Layout takes place in the context of a View Hierarchy, but generally we only need to concern ourselves with the relationships between a single parent view and its children at a time.
If your layout logic doesn’t have this kind of locality, there is probably a better approach.
In the context of layout, we call a parent view the superview and its children the subviews. When using WeView 2, the superview will generally be a WeView. Subviews can be any kind of UIView.
Subviews don’t need to extend any class or implement any interface to work with WeView 2.
We also distinguish between widgets - the visible elements of the interface, ie. buttons and labels - and containers, mostly invisible views that we use to structure and layout other views. In HTML, DIVs are usually used as containers; in UIKit, plain UIViews are often used. The core of WeView 2 is the WeView container class.
We refer to a given container’s subviews collectively as its contents.
There are two complementary aspects of layout: measurement and arrangement. Measurement is the process of determining what size each view should be - perhaps in the abstract, or perhaps in the context of specific amount of space. Arrangement is the process of actually sizing and positioning views and always takes place in the context of a specific space.
In UIKit, measurement centers around the [UIView sizeThatFits:] method and arrangement is driven by the [UIView layoutSubviews] method.
In terms of measurement, many views have a specific size they want to have. Ie. a button might want to be the size of its icon and a UILabel might want to be the size of its text, as rendered with its font. UIKit refers to this as a view’s desired size. (When using WeView 2, we can safely ignore the related concept of intrinsic size.) A view’s desired size corresponds to the size returned by [UIView sizeThatFits:].
Note that the desired size of a view can depend on the argument passed to that method and is a surprisingly complicated concept.
More information can be found in the Sizing section of the tutorial.
UIViews use two coordinate systems. A UIView’s frame specifies its size and position in the coordinate system of its parent. A UIView’s bounds specifies its size and position in its own local coordinate system. Layout of subviews always takes place in the coordinate system of the superview and therefore manipulates the subviews’ frames. Changes to a view’s frame affects the bounds and vice versa. UIView properties (ie. frame) and WeView properties (ie. spacing) are expressed in points, not pixels. On a Retina device, a point is 2 pixels.
Next: Tutorial 4: Basics