iPhone全机型背景图适配：屏幕尺寸、图片规格及命名规范问询

Hey Jeff, let's work through your iPhone background image adaptation questions—this is such a common headache after iPhone X introduced the notch and new aspect ratios, so I feel your pain! Here's a straightforward breakdown:

Short answer: Yes, you can, but there are tradeoffs to consider:

• First, a quick recap of pixel densities: 4.7" iPhones (like 8/SE 2) are @2x devices, while 5.5" (8 Plus) and 5.8" (X/XS) are @3x.
• If you only ship 3x images, iOS will automatically downscale them for @2x devices. This won’t cause blurriness (since you’re starting with more pixels than needed), but it will use slightly more memory than shipping optimized @2x versions. For most apps, this is negligible—modern iPhones have plenty of RAM, and the simplicity of maintaining one set of images is often worth it.
• That said, if your app uses tons of large background images, splitting into @2x and @3x will help keep memory usage in check. It’s a balance between development ease and performance.

You need to distinguish between logical points (pt) (iOS's device-agnostic measurement) and physical pixels (px). Here's the breakdown for each screen:

Logical dimensions (use these for Auto Layout) #

These are what you’ll set in Interface Builder or code, since they scale with pixel density:

• 4.7" iPhones: 375pt × 667pt
• 5.5" iPhones: 414pt × 736pt
• 5.8" iPhones (X/XS): 375pt × 812pt

Physical pixel dimensions (for image assets) #

Multiply the logical dimensions by the device's scale factor (@2x = ×2, @3x = ×3):

• 4.7" (@2x): 750px × 1334px
• 5.5" (@3x): 1242px × 2208px (note: the actual display is 1080x1920, but iOS scales this image down automatically—always design to the full @3x size)
• 5.8" (@3x): 1125px × 2436px

• Use Auto Layout: Pin your ImageView to all four edges of the screen (or superview) with zero constraints. This ensures it stretches to fit any screen size automatically.
• Pick the right contentMode: For full-screen backgrounds, use UIViewContentModeScaleAspectFill—this scales the image to fill the entire view while maintaining aspect ratio, cropping any excess. If you want the whole image visible without cropping, use UIViewContentModeScaleAspectFit, but this will leave letterboxing if the image's aspect ratio doesn't match the screen.
• Handle the notch and safe area: To make your background extend under the notch and home indicator, set edgesForExtendedLayout = .all (Swift) or self.edgesForExtendedLayout = UIRectEdgeAll (Objective-C) in your view controller. This tells iOS to let your content fill the entire screen, not just the safe area.
• Stick to Apple's naming convention: Name your assets like background.png (legacy @1x, rarely needed now), background@2x.png, background@3x.png. Xcode's asset catalog will automatically map these to the correct devices.

内容的提问来源于stack exchange，提问作者Jeff