Fix indentation handling

The previous implementation did not support tab indentations or deeply
nested types. Instead of using indentationWidth to pad the generated
code, we deduce the current indentation depth from the first line in the
selection and check if the user prefers tabs for the initializer body.

This commit also inserts a new line between the variables and generated
initializers, as this seems to be a common Swift design choice.

SourceEditorExtension/SIG.swift

@@ -17,7 +17,7 @@ enum SIGError: Swift.Error {
 
 let accessModifiers = ["open", "public", "internal", "private", "fileprivate"]
 
-func generate(selection: [String], tabWidth: Int, indentationWidth: Int) throws -> [String] {
+func generate(selection: [String], indentation: String, leadingIndent: String) throws -> [String] {
     var variables = [(String, String)]()
 
     for line in selection {
@@ -51,11 +51,9 @@ func generate(selection: [String], tabWidth: Int, indentationWidth: Int) throws
 
     let arguments = variables.map { "\($0.0): \(addEscapingAttributeIfNeeded(to: $0.1))" }.joined(separator: ", ")
 
-    let indentExpressions = String(repeating: " ", count: tabWidth)
-    let expressions = variables.map { "\(indentExpressions)self.\($0.0) = \($0.0)" }
+    let expressions = variables.map { "\(indentation)self.\($0.0) = \($0.0)" }
 
-    let indentLines = String(repeating: " ", count: indentationWidth)
-    let lines = (["public init(\(arguments)) {"] + expressions + ["}"]).map { "\(indentLines)\($0)" }
+    let lines = (["public init(\(arguments)) {"] + expressions + ["}"]).map { "\(leadingIndent)\($0)" }
 
     return lines
 }

SourceEditorExtension/SourceEditorCommand.swift

@@ -10,12 +10,22 @@ import Foundation
 import XcodeKit
 
 class SourceEditorCommand: NSObject, XCSourceEditorCommand {
-    func perform(with invocation: XCSourceEditorCommandInvocation, completionHandler: @escaping (Swift.Error?) -> Void ) -> Void {
+
+    func perform(with invocation: XCSourceEditorCommandInvocation, completionHandler: @escaping (Swift.Error?) -> Void) {
+        do {
+            try generateInitializer(invocation: invocation)
+            completionHandler(nil)
+        } catch {
+            completionHandler(error)
+        }
+    }
+
+    private func generateInitializer(invocation: XCSourceEditorCommandInvocation) throws {
         guard invocation.buffer.contentUTI == "public.swift-source" else {
-            return completionHandler(SIGError.notSwiftLanguage)
+            throw SIGError.notSwiftLanguage
         }
         guard let selection = invocation.buffer.selections.firstObject as? XCSourceTextRange else {
-            return completionHandler(SIGError.noSelection)
+            throw SIGError.noSelection
         }
 
         print(selection.start.line, selection.start.column)
@@ -34,18 +44,31 @@ class SourceEditorCommand: NSObject, XCSourceEditorCommand {
                 + [String((invocation.buffer.lines[selection.end.line] as! String).utf8.prefix(selection.end.column))!]
         }
 
-        let initializer: [String]
-        do {
-            initializer = try generate(selection: selectedText,
-                                       tabWidth: invocation.buffer.tabWidth,
-                                       indentationWidth: invocation.buffer.indentationWidth)
-        } catch {
-            return completionHandler(error)
-        }
+        var initializer = try generate(
+            selection: selectedText,
+            indentation: indentSequence(for: invocation.buffer),
+            leadingIndent: leadingIndentation(from: selection, in: invocation.buffer)
+        )
+
+        initializer.insert("", at: 0) // separate from selection with empty line
 
-        let targetRange = selection.end.line + 1..<selection.end.line + 1 + initializer.count
+        let targetRange = selection.end.line + 1 ..< selection.end.line + 1 + initializer.count
         invocation.buffer.lines.insert(initializer, at: IndexSet(integersIn: targetRange))
-
-        completionHandler(nil)
+    }
+}
+
+private func indentSequence(for buffer: XCSourceTextBuffer) -> String {
+    return buffer.usesTabsForIndentation
+        ? "\t"
+        : String(repeating: " ", count: buffer.indentationWidth)
+}
+
+private func leadingIndentation(from selection: XCSourceTextRange, in buffer: XCSourceTextBuffer) -> String {
+    let firstLineOfSelection = buffer.lines[selection.start.line] as! String
+
+    if let nonWhitespace = firstLineOfSelection.rangeOfCharacter(from: CharacterSet.whitespaces.inverted) {
+        return String(firstLineOfSelection.prefix(upTo: nonWhitespace.lowerBound))
+    } else {
+        return ""
     }
 }

SwiftInitializerGeneratorTests/SwiftInitializerGeneratorTests.swift

@@ -11,7 +11,7 @@ import XCTest
 class SwiftInitializerGeneratorTests: XCTestCase {
     func assert(input: [String], output: [String], file: StaticString = #file, line: UInt = #line) {
         do {
-            let lines = try generate(selection: input, tabWidth: 4, indentationWidth: 0)
+            let lines = try generate(selection: input, indentation: "    ", leadingIndent: "")
             if(lines != output) {
                 XCTFail("Output is not correct; expected:\n\(output.joined(separator: "\n"))\n\ngot:\n\(lines.joined(separator: "\n"))", file: file, line: line)
             }