必承其重 | 欲带皇冠
2014-09-10 13:28

在 Xcode 6中使用 AutoLayout 约束模拟方面适配行为

I want to use AutoLayout to size and layout a view in a manner that is reminiscent of UIImageView's aspect-fit content mode.

I have a subview inside a container view in Interface Builder. The subview has some inherent aspect ratio which I wish to respect. The container view's size is unknown until runtime.

If the container view's aspect ratio is wider than the subview, then I want the subview's height to equal the parent view's height.

If the container view's aspect ratio is taller than the subview, then I want the subview's width to equal the parent view's width.

In either case I wish the subview to be centered horizontally and vertically within the container view.

Is there a way to achieve this using AutoLayout constraints in Xcode 6 or in previous version? Ideally using Interface Builder, but if not perhaps it is possible to define such constraints programmatically.


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  • csdnceshi70 笑故挽风 7年前

    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:

    some views

    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:

    blue constraints

    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.

    pink constraints

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

    enter image description here

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

    containers laid out

    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:

    top aspect

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

    top equal size

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

    top less than or equal size

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

    top equal size again

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

    top equal not required

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

    top centered

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

    incorrect top layout

    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:

    fix top constraints

    Xcode updates the layout:

    correct top layout

    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:

    correct top and bottom layout

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

    iphone 4s test

    And I can test in in the iPhone 6 simulator:

    iphone 6 test

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

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  • weixin_41568131 10.24 6年前

    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) {
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  • csdnceshi71 Memor.の 5年前

    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!

    enter image description here enter image description here

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  • csdnceshi50 三生石@ 5年前

    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.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 []
            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
                //Center in parent
            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 []
            //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
                //Center in parent
            //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 []
            //Lower-priority than above
            foreach (var c in greenLowPriorityConstraints) 
                c.Priority = 750;
            NSLayoutConstraint.ActivateConstraints (greenLowPriorityConstraints);
            this.View = view;
            view.LayoutIfNeeded ();
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  • weixin_41568184 叼花硬汉 6年前

    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:

    Content Compression Resistance

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  • csdnceshi78 程序go 7年前

    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:


    #import <UIKit/UIKit.h>
    @interface AspectFitView : UIView
    @property (nonatomic, strong) UIView *childView;


    #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);

    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

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  • csdnceshi74 7*4 4年前

    Maybe this is the shortest answer, with Masonry.

    [containerView addSubview:subview];
    [subview mas_makeConstraints:^(MASConstraintMaker *make) {
        if (contentMode == ContentMode_scaleToFill) {
        else {
            make.width.equalTo(content.mas_height).multipliedBy(4.0 / 3);
            if (contentMode == ContentMode_scaleAspectFit) {
            else { // contentMode == ContentMode_scaleAspectFill
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