两种方法设置：
LaunchScreen.storyboard
在LaunchScreen.storyboard中拖拽imageView并设置约束，勾选右侧Use as launch Screen
工程—General—App Icons and Launch Images选项中设置Launch Screen File为所制作的LaunchScreen.storyboard或者LaunchScreen.xib
Assets.xcassets
设置LaunchImage
准备:
各种尺寸启动图：640 × 960，640 × 1136，750 × 1334，1242 × 2208，（横平需要2208 ×1242）
iPhone Portrait iOS5,6(1x:320 × 480 pixels, 2x:640 × 960 pixels, Retina 4:640 × 1136 pixels)
iPhone Portrait iOS8,9(Retina HD 5.5”:1242 × 2208 pixels, Retina HD 4.7”:750 × 1334 pixels)
iPhone Landscape iOS 8,9(Retina HD 5.5”:2208 × 1242 pixels)
iPhone Portrait iOS7,9(2x:640 × 960 pixels, Retina 4:640 × 1136 pixels)
iPhone X Portrait iOS 11+ (3x:1125 x 2436 pixels)
配置
工程—General—App Icons and Launch Images选项中设置Launch Image Source 为LaunchImage
LaunchScreen.storyboard 取消勾选Use as Launch Screen
置空路径Launch Screen File