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Lesson 3 - Swift UI for different screen sizes and Autolayout

In the previous lesson, Building the UI and introduction to basic components, we created our first Swift app in Xcode. However, it could only display a text label or some other Apple control which we described last time.

In today's tutorial, we'll look at very important part of building the UI, which is making the components on the screen aligned where we want them to be.

You probably know that modern websites nowadays can handle different screen sizes (desktop, tablet, cell phone..) and adapt to them. The situation is not that extreme for iOS, but we still have two iPhone sizes (+ the new X model), several iPads and iPhone SE is also still relevant (has the same size as the 5 and 5s models). There are already many display sizes and we should take them into account.

We can make our work easier making the app available for iPhones only. After all, there are less iPads there, but we still have to have different screen sizes in mind. So what should we do? The answer is Autolayout which is the way of defining a responsive UI, adapting to the display size.


If all devices had the same screen size, we could simply put the components at fixed coordinates (e.g. put a label to 20px on the X axis and 40px on the Y axis). As soon as the screen size differs, we have to reposition the controls in the controller to make them fit or to avoid them using only a small portion of the screen. Layouts provide us with this mechanism.


The first layout we'll introduce is Autolayout. The position of the controls is defined by constraints. In some programming languages, these are called anchors, because constraints are often used to attach a side of the component to the outer container side. We'll stick with the "constraint" term here.

To make the Autolayout work correctly, every component must define how it should be positioned in consideration to other components or to the edges of the particular controller's UI. There are several ways to achieve this. We can align a component to both axes, define its distance from other UI controls and so on. We'll show everything step by step. In different situations, different approaches are needed.

Autolayout can be complex for beginners, but it's still the only reasonable way to design the UI so don't avoid it and don't give up on Autolayout after several unsuccessful attempts.

Practical example

Let's open Main.storyboard, we can use the project with the Label from the last lesson. Or you can create a new project and add some UI components to it. It doesn't matter since we're going to work within just this single file for now. Let's select Label, for example, and align it to the center using constraints. We can do so using the icons in the bottom right corner of the editor.

Constraints in Xcode - Developing iOS Applications in Swift

Component alignment

We use the Align constraints icon with two rectangles in the center for alignment.

Centering using Autolayout and constraints in Xcode - Developing iOS Applications in Swift

We'll check Vertically in Container and Horizontally in Container which makes the Label centered in both directions. Then we'll just click the Add 2 Constraints assigning the appropriate alignment constraints to it. The term container refers to the component in which the control is nested. In this case, this refers to the controller itself. If the Label was e.g. inside of the View component, which is by the way often used as the container for other controls or e.g. for detecting touch gestures, the View would be the container. This way, the Label is aligned and will be displayed in the center on all screen sizes. It's real position (X and Y) will be changed depending on the screen size of the given device.

The result:

Label aligned using the Autolayout in Xcode - Developing iOS Applications in Swift

Have you been wondering why are we allowed to enter a particular number, which was 0 by default? In case we wanted to move the Label to one side, we'd set such a value. E.g. move it a few pixels vertically above the center. You can try editing the already assigned constraints in the Size inspector. Since the coordinates start from the top left corner, we would move the Label upwards by setting the Y axis constraint to e.g. -30.

Attaching to sides and limiting the size

Align constraints are especially useful for centering. Now, let's have a look at position and size constraints. Here, constraints for all four sides have to be set. Or you can set only two side constraints and define the width and height of the component. (We can avoid specifying the height using Aspect ratio.) Only like this the Autolayout will know where to put our component at and what size set it to. Of course, you can't set, for example, constraints for left and right sides and set a fixed width at the same time. These constraints would be in conflict with each other because attaching the component to the container on both left and right would make the component stretch.

Let's try to position our Label to the top right corner. Select it and then click the icon next to the alignment one, which kinda looks like a Tie Fighter.

Setting constraints for iOS apps in Xcode - Developing iOS Applications in Swift

This dialog box looks more complicated, but there's nothing to be afraid of. We're interested in the top right corner, so we're going to align from the top and right side. I recommend unchecking Constrain to margins which counts with some inner edges and sometimes doesn't work correctly. I personally encountered the margins not being registered. Also, if you don't use them, you won't make a mistake in sizes etc.

Let's position the Label e.g. 40 pixels from the top edge and 40 pixels from the right one. You can recognize active constraint by the solid red line. Clicking in toggles between active/inactive and also activates entering a new value. Don't forget to set the height and weight, since we've attached the Label only to the top and bottom edges. Otherwise, its size wouldn't be defined. Then just click Add 4 Constraints to anchor the Label. Now it stays in the top right corner. You can try it out by switching between different screen sizes - you can set this in the bottom part of the editor.

Switching display sizes in Xcode - Developing iOS Applications in Swift

Now we introduced the two basic approaches, but that's not all Autolayout can do.

More components

If you have multiple components in the container, Autolayout takes them into account when creating constraints. Now, if we add another component, e.g. a Button, the distance constraint for the right edge of the Button will be calculated from the Label, which makes sense. However, you can choose to make it calculate from the controller's edge again (the down arrow in the value field). I don't recommend it though.

Before you start setting constraints to other elements like this, it's worth thinking about the overall design and if you will add more components soon or change their position. Don't be afraid of using View as the container for components that relate to each other. You'll then set the constraints from the edges of this View and you can move the view itself easily. You can also temporarily set distinct background colors to different Views to see how they are positioned.

Conflicts between constraints

Sometimes it may happen that Autolayout will get "mad at us" for bad constraints. In that case, you may have forgotten to set everything needed or you accidentally added duplicate constraint. Button can't be e.g. 0 from the left edge and at the same time have a different constraint set to 20 also from the left edge. Xcode will fairly well show you where's the problem and hints a fix "with one click". I personally don't use these fixes because it often ends up differently than what I wanted. I also almost never use Reset to suggested constraints which sets the constraints for you. Maybe you'll be comfortable with these options, just try them out. Remember that you will find constraints in the Size inspector of individual components.

In the next lesson, Simple iOS calculator in Swift, we'll finally look at something more real-world-ish and create a simple iOS calculator. We'll learn to use StackView and connect the UI with the code.


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