Auto Layout Basics - codepath/ios_guides GitHub Wiki

This guide provides a quick overview and basic examples of the most common uses cases for using Auto Layout in Interface Builder. You can also check out Apple's official Auto Layout documentation.

What is Auto Layout and why use it?

In the first few years of iOS, every iPhone screen had the same point dimensions of 320x480 width x height, and it was often possible to describe an app's layout by specifying the absolute position and size of views. Screen sizes have diversified ever since: the original iPad (2010) introduced a 768x1024-point display, the iPhone 5 (2012) stretched the iPhone screen to 320x568 points, and today's lineup spans many more sizes.

These days, most applications will want their layout to be responsive to changes in the screen size or the content they are displaying. Auto Layout provides a convenient way for you to describe how the size and position of your views should change when the size and position of their parent views or neighboring views change. This can happen for example when:

your application is run on different devices
the size of or number of neighboring views change to reflect a change in the content the application is displaying
the user rotates the orientation of the device

In Auto Layout you describe a view's layout by providing one or more constraints that describe how its size and position are related to the size and position of other views.

Example of layout problem

To get a sense of the kind of problem that arises when laying out views and that Auto Layout can solve for us, consider the following example. Using Interface Builder we've added a single view (colored red) inside the view controller's root view.

Without adding any layout constraints here is what this interface looks likes in some situations where this app might run.

Depending on the use case, chances are this is not behavior that we want. For example, if the red view were meant to be a button that is always pinned to the upper right corner of the screen, the behavior would not be consistent for landscape orientations.

Basic constraints

In Auto Layout, you describe your application's layout by adding constraints that define relationships between the size and position of its views. In order for Auto Layout to function properly you'll need to provide enough constraints for each view so that the system can determine its size (width and height) and location (the x and y coordinates of the top left corner of the view).

You can add constraints in Interface Builder by selecting one or more views and using the Auto Layout controls (boxed in red below). The buttons are from left to right:

Update Frames - When adding constraints, this will apply the change to the view frames.
Align tool - Use to align view in relation to the main container, or to each other.
Add New Constraints- The main interface for adding new constraints. Note that if you want to edit existing constraints, you must do this from the Size inspector. Adding constraints on views you already worked on will simply add more constraints, leading to conflicts.
Resolve Auto Layout Issues - Use this as a last resort to let Xcode try its best to figure out how to handle constraints.
Embed in - Used to Embed a view in another view or a Navigation or Tab Controller.

You can also add constraints by control-dragging from one view to the relevant area of another view and selecting the appropriate item in the context menu.

Here are some of the most basic situations you'll come across.

Pinning to one or more edges

You'll often find yourself wanting to position a view to be a fixed distance from an edge of its parent view or one of its neighboring views.

Here we pin the red view to the top right corner of its parent view. This might be useful for example if the red view is a menu button that we want to always be accessible from the same location. Notice that we had to specify the height and width of the view as well so that Auto Layout would have enough information to figure out both the location and size of the red view.

The result when we run our app is as follows.

Resize with parent view

Another common situation is for a view to resize its dimensions (either height, width or both) to match the parent view's dimensions.

Here we specify that the red view should be pinned to the top of the screen with fixed height, but it should resize to span the width of the screen. This might be useful for example in a situation where the red view contains an alert message.

Here's what the result is when running our app:

Center a view within a parent view

Sometimes you'll want to center a view (either vertically, horizontally, or both) within another view.

Here we center the red view vertically on the screen. This might be useful if we need to show our logo in the Launch Screen.

Working with constraints in Interface Builder

Here are a few common situations that will come up as you add and modify constraints in Interface Builder

Specifying the second view to which a constraint should be relative

The pin tool by default will try to create a constraint relative to the nearest neighbor. You can change which view a constraint is relative to by clicking on the small arrow and selecting the right view in the drop down menu.

Editing constraints

Note that constraints added using the align and pin buttons are additive. They do not update existing constraints, but rather create entirely new ones. For example, here we try to set a second height constraint which results in a case of conflicting constraints.

You can edit a constraint by selecting the view associated with that constraint and using the Size inspector, or simply selecting the constraint directly in the Scene Outline.

Here we update our red view to have a different height and different inset distance from its parent view.

Constraint Errors and Warnings

The Auto Layout system will give an error if it is unable to determine the correct position and size of any of the views in your scene. It will provide you with a warning when there is an issue that may result in unexpected behavior—for example something that would result in your interface not looking like it appears in interface builder.

Misplaced Views

As you edit your constraints you'll run into situations where the position and/or size of your views as they appear in Interface Builder no longer match what would be the result of the constraints you've created. In this case Auto Layout will give you a "Misplaced Views" warning.

Update frames

One way to fix this warning is to update the views' sizes and locations in the Interface Builder to match the constraints. You can do this by selecting "Update frames" from the issues button or in the Auto Layout error inspector. You should use this option when you know that your constraints are correct, and the way the views are laid out on the canvas is wrong. You should not select this option if you suspect one of the constraints is wrong. In particular, do not select this option if Interface Builder says that you have "Missing Constraints" as this will result in confusing placement of your view off screen or having it be sized to zero.

Here we edit the red view's position (which could happen if you accidentally move it). After selecting "Update Frames" the view returns to its defined position.

Update Constraints

Other times you'll be editing a view's location or size independently of manipulating constraints, and the view's location on the canvas is the location you want to keep. You can update existing constraints to match the view's location on the canvas by selecting "Update Constraints" from the issues button or in the Auto Layout error inspector.

After "updating constraints", you should check to see if the system modified your constraints in a sensible way since sometimes constraints will be updated in a way you did not intend. In particular, do not use this option if Interface Builder tells you that you have "Conflicting Constraints" since this will update all constraints to fit the location of the view on the canvas, and you will end up having duplicate or redundant constraints.

Here, we've modified the x position and height of the red view. Selecting "Update Constraints" changes the space constraints between the red view and the parent view to match the new layout.

Conflicting Constraints

If you create constraints such that Auto Layout cannot simultaneously satisfy all of your constraints (i.e. your system is overconstrained), then it will give you an error about "Conflicting Constraints". You'll have to remove at least one constraint to resolve the issue. Sometimes it's helpful to remove all constraints for a particular view and start over. You can do this by selecting the view and choosing "Clear Constraints" from the issues button.

Missing Constraints

If you do not provide enough information for Auto Layout to determine both the x-coordinate and y-coordinate of the top left corner of your view and the width and height of your view, then it will give you an error about "Missing Constraints". This can be resolved by adding an appropriate constraint. Be careful when using the automatic issue resolver since this may not add the constraint you expected or it may add a relatively unintuitive constraint.

It's important to understand what Xcode needs to know about your view in order to place it with Auto Layouts:

The X and Y position of the top left corner of the view's frame.
Its width, which can be either set as constraint, or dynamic based on the spacing to the left and right.
Its height, which can be set, or also based on top and bottom spacing.

Dealing with flexible content size

So far we have been only dealing with views whose content (and hence size) does not change during run time. However, many views that we end up working with will have dynamic size depending on their content. The most prominent examples are UILabels, UIButtons, and UIImageViews.

Intrinsic content size of a view

Views that can determine the size they "should be" have what is known as an intrinsic content size. This size is the size the view has determined would be best to display its current content. For example a UILabel's intrinsic content size will change depending on the text in the label, and a UIImageView's intrinsic content size will depend on the image it has loaded.

When using Auto Layout, you do not necessarily have to provide width and height constraints for views with an intrinsic content size since the system will take this into account when computing the final layout of the views. To get a sense of how intrinsic content size works we can consider some examples.

Two labels: one on top of another

Here we add constraints for two labels that are located vertically adjacent to each other. We pin the first label to the top left corner, specify the vertical space between the labels, pin the second label to the left margin and also give both labels a width. Notice that we did not have to specify the height for either label.

Now we make one of the labels a multi-line label by setting its "Lines" property to 0 in the attributes inspector—this means that the label can have an arbitrary number of lines. Once we add sufficiently long text and update frames, the first label changes its height to match the content and the second label gets automatically pushed lower down the canvas so that the vertical space between the two labels is maintained. We had to update the frames manually here, but this would be done automatically for us at runtime when we change the content of the label.

Auto Layout offers some suggestions as warnings, and you can choose the right one depending on the desired behavior or use case.

Left aligned label next to right aligned label

Consider another example where we have two single-line labels horizontally adjacent to each other. We want to pin one label to the left margin and the other to the right margin—this is common for example in the design of many table view cells.

Inequality constraints

We also want to specify that the labels should have a minimum amount of horizontal space between them so that they do not run into each other. We do not know the exact amount of horizontal space since the contents of the labels might change at run time. One way to accomplish this is to define an inequality constraint where we can specify that a certain constraint's value be greater than or less than a constant.

Compression resistance

In this same example, what if the text in our labels becomes long enough so overlap is unavoidable? As seen below, at least one of the labels will start to shrink and compress its content (in this case by using an ellipsis).

How do we control this shrinking behavior? Each view has a horizontal and vertical content compression resistance priority that can be modified. Higher compression resistance means the view is less likely to shrink its content.

In this case we specify that the green label is the one whose content should be compressed if there is a conflict by lowering its compression resistance priority.

Content hugging

Sometimes you'll want views to be a fixed distance from each other and for one of the views to expand to fill the available space — this is common for example with buttons. This can be accomplished by pinning the views to the surrounding views and adding a fixed constraint for the space between them. You can specify which view should fill the available space by changing the content hugging priority of a view. A lower content hugging priority means the view is more likely to expand to match constraints, whereas a higher content hugging priority means a view wants to be as close to its intrinsic content size as possible.

Manipulating constraints programmatically

Everything Interface Builder does with constraints can also be done in code. The most common reasons to work with constraints programmatically are laying out views you create in code and animating a layout by changing a constraint's constant at runtime.

Two things to know before creating constraints in code:

Set translatesAutoresizingMaskIntoConstraints to false on any view you lay out with your own constraints. Views you create in code have this property set to true by default, which makes the system generate constraints from the view's frame — those generated constraints will conflict with the ones you add. (Views you add in Interface Builder get false automatically.)
Constraints only work once the views share a common ancestor, so add the view to its superview before activating constraints.

The preferred API is the layout anchor API: each view exposes anchors (topAnchor, leadingAnchor, widthAnchor, centerXAnchor, ...) whose constraint(...) methods build NSLayoutConstraint objects. Activate them in a batch with NSLayoutConstraint.activate(_:). For example, this pins a view to the top-right corner of the safe area with a fixed size — the programmatic equivalent of the "Pinning to one or more edges" example above:

let redView = UIView()
redView.backgroundColor = .red
redView.translatesAutoresizingMaskIntoConstraints = false
view.addSubview(redView)

NSLayoutConstraint.activate([
    redView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 20),
    redView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor, constant: -20),
    redView.widthAnchor.constraint(equalToConstant: 100),
    redView.heightAnchor.constraint(equalToConstant: 100)
])

To modify a constraint later (for example to animate a view to a new position), keep a reference to it in a property and change its constant, then call layoutIfNeeded() inside an animation block. You can also connect an @IBOutlet to a constraint created in Interface Builder and adjust its constant the same way.

Visual Format Language (VFL)

The Visual Format Language is an older, string-based way to define Auto Layout constraints for views.

As with anchors, ensure translatesAutoresizingMaskIntoConstraints is set to false.

  func addConstraints() {
    //Collect Views to apply VFL
    let buttonsDictionary = ["button1": flagButton1,
                         "button2": flagButton2,
                         "button3": flagButton3]

    //Metrics establish Fixed Constants
    let metrics = ["topSpacing": 80, "bottomSpacing": 20, "buttonHeight": 20, "buttonSpacing": 20]

    //Note that priorities can be set using @. 1000 for Required. < 100 for Optional. Example: @999

    //Horizontal constraints
    for buttonName in buttonsDictionary.keys {
      view.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "H:|-[\(buttonName)]-|", options: [], metrics: nil, views: buttonsDictionary))
    }

    //Vertical constraints
    view.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|-(==topSpacing)-[button1(>=buttonHeight@997)]-(==buttonSpacing@999)-[button2(==button1)]-(==buttonSpacing@999)-[button3(==button1)]-(>=bottomSpacing@998)-|", options: [], metrics: metrics, views: buttonsDictionary))
  }

Understanding debug output

When Auto Layout cannot satisfy all of your active constraints at runtime, it does not crash. Instead it prints a report to the console that starts with:

Unable to simultaneously satisfy constraints.
	Probably at least one of the constraints in the following list is one you don't want.

followed by the list of conflicting NSLayoutConstraint objects, and a line beginning Will attempt to recover by breaking constraint — the system then discards one of the constraints so it can continue, which is why an unsatisfiable layout usually shows up as one view being the wrong size or in the wrong place rather than as a crash.

Tips for reading the dump:

Each constraint is printed in an equation-like form, e.g. <NSLayoutConstraint:0x... UIView:0x....height == 100>. Views appear as class names plus memory addresses, which makes big dumps hard to read. You can give a constraint a name by setting its identifier property (also editable in the constraint's Attributes inspector in Interface Builder); the identifier is then printed in the log in place of the address soup.
A constraint of class NSAutoresizingMaskLayoutConstraint in the list is a strong hint that you forgot to set translatesAutoresizingMaskIntoConstraints = false on a view you are constraining in code — the conflicting constraints were generated from the view's frame, not added by you.
The last line of the report suggests making a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints. Add one via Breakpoint Navigator → + → Symbolic Breakpoint with that symbol name, and the debugger will pause at the moment the conflict happens so you can inspect the views involved instead of reading addresses out of a log after the fact.

The other class of layout problem, ambiguity (too few constraints — the runtime counterpart of Interface Builder's "Missing Constraints"), produces no console output at all; the system silently picks one of the possible layouts. While paused in the debugger you can check hasAmbiguousLayout on a view and call constraintsAffectingLayout(for:) (e.g. po view.constraintsAffectingLayout(for: .vertical) in the LLDB console) to see which constraints the layout engine is actually using for an axis.