A zoomable index bar for use with UICollectionView or similar.

KFIndexBar is a control which may be placed over the side of a collection view. It may be used to give a swipeable index, allowing the user to quickly scroll the collection view to any section by touching, say, the initial letter of an alphabetically sorted list. (The contents of the index bar are completely configurable.) Unlike the UITableView's built-in index, KFIndexBar may be either horizontal (running along the bottom of the view) or vertical (along the right). Also, KFIndexBar allows the index to have two levels, with the user dragging left (on a vertical bar; or up, on a horizontal bar) to open the second level of markers between two markers. For example, in an alphabetical index bar, touching the label for "A" and dragging left might open a set of secondary labels reading "AA", "AD", "AF", and so on; once opened, dragging over these will scroll to the relevant location.

To place in your interface, add to your UICollectionView's parent view, above the collection view, and use constraints to attach to the appropriate side of the screen. Be sure to set KFIndexBar's isHorizontal variable to indicate its orientation. (Horizontal index bars, running along the bottom of the screen, are recommended for horizontally-scrolling collection views.)

KFIndexBar defines a protocol named KFIndexBarDataSource, which your code needs to implement to populate the bar with markers. Each marker is represented by a structure, KFIndexBar.Marker, which contains the label text to display and an offset in the data to scroll to. The KFIndexBarDataSource specifies two methods you need to implement, which are:

• topLevelMarkers(forIndexBar:) — return a list of the markers which will be seen in the initial, not zoomed-in state. In an example in alphabetical order, these would have the labels 'A', 'B' and so on, each with an offset pointing to the index of the data item to scroll to.
• indexBar(: markersBetween: and: Int) - this returns second-level markers when the user zooms in between two markers. It should return all the second-level markers from the first offset to just before the second offset (which may be infinity, if the user is zooming in below the last top-level marker). If there are no second-level markers, return an empty array.

KFIndexBar is a UIControl, and sends a valueChanged UIControl event when the offset it points to has changed. This offset may be read from its currentOffset instance variable.

An example, allowing the user to navigate an alphabetical list (populated with over 1,100 surnames), is included in this repository.

KFIndexBar is written in Swift 4. Due to its use of Swift's type system, it probably won't ever be compatible with Objective C, but if you're writing new code in Objective C, you should probably ask yourself why.

KFIndexBar currently assumes that the data being scrolled over is in one section, with an integer index; multi-part IndexPaths are not yet supported, though may be added in future.

The example project contains unit tests for KFIndexBar's internal layout logic.

• Andrew Bulhak - initial development - GitHub/Technical blog

KFIndexBar is licenced under the MIT License