you want but you can of course calculate the margins and use them to create the visual format string ;)

Anyway, no. Unfortunatly it's not possible to do that 'automatically' with VFL - at least not yet.

回答7:

Thanks to the answer from @Evgenii, I create a full example in gist:

center the red square vertically with custom height

https://gist.github.com/yallenh/9fc2104b719742ae51384ed95dcaf626

You can center a view vertically by useing either VFL (which is not quite intuitive) or use constraints manually. By the way, I cannot combine both centerY and height together in VFL like:

"H:[subView]-(<=1)-[followIcon(==customHeight)]"

In the current solution VFL constraints are added separately.

回答8:

What it have to be done is that superview should be declared in the dictionary. Instead of using | you use @{@"super": view.superview};.

And NSLayoutFormatAlignAllCenterX for vertical and NSLayoutFormatAlignAllCenterY for horizontal as options.

回答9:

You can use extra views.

NSDictionary *formats =
@{
  @"H:|[centerXView]|": @0,
  @"V:|[centerYView]|": @0,
  @"H:[centerYView(0)]-(>=0)-[yourView]": @(NSLayoutFormatAlignAllCenterY),
  @"V:[centerXView(0)]-(>=0)-[yourView]": @(NSLayoutFormatAlignAllCenterX),
};
NSDictionary *views = @{
  @"centerXView": centerXView, 
  @"centerYView": centerYView, 
  @"yourView": yourView,
};
 ^(NSString *format, NSNumber *options, BOOL *stop) {
     [superview addConstraints:
      [NSLayoutConstraint constraintsWithVisualFormat:format
                                              options:options.integerValue
                                              metrics:nil
                                                views:views]];
 }];

If you want it neat, then centerXView.hidden = centerYView.hidden = YES;.

PS: I am not sure I can call the extra views "placeholders", because English is my second language. It will be appreciated if someone can tell me that.

回答10:

For those who came here for an Interface Builder based solution (Google leads me here), just add const width/height constraints, and select the subview -> control drag to it's superview -> select "center vertically/horizontally in superview".

回答11:

Just need to say, that u can use this instead of constraints:

child.center = [parent convertPoint:parent.center fromView:parent.superview];

回答12:

@igor-muzyka answer in Swift 2:

NSLayoutConstraint.constraintsWithVisualFormat("H:[view]-(<=1)-[subview]", 
                                               options: .AlignAllCenterY,
                                               metrics: nil,
                                               views: ["view":view, "subview":subview])

回答13:

Instead of using VFL, you can easily do this in interface builder. In the Main Storyboard, ctrl+click on the view you want to center. Drag to the superview (while holding ctrl) and select "Center X" or "Center Y". No code needed.