[Rule Request] Prefer new block based KVO API in Swift 3.2 or 4

[marcelofabri]

When using Swift 3.2 or 4, the new block based API KVO API is preferred.

// should trigger
func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {

}

// ok
let observer = foo.observe(\.value, options: [.new]) { (foo, change) in
    print(change.newValue)
}

[marcelofabri]

Probably the most trick thing here is to detect the Swift version.

We can the new keypath syntax and the structure to find if it's Swift >= 3.2.

• file.swift

let nameKeyPath = \ForceUser.name

• Command
  sourcekitten structure --file file.swift

• Result with Swift 3.1:

{
  "key.offset" : 0,
  "key.diagnostic_stage" : "source.diagnostic.stage.swift.parse",
  "key.length" : 34,
  "key.diagnostics" : [
    {
      "key.filepath" : "\/Users\/marcelofabri\/dev\/SwiftLint\/file.swift",
      "key.column" : 19,
      "key.description" : "invalid character in source file",
      "key.fixits" : [
        {
          "key.offset" : 18,
          "key.sourcetext" : " ",
          "key.length" : 1
        }
      ],
      "key.line" : 1,
      "key.diagnostic_stage" : "source.diagnostic.stage.swift.parse",
      "key.severity" : "source.diagnostic.severity.error"
    }
  ],
  "key.substructure" : [
    {
      "key.nameoffset" : 4,
      "key.accessibility" : "source.lang.swift.accessibility.internal",
      "key.length" : 33,
      "key.name" : "nameKeyPath",
      "key.kind" : "source.lang.swift.decl.var.global",
      "key.namelength" : 11,
      "key.offset" : 0
    }
  ]
}

• Result with Swift 3.2 or 4:

{
  "key.diagnostic_stage" : "source.diagnostic.stage.swift.parse",
  "key.substructure" : [
    {
      "key.nameoffset" : 4,
      "key.accessibility" : "source.lang.swift.accessibility.internal",
      "key.length" : 33,
      "key.name" : "nameKeyPath",
      "key.kind" : "source.lang.swift.decl.var.global",
      "key.namelength" : 11,
      "key.offset" : 0
    }
  ],
  "key.offset" : 0,
  "key.length" : 34
}

Unfortunately, I couldn't find a way do identify whether it's Swift 3.2 or 4.

[ornithocoder]

Maybe using #if swift()?

$ swift --version
Apple Swift version 3.1 (swiftlang-802.0.53 clang-802.0.42)
Target: x86_64-apple-macosx10.9

$ cat file.swift 
#if swift(>=3.1)
    print("3.1.x")
#endif

$ sourcekitten structure --file file.swift
{
  "key.diagnostic_stage" : "source.diagnostic.stage.swift.parse",
  "key.substructure" : [
    {
      "key.namelength" : 5,
      "key.nameoffset" : 21,
      "key.length" : 14,
      "key.name" : "print",
      "key.bodyoffset" : 27,
      "key.bodylength" : 7,
      "key.kind" : "source.lang.swift.expr.call",
      "key.offset" : 21
    }
  ],
  "key.offset" : 0,
  "key.length" : 43
}

$ cat file.swift 
#if swift(>=4.0)
    print("4.0.x")
#endif

$ sourcekitten structure --file file.swift
{
  "key.diagnostic_stage" : "source.diagnostic.stage.swift.parse",
  "key.substructure" : [

  ],
  "key.offset" : 0,
  "key.length" : 43
}

[marcelofabri]

@ornithocoder Nice! Unfortunately, we still can't tell if it's 3.2 or 4. But I guess in most cases it won't matter anyway. We can leave that for later if needed.