Static Launch Images
Launch images are what iOS displays whilst loading an App to give the impression of a responsive system. Creating these static launch images for the growing number of screen resolutions has become something of a pain in recent years. At the time of writing the list of possible launch image sizes is below (sizes include the status bar region). I have omitted the landscape versions for brevity:
- iPad 2 and iPad mini (@1x): 768 x 1024
- iPad and iPad mini (retina @2x): 1536 x 2048
- iPhone 4s (retina @2x) 640 x 960
- iPhone 5 (@2x): 640 x 1136
- iPhone 6 (@2x): 750 x 1334
- iPhone 6 Plus (@3x): 1242 x 2208
There is some good news with Xcode 6 and iOS 8 which allow a NIB or storyboard launch screen file to be used. By taking advantage of auto layout and size classes a single NIB or storyboard file automatically creates the launch images at runtime. Note that if you want to properly support the larger iPhone 6 and iPhone 6 Plus screen sizes in fullscreen mode without scaling you must supply the appropriate launch images either as static images of dimensions as listed above or with a storyboard launch screen file.
(Updated 26-Dec-2014: made it clearer that supporting iPhone 6 and iPhone 6 Plus requires you to provide the launch images but they can be static image files or from a launch screen file).
Using a Launch Image File
Xcode 6 adds a LaunchScreen.xib file by default to new projects. For an existing project add a new file using the Launch Screen template:
Note that this will add a NIB file to the project which is fine if you have a single view or view controller on the initial launch screen. If you have multiple views you will need to ignore the launch screen template and add a storyboard. You should also specify the launch screen file in the project settings for the target:
This will add the Launch screen interface file base name (UILaunchStoryboardName) key to the application plist file:
At this point you can layout the launch view in Interface Builder using autolayout and size classes as necessary to create suitable images for each screen resolution. The Xcode template does not provide a very good example as it provides a splash screen style layout with the app name and copyright statement that you will probably want to delete before adding your own view layout:
You can preview the storyboard in Xcode or test it in the simulator or on an actual device. Since the launch screen is only briefly displayed you may find it useful to set a breakpoint on application:didFinishLaunchingWithOptions: in the App delegate.
Launch Screen Constraints
The system loads the launch file before launching the app which creates some constraints on what it can contain (some of which may force you back to static image files):
The app is not yet loaded so the view hierarchy does not exist and the system can not call any custom view controller setup code you may have in the app (e.g. viewDidLoad)
You can only use standard UIKit classes so you can use UIView or UIViewController but not a custom subclass. If you try to set a custom class you will get an Illegal Configuration error in Xcode.
The launch file can only use basic UIKit views such as UIImageView and UILabel. You cannot use a UIWebView.
If you are using a storyboard you can specify multiple view controllers but there are again some limitations. For example you can embed view controllers in a navigation or tab bar controller but more complex container classes such as UISplitViewController do not work (at least not yet).
Localizing the launch file does not currently seem to have any effect. The base localization is always used so you will probably want to avoid text on the launch screen.
You cannot specify different launch files for iPad and iPhone. This may be a problem if you have significantly different interfaces for those devices as there is only so much you can do with auto layout and size classes.
Note that if you are deploying to iOS 7 you will still need to include the static launch image files. You can include both a launch image file and static launch images. Devices such as the iPhone 6 running iOS 8 will use the launch image file whilst iOS 7 devices will fallback to the launch images.
Split View Controllers
If your root view controller is a split view controller you do not have too many options at least with iOS 8.1. If you add a split view controller to the launch screen storyboard it will not load. The increased flexibility of split view controllers in iOS 8 also makes me suspect they will not be supported any time soon.
Other than going back to static launch images the only alternative seems to be to simplify the user interface by ignoring the split screen. For example consider the following iPhone and iPad launch screens that use a split view controller. On the iPhone (compact width) device the initial screen shows the master view controller (a table view controller embedded in a navigation controller in this case):
On the iPad (regular width) device the initial screen after launch shows the master and detail view controllers in a split screen layout:
This is a very common setup but there is no good way to use a Launch screen file in this case. I am open to suggestions but the closest I can get is to ignore the split screen and use a view controller embedded in a navigation controller as the launch screen.
It is far from perfect but it does as least more or less match the initial user interface on all devices (albeit without the split on the iPad). As a launch image intended to give the user the impression that the app is loading it may just about be good enough but you will have to judge for yourself.
Runtime Generation (added 28-Dec-2014)
The Apple documentation does not make it clear but the required launch images are generated by the system at runtime. This was briefly mentioned in the WWDC 2014 Platform State of the Union session (at about 1hour,22minutes). You can verify it by looking at the application container of an App deployed to a device or the simulator. The launch images required for a specific device are cached in Library/Caches/LaunchImages. The following screenshot shows the launch images generated on an iPad Air 2:
For comparison the following screenshot shows the launch images generated for an iPhone 6 Plus: