Auto Layout Basics - sammanthp007/Linux-Kernel-Development 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 guide.

What is Auto Layout and why use it?

Until the introduction of iPad and iPhone 5, all screens in the iOS world had the same point dimensions of 320x480 width x height. At this point in time it was often possible to describe an app's layout by specifying the absolute position and size of views.

These days, most applications will want their layout to be responsive to changes 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 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 it is 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 is incorrect for the landscape and iPhone 6 sizes.

Basic constraints

In Auto Layout, you describe your application's layout by adding constraints that define relationships between the size and position 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 the Auto Layout controls (boxed in red below). The buttons are from left to right:

Aligntool - specify how views should be aligned relative to each other
Pin tool - specify that a view should be a fixed distance from another view
Issues tool - quick fixes for errors or warnings that may come up as you add constraints.
Resizing settings - changes the way Interface Builder updates constraints when you resize views. You probably won't use this.

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 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 informatin 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. Notice that we had select "Update Frames" after setting our constraints. More on this [below].

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 an 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 it's parent view.

Constraint Errors and Warnings

The Auto Layout system will give an error if it us unable to determine the correct position and size of any of the views in your scene. It will provide you with a warning there is issue that may result 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 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 constraints for height, but this is not properly updated on our canvas. After selecting "Update Frames" the new height is properly reflected.

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 the 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 decreased the width of the red view. Selecting "Update Constraints" changes the horizontal 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.

Here our constraints on the red view to be centered along the y axis and to be a specific distance from the top of the parent view conflict with each other. We resolve the issue by removing the vertical centering constraint and updating the frame of the view.

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.

Here when we remove the constraint specifying the distance the red view has to be away from the right side of the parent view, Auto Layout can no longer determine the width of the red view. Note that when we use the automatic issue resolver it adds a width constraint instead of the horizontal space constraint we originally had.

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 that "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 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 necessary 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 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 the labels a multi-line label by setting it's "Lines" property to 0 in the attributes inspector—this means that the label can have an arbitray 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.

Left aligned label next to right aligned label

Consider another example where we have two single-line labels horizontally adjancent 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 to views to be a fixed distance from each other and for one of the views to expand to fill the available space&emdash;this is common for example with buttons. This can be accomplished by pinning the views to the surround 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.

Here we specify that the red label should expand to fill the available space by lowering its content hugging priority.

Manipulating constraints programmatically

VFL

The Visual Format Language is a declarative language that is used to define Auto Layout constraints for views.

To use VFL, 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.constraintsWithVisualFormat("H:|-[\(buttonName)]-|", options: .allZeros, metrics: nil, views: buttonsDictionary))
    }

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