Swift 4中substring等效替代代码求助（零基础修改现有项目）

Hey there! No worries at all—let’s get that substring warning sorted out quickly. Since you didn’t share the exact code triggering the warning, I’ll walk through the most common old substring patterns and their direct Swift 4 equivalents, which should match what you’re working with.

Common Old Substring Syntax & Swift 4 Replacements #

1. Taking a substring from a specific index to the end #

Old (warning-triggering) code example:

let myString = "Hello Swift"
let mySubstring = myString.substring(from: myString.index(myString.startIndex, offsetBy: 6))

Swift 4 equivalent:

let myString = "Hello Swift"
// Option 1: Using range syntax
let mySubstring = String(myString[myString.index(myString.startIndex, offsetBy: 6)...])
// Option 2: Shorter suffix approach (if you know the length to take from the end)
let mySubstring = String(myString.suffix(myString.count - 6))

2. Taking a substring from the start up to a specific index #

Old (warning-triggering) code example:

let myString = "Hello Swift"
let mySubstring = myString.substring(to: myString.index(myString.startIndex, offsetBy: 5))

Swift 4 equivalent:

let myString = "Hello Swift"
// Option 1: Using range syntax
let mySubstring = String(myString[..<myString.index(myString.startIndex, offsetBy: 5)])
// Option 2: Shorter prefix approach
let mySubstring = String(myString.prefix(5))

3. Taking a substring from a start index to an end index (middle slice) #

Old (warning-triggering) code example:

let myString = "Hello Swift"
let startIndex = myString.index(myString.startIndex, offsetBy: 2)
let endIndex = myString.index(myString.startIndex, offsetBy: 7)
let mySubstring = myString.substring(with: startIndex..<endIndex)

Swift 4 equivalent:

let myString = "Hello Swift"
let startIndex = myString.index(myString.startIndex, offsetBy: 2)
let endIndex = myString.index(myString.startIndex, offsetBy: 7)
let mySubstring = String(myString[startIndex..<endIndex])

Quick Note #

The main difference in Swift 4 is that slicing a string now returns a Substring type (instead of a full String). Wrapping the slice in String() converts it back to a regular String, which is almost always what you need to eliminate the warning and keep your code working as expected.

内容的提问来源于stack exchange，提问作者Kardo