Emulating aspect-fit behaviour using AutoLayout constraints in Xcode 6

You're not describing scale-to-fit; you're describing aspect-fit. (I have edited your question in this regard.) The subview becomes as large as possible while maintaining its aspect ratio and fitting entirely inside its parent.

Anyway, you can do this with auto layout. You can do it entirely in IB as of Xcode 5.1. Let's start with some views:

https://i.stack.imgur.com/hHDxR.png

The light green view has an aspect ratio of 4:1. The dark green view has an aspect ratio of 1:4. I'm going to set up constraints so that the blue view fills the top half of the screen, the pink view fills the bottom half of the screen, and each green view expands as much as possible while maintaining its aspect ratio and fitting in its container.

First, I'll create constraints on all four sides of the blue view. I'll pin it to its nearest neighbor on each edge, with a distance of 0. I make sure to turn off margins:

https://i.stack.imgur.com/Yo2JF.gif

Note that I don't update the frame yet. I find it easier to leave room between the views when setting up constraints, and just set the constants to 0 (or whatever) by hand.

Next, I pin the left, bottom, and right edges of the pink view to its nearest neighbor. I don't need to set up a top edge constraint because its top edge is already constrained to the bottom edge of the blue view.

https://i.stack.imgur.com/SPiNs.gif

I also need an equal-heights constraint between the pink and blue views. This will make them each fill half the screen:

https://i.stack.imgur.com/oelrv.gif

If I tell Xcode to update all the frames now, I get this:

https://i.stack.imgur.com/9fbwL.png

So the constraints I've set up so far are correct. I undo that and start work on the light green view.

Aspect-fitting the light green view requires five constraints:

A required-priority aspect ratio constraint on the light green view. You can create this constraint in a xib or storyboard with Xcode 5.1 or later.

A required-priority constraint limiting the width of the light green view to be less than or equal to the width of its container.

A high-priority constraint setting the width of the light green view to be equal to the width of its container.

A required-priority constraint limiting the height of the light green view to be less than or equal to the height of its container.

A high-priority constraint setting the height of the light green view to be equal to the height of its container.

Let's consider the two width constraints. The less-than-or-equal constraint, by itself, is not sufficient to determine the width of the light green view; many widths will fit the constraint. Since there's ambiguity, autolayout will try to choose a solution that minimizes the error in the other (high-priority but not required) constraint. Minimizing the error means making the width as close as possible to the container's width, while not violating the required less-than-or-equal constraint.

The same thing happens with the height constraint. And since the aspect-ratio constraint is also required, it can only maximize the size of the subview along one axis (unless the container happens to have the same aspect ratio as the subview).

So first I create the aspect ratio constraint:

https://i.stack.imgur.com/4IhOw.gif

Then I create equal width and height constraints with the container:

https://i.stack.imgur.com/vycag.gif

I need to edit these constraints to be less-than-or-equal constraints:

https://i.stack.imgur.com/BC7rh.gif

Next I need to create another set of equal width and height constraints with the container:

https://i.stack.imgur.com/U9HkM.gif

And I need to make these new constraints less than required priority:

https://i.stack.imgur.com/I7DTE.gif

Finally, you asked for the subview to be centered in its container, so I'll set up those constraints:

https://i.stack.imgur.com/4DTr1.gif

Now, to test, I'll select the view controller and ask Xcode to update all the frames. This is what I get:

https://i.stack.imgur.com/KmtoY.png

Oops! The subview has expanded to completely fill its container. If I select it, I can see that in fact it's maintained its aspect ratio, but it's doing an aspect-fill instead of an aspect-fit.

The problem is that on a less-than-or-equal constraint, it matters which view is at each end of the constraint, and Xcode has set up the constraint opposite from my expectation. I could select each of the two constraints and reverse its first and second items. Instead, I'll just select the subview and change the constraints to be greater-than-or-equal:

https://i.stack.imgur.com/Mh4XM.gif

Xcode updates the layout:

https://i.stack.imgur.com/sRvqg.png

Now I do all the same things to the dark green view on the bottom. I need to make sure its aspect ratio is 1:4 (Xcode resized it in a weird way since it didn't have constraints). I won't show the steps again since they're the same. Here's the result:

https://i.stack.imgur.com/kDj1r.png

Now I can run it in the iPhone 4S simulator, which has a different screen size than IB used, and test rotation:

https://i.stack.imgur.com/HpA6w.gif

And I can test in in the iPhone 6 simulator:

https://i.stack.imgur.com/OtBQ9.gif

I've uploaded my final storyboard to this gist for your convenience.

---

Rob, your answer is awesome! I also know that this question is specifically about achieving this by using auto-layout. However, just as a reference, I'd like to show how this can be done in code. You set up the top and bottom views (blue and pink) just like Rob showed. Then you create a custom AspectFitView:

AspectFitView.h:

#import <UIKit/UIKit.h>

@interface AspectFitView : UIView

@property (nonatomic, strong) UIView *childView;

@end

AspectFitView.m:

#import "AspectFitView.h"

@implementation AspectFitView

- (void)setChildView:(UIView *)childView
{
    if (_childView) {
        [_childView removeFromSuperview];
    }

    _childView = childView;

    [self addSubview:childView];
    [self setNeedsLayout];
}

- (void)layoutSubviews
{
    [super layoutSubviews];

    if (_childView) {
        CGSize childSize = _childView.frame.size;
        CGSize parentSize = self.frame.size;
        CGFloat aspectRatioForHeight = childSize.width / childSize.height;
        CGFloat aspectRatioForWidth = childSize.height / childSize.width;

        if ((parentSize.height * aspectRatioForHeight) > parentSize.height) {
            // whole height, adjust width
            CGFloat width = parentSize.width * aspectRatioForWidth;
            _childView.frame = CGRectMake((parentSize.width - width) / 2.0, 0, width, parentSize.height);
        } else {
            // whole width, adjust height
            CGFloat height = parentSize.height * aspectRatioForHeight;
            _childView.frame = CGRectMake(0, (parentSize.height - height) / 2.0, parentSize.width, height);
        }
    }
}

@end

Next, you change the class of the blue and pink views in the storyboard to be AspectFitViews. Finally you set two outlets to your viewcontroller topAspectFitView and bottomAspectFitView and set their childViews in viewDidLoad:

- (void)viewDidLoad {
    [super viewDidLoad];

    UIView *top = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 500, 100)];
    top.backgroundColor = [UIColor lightGrayColor];

    UIView *bottom = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 100, 500)];
    bottom.backgroundColor = [UIColor greenColor];

    _topAspectFitView.childView = top;
    _bottomAspectFitView.childView = bottom;
}

So it's not hard to do this in code and it is still very adaptable and works with variably-sized views and different aspect ratios.

Update July 2015: Find a demo app here: https://github.com/jfahrenkrug/SPWKAspectFitView

---

This is for macOS.

I have problem to use Rob's way to achieve aspect fit on the OS X application. But I made it with another way -- Instead of using width and height, I used leading, trailing, top and bottom space.

Basically, add two leading spaces where one is >= 0 @1000 required priority and another one is = 0 @250 low priority. Do same settings to trailing, top and bottom space.

Of course, you need to set aspect ratio and centre X and centre Y.

And then job's done!

https://i.stack.imgur.com/Mo1Gs.png

---

I needed a solution from the accepted answer, but executed from the code. The most elegant way I've found is using Masonry framework.

#import "Masonry.h"

...

[view mas_makeConstraints:^(MASConstraintMaker *make) {
    make.width.equalTo(view.mas_height).multipliedBy(aspectRatio);
    make.size.lessThanOrEqualTo(superview);
    make.size.equalTo(superview).with.priorityHigh();
    make.center.equalTo(superview);
}];

---

I found myself wanting aspect-fill behavior so that a UIImageView would maintain its own aspect ratio and entirely fill the container view. Confusingly, my UIImageView was breaking BOTH high-priority equal-width and equal-height constraints (described in Rob's answer) and rendering at full resolution.

The solution was simply to set the UIImageView's Content Compression Resistance Priority lower than the priority of the equal-width and equal-height constraints:

https://i.stack.imgur.com/LgjHR.png

---

This is a port of @rob_mayoff's excellent answer to a code-centric approach, using NSLayoutAnchor objects and ported to Xamarin. For me, NSLayoutAnchor and related classes have made AutoLayout much easier to program:

public class ContentView : UIView
{
        public ContentView (UIColor fillColor)
        {
            BackgroundColor = fillColor;
        }
}

public class MyController : UIViewController 
{
    public override void ViewDidLoad ()
    {
        base.ViewDidLoad ();

        //Starting point:
        var view = new ContentView (UIColor.White);

        blueView = new ContentView (UIColor.FromRGB (166, 200, 255));
        view.AddSubview (blueView);

        lightGreenView = new ContentView (UIColor.FromRGB (200, 255, 220));
        lightGreenView.Frame = new CGRect (20, 40, 200, 60);

        view.AddSubview (lightGreenView);

        pinkView = new ContentView (UIColor.FromRGB (255, 204, 240));
        view.AddSubview (pinkView);

        greenView = new ContentView (UIColor.Green);
        greenView.Frame = new CGRect (80, 20, 40, 200);
        pinkView.AddSubview (greenView);

        //Now start doing in code the things that @rob_mayoff did in IB

        //Make the blue view size up to its parent, but half the height
        blueView.TranslatesAutoresizingMaskIntoConstraints = false;
        var blueConstraints = new []
        {
            blueView.LeadingAnchor.ConstraintEqualTo(view.LayoutMarginsGuide.LeadingAnchor),
            blueView.TrailingAnchor.ConstraintEqualTo(view.LayoutMarginsGuide.TrailingAnchor),
            blueView.TopAnchor.ConstraintEqualTo(view.LayoutMarginsGuide.TopAnchor),
            blueView.HeightAnchor.ConstraintEqualTo(view.LayoutMarginsGuide.HeightAnchor, (nfloat) 0.5)
        };
        NSLayoutConstraint.ActivateConstraints (blueConstraints);

        //Make the pink view same size as blue view, and linked to bottom of blue view
        pinkView.TranslatesAutoresizingMaskIntoConstraints = false;
        var pinkConstraints = new []
        {
            pinkView.LeadingAnchor.ConstraintEqualTo(blueView.LeadingAnchor),
            pinkView.TrailingAnchor.ConstraintEqualTo(blueView.TrailingAnchor),
            pinkView.HeightAnchor.ConstraintEqualTo(blueView.HeightAnchor),
            pinkView.TopAnchor.ConstraintEqualTo(blueView.BottomAnchor)
        };
        NSLayoutConstraint.ActivateConstraints (pinkConstraints);


        //From here, address the aspect-fitting challenge:

        lightGreenView.TranslatesAutoresizingMaskIntoConstraints = false;
        //These are the must-fulfill constraints: 
        var lightGreenConstraints = new []
        {
            //Aspect ratio of 1 : 5
            NSLayoutConstraint.Create(lightGreenView, NSLayoutAttribute.Height, NSLayoutRelation.Equal, lightGreenView, NSLayoutAttribute.Width, (nfloat) 0.20, 0),
            //Cannot be larger than parent's width or height
            lightGreenView.WidthAnchor.ConstraintLessThanOrEqualTo(blueView.WidthAnchor),
            lightGreenView.HeightAnchor.ConstraintLessThanOrEqualTo(blueView.HeightAnchor),
            //Center in parent
            lightGreenView.CenterYAnchor.ConstraintEqualTo(blueView.CenterYAnchor),
            lightGreenView.CenterXAnchor.ConstraintEqualTo(blueView.CenterXAnchor)
        };
        //Must-fulfill
        foreach (var c in lightGreenConstraints) 
        {
            c.Priority = 1000;
        }
        NSLayoutConstraint.ActivateConstraints (lightGreenConstraints);

        //Low priority constraint to attempt to fill parent as much as possible (but lower priority than previous)
        var lightGreenLowPriorityConstraints = new []
         {
            lightGreenView.WidthAnchor.ConstraintEqualTo(blueView.WidthAnchor),
            lightGreenView.HeightAnchor.ConstraintEqualTo(blueView.HeightAnchor)
        };
        //Lower priority
        foreach (var c in lightGreenLowPriorityConstraints) 
        {
            c.Priority = 750;
        }

        NSLayoutConstraint.ActivateConstraints (lightGreenLowPriorityConstraints);

        //Aspect-fit on the green view now
        greenView.TranslatesAutoresizingMaskIntoConstraints = false;
        var greenConstraints = new []
        {
            //Aspect ratio of 5:1
            NSLayoutConstraint.Create(greenView, NSLayoutAttribute.Height, NSLayoutRelation.Equal, greenView, NSLayoutAttribute.Width, (nfloat) 5.0, 0),
            //Cannot be larger than parent's width or height
            greenView.WidthAnchor.ConstraintLessThanOrEqualTo(pinkView.WidthAnchor),
            greenView.HeightAnchor.ConstraintLessThanOrEqualTo(pinkView.HeightAnchor),
            //Center in parent
            greenView.CenterXAnchor.ConstraintEqualTo(pinkView.CenterXAnchor),
            greenView.CenterYAnchor.ConstraintEqualTo(pinkView.CenterYAnchor)
        };
        //Must fulfill
        foreach (var c in greenConstraints) 
        {
            c.Priority = 1000;
        }
        NSLayoutConstraint.ActivateConstraints (greenConstraints);

        //Low priority constraint to attempt to fill parent as much as possible (but lower priority than previous)
        var greenLowPriorityConstraints = new []
        {
            greenView.WidthAnchor.ConstraintEqualTo(pinkView.WidthAnchor),
            greenView.HeightAnchor.ConstraintEqualTo(pinkView.HeightAnchor)
        };
        //Lower-priority than above
        foreach (var c in greenLowPriorityConstraints) 
        {
            c.Priority = 750;
        }

        NSLayoutConstraint.ActivateConstraints (greenLowPriorityConstraints);

        this.View = view;

        view.LayoutIfNeeded ();
    }
}

---

Maybe this is the shortest answer, with Masonry, which also supports aspect-fill and stretch.

typedef NS_ENUM(NSInteger, ContentMode) {
    ContentMode_aspectFit,
    ContentMode_aspectFill,
    ContentMode_stretch
}

// ....

[containerView addSubview:subview];

[subview mas_makeConstraints:^(MASConstraintMaker *make) {
    if (contentMode == ContentMode_stretch) {
        make.edges.equalTo(containerView);
    }
    else {
        make.center.equalTo(containerView);
        make.edges.equalTo(containerView).priorityHigh();
        make.width.equalTo(content.mas_height).multipliedBy(4.0 / 3); // the aspect ratio
        if (contentMode == ContentMode_aspectFit) {
            make.width.height.lessThanOrEqualTo(containerView);
        }
        else { // contentMode == ContentMode_aspectFill
            make.width.height.greaterThanOrEqualTo(containerView);
        }
    }
}];

---

I couldn't find any ready-to-go fully programatical solution, so here is my take in swift 5 for the aspect fill extension to a view:

extension UIView {

    public enum FillingMode {
        case full(padding:Int = 0)
        case aspectFit(ratio:CGFloat)
        // case aspectFill ...
    }

    public func addSubview(_ newView:UIView, withFillingMode fillingMode:FillingMode) {
        newView.translatesAutoresizingMaskIntoConstraints = false
        addSubview(newView)

        switch fillingMode {
        case let .full(padding):
            let cgPadding = CGFloat(padding)

            NSLayoutConstraint.activate([
                newView.leadingAnchor.constraint(equalTo: leadingAnchor, constant: cgPadding),
                newView.trailingAnchor.constraint(equalTo: trailingAnchor, constant: -cgPadding),
                newView.topAnchor.constraint(equalTo: topAnchor, constant: cgPadding),
                newView.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -cgPadding)
            ])
        case let .aspectFit(ratio):
            guard ratio != 0 else { return }

            NSLayoutConstraint.activate([
                newView.centerXAnchor.constraint(equalTo: centerXAnchor),
                newView.centerYAnchor.constraint(equalTo: centerYAnchor),

                newView.leadingAnchor.constraint(greaterThanOrEqualTo: leadingAnchor),
                newView.leadingAnchor.constraint(equalTo: leadingAnchor).usingPriority(900),

                newView.trailingAnchor.constraint(lessThanOrEqualTo: trailingAnchor),
                newView.trailingAnchor.constraint(equalTo: trailingAnchor).usingPriority(900),

                newView.topAnchor.constraint(greaterThanOrEqualTo: topAnchor),
                newView.topAnchor.constraint(equalTo: topAnchor).usingPriority(900),

                newView.bottomAnchor.constraint(lessThanOrEqualTo: bottomAnchor),
                newView.bottomAnchor.constraint(equalTo: bottomAnchor).usingPriority(900),

                newView.heightAnchor.constraint(equalTo: newView.widthAnchor, multiplier: CGFloat(ratio)),
            ])
        }
    }
}

And here is the priority extension (comes from another thread but I "secured it" with some pattern matching between 1...1000:

extension NSLayoutConstraint {
    /// Returns the constraint sender with the passed priority.
    ///
    /// - Parameter priority: The priority to be set.
    /// - Returns: The sended constraint adjusted with the new priority.
    func usingPriority(_ priority: Int) -> NSLayoutConstraint {
        self.priority = UILayoutPriority( (1...1000 ~= priority) ? Float(priority) : 1000 )
        return self
    }
}

Hope it helps ~

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