为什么选择iOS启动屏幕，然后是快照，然后是主屏幕？

Question

When I kill my iOS app and relaunch it, the quick sequence of screens I see are

1. Launch Screen. The screen defined in LaunchScreen.storyboard .
2. Snapshot. The screen image the iPhone created the last time my app was put in the background.
3. Home Screen. The ViewController that I set in AppDelegate 's application(_:didFinishLaunchingWithOptions:) method.

From the Apple documentation that I've read, it seemed that during application launch, if a snapshot exists then iOS uses that snapshot instead of the launch screen. Why am I seeing both the launch screen and the snapshot?

This screen recording shows the issue. The launch screen is red. The home screen is white. The second screen is green. I navigate to the green screen and then put the app in the background to get iOS to take a green screenshot. Then I kill the app. When the app is relaunched, the sequence is launch/red, flash of screenshot/green, home/white. I'm testing on an iPhone 6 with iOS 12.1.1.

https://imgur.com/a/EMX4dM1

From https://developer.apple.com/documentation/uikit/uiapplication/1623097-ignoresnapshotonnextapplicationl

As part of the state preservation process, UIKit captures your app's user interface and stores it in an image file. When your app is relaunched, the system displays this snapshot image in place of your app's default launch image to preserve the notion that your app was still running.

Answer 1

I can't confirm your description. My testing shows that if you kill your app prematurely, then on the next launch the previous backgrounding screen shot is thrown away and the launch screen alone is used.

Here is my test:

• The launch screen is red

• The initial view controller's view is white

• The secondary view controller's view is green

Here is what happens:

1. You will see me launch and then move from the white view to the green view.

2. I then background the app to make sure the snapshot is taken.

3. I then bring the app back to the foreground and then go into the app launcher so you can see the snapshot, which is indeed green.

4. Then I kill the app and relaunch it. I used slow motion in this part of the movie, and you can see clearly that we use the red launch screen and fade to the white of the initial view controller; the green never reappears.

Perhaps your testing procedure is faulty (test only on a device). Perhaps you are launching into the wrong screen and then jumping back to the opening screen (that seems the most likely explanation).