What Is The Purpose of Launch Images in an iOS Application

What is the purpose of launch images in an iOS application?

Note: as Zev mentions below, the Apple HIG recommendation has changed since I originally answered this question. Apple now recommends that you take all of the static elements of your first screen and make that into your default or launch page. All dynamic elements are to be removed, to give the impression that your app is going through an initialization process -- which in some cases may mean that your default screen is simply a background from your app. Thanks to Zev Eisenberg for pointing that out, "splash" pages are not recommended.

The launch image will show while your app is initializing, it's a good idea to have one. If you don't have one the user is likely to stare at a blank screen while waiting for your app to load. I believe the recommendation is to use an image that looks like the first page of your app. Here is the reference:

http://developer.apple.com/library/ios/#documentation/UserExperience/Conceptual/MobileHIG/IconsImages/IconsImages.html#//apple_ref/doc/uid/TP40006556-CH14-SW5

Xcode | Why use a launch screen file over launch images source?

This answer assumes you're talking about iOS, even though you didn't mention or tag iOS in your original question.

A launch image is simply a static (non-changing) placeholder image.

"Launch Screen" (or more accurately "LaunchScreen" or "LaunchScreen.storyboard"), is the default name of the storyboard for a Launch Screen File; it is a very simple xib or storyboard file that appears before the rest of the app gets loaded up. The advantage to this is that a dev can use auto layout to reposition things subtly depending on the size of the screen (iPhone vs. iPad, iPhone 6 vs iPhone 6+, etc.).

Launch Images vs Storyboards in iOS app development

• Q1: Basically, yes.

• Q2: You do not need to "remove" anything. This a target-level setting; specify a launch screen file instead of a launch images source:

Note that this is the default in a new project, so it's easy to try it out and see for yourself.

Is iOS app Launch Screen (splash screen) a must?

You must have storyboard or Launch Image.You can remove LaunchScreen.xib, but you have to add Launch Images. For using Launch Images you can try rn-splash-screen.

IOS launch images - driving me crazy

I use a LaunchImage set. Click Images.xcassets from the files list. In the next column to the right, you may need to use the "+" button to add AppIcon and LaunchImages image sets. These will have slots for every supported (expected) screen size. If you use these sets, you don't have to worry about encrypting your image files with Apple's indecipherable naming scheme, you just drag each image file to the corresponding slot in the set.

Of course, one still has to know the actual dimensions associated with 1x, 2x, retina4, Retina 4.7, Retina 5.5, etc. I'd certainly prefer Xcode to just show the required dimensions of each image, but it doesn't. Barring that, I'd love a single table from Apple listing all the expected dimensions, but that doesn't seem to exist, either. It looks like Apple has, thankfully, consolidated all of this information in the https://developer.apple.com/ios/human-interface-guidelines/graphics/launch-screen/ under the "Graphics" section. Specifically, as of this writing for devices supported on IOS10:

Device                          Portrait size       Landscape size
iPhone 6s Plus, iPhone 6 Plus   1080px by 1920px    1920px by 1080px
iPhone 6s, iPhone 6             750px by 1334px     1334px by 750px
iPhone SE                       640px by 1136px     1136px by 640px
12.9-inch iPad Pro              2048px by 2732px    2732px by 2048px
9.7-inch iPad Pro, iPad Air 2, 
     iPad mini 4, iPad mini 2   1536px by 2048px    2048px by 1536px

Can I display a snapshot instead of the launch image

No, by default app will display launch screen.

Above mentioned is to ignore snapshot on next launch if you are using state preservation process

See more details on state preservation process here:
https://developer.apple.com/documentation/uikit/view_controllers/preserving_your_app_s_ui_across_launches?language=objc

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