Saying “Yes” to Auto Layout
Auto Layout revolutionizes view layout with something wonderful, fresh, and new. Apple’s layout features make your life easier and your interfaces more consistent, and they add resolution-independent placement for free. You get all this regardless of device geometry, orientation, and window size.
Auto Layout works by creating relationships between onscreen objects. It specifies the way the runtime system automatically arranges your views. The outcome is a set of robust rules that adapt to screen and window geometry. With Auto Layout, you describe requirements (called constraints) that specify how views relate to one another and you set view properties that describe a view’s relationship to its content. With Auto Layout, you can make requests such as the following:
- Match one view’s size to another view’s size so that they always remain the same width.
- Center a view in its parent no matter how much the parent reshapes.
- Align one view to another view’s bottom while laying out a row of items.
- Offset a pair of items by some constant distance (for example, pad with a standard 8-point space).
- Tie the bottom of one view to another view’s top so that when you move one, you move them both.
- Don’t allow an image view to shrink to the point where the image cannot be fully seen at its natural size. (That is, don’t compress or clip the view’s content.)
- Prevent a button from showing too much padding around its text.
The first five items in this list describe constraints that define view geometry and layout, establishing visual relationships between views. The last two items reference view properties that relate each view to the content it presents. When working with Auto Layout, you negotiate both of these kinds of tasks.
Figure 1-3 shows a custom iOS control built entirely with Auto Layout. This picker enables users to select a color. Each pencil consists of a fixed-size tip view placed directly above a stretchable bottom view. As users make selections, items move up and down to indicate their current choice. Auto Layout constraints ensure that the each tip stays exactly on top of its base, that each “pencil” is sized to match, and that the paired tip/base items are laid out in a bottom-aligned row.
Figure 1-3. This pencil-picker custom control was built entirely with Auto Layout.
This particular pencil picker is built programmatically; that is, a data source supplies the number of pencils and the art for each tip. By describing the relationships between each item, Auto Layout simplifies the process of extending this control. You need only say “place each new item to the right, match its width to the existing pencils, and align its bottom” to grow this picker from 10 items to 11, 12, or more. Best of all, constraint changes can be animated. The pencil tip animates up and down as the base reshapes to new constraint offsets.
Auto Layout can also consider a view’s content during layout. To accomplish this, it may need to negotiate competing requests. For example, imagine a resizable content view with several subviews, like the one shown in Figure 1-4. Suppose that you want to be able to resize this view but don’t want to clip any subview content while doing so. Auto Layout helps you express these desires and rank them so that the system makes sure not to clip when resizing.
Figure 1-4 shows a small OS X application whose primary window protects the content of its two subviews. These include a label whose content is the string Label and a resizable button whose content is, similarly, the string Button. The original content view as the application launches is shown in the left screenshot.
Figure 1-4. Auto Layout can ensure that the stretchable button shown in the original view (left) won’t clip its subviews while resizing. The window cannot resize any smaller than the small view (right) because doing so would cause either the label or button to clip.
At the right of Figure 1-4, you see the smallest possible version of this view. Because its Auto Layout rules resist clipping (these rules are called compression resistance), the window cannot resize any further. The only way to allow it to shrink beyond this size is to demote one or both of its “do not clip” subview rules.
A similar rule called content hugging allows a view to resist padding and stretching, keeping the frame of each view close to the natural size of the content it presents. Both compression resistance and content hugging are demonstrated further later in this chapter as well as in Chapter 2.
Rule-balancing forms an important backbone of Auto Layout design work. You not only specify the layout qualities of each view, you also prioritize them. When rules come into conflict—and they do quite regularly—the system uses these rankings to select the most important layout qualities to preserve.
In the example of Figure 1-4, the integrities of the label and button contents have priority over any request for a smaller window. That forces a natural minimum on the window size, preventing it from resizing any further than the smaller version shown at the bottom of the figure.
Auto Layout Strengths
To summarize, you can do a lot with Auto Layout. Auto Layout offers a flexible and powerful descriptive system that updates and strengthens view layout. You can make rules about how views relate to each other and to their content, and you can prioritize rules to determine which rules prevail in the event of conflict.
You’ve seen just a taste of what Auto Layout can do. If you’re not using Auto Layout, you’re missing out on one of the best tools Apple has delivered to developers in years.