Handle settings that are arrays

Sources/xcodeproj/Extensions/Array+Extras.swift

@@ -0,0 +1,15 @@
+import Foundation
+
+extension Array where Element: Hashable {
+
+    /// Return the array with all duplicates removed.
+    ///
+    /// i.e. `[ 1, 2, 3, 1, 2 ].uniqued() == [ 1, 2, 3 ]`
+    ///
+    /// - note: Taken from stackoverflow.com/a/46354989/3141234, as
+    ///         per @Alexander's comment.
+    public func uniqued() -> [Element] {
+        var seen = Set<Element>()
+        return self.filter { seen.insert($0).inserted }
+    }
+}

Sources/xcodeproj/Objects/Configuration/XCBuildConfiguration.swift

@@ -74,14 +74,20 @@ public final class XCBuildConfiguration: PBXObject {
     ///   - value: Value to be appended.
     public func append(setting name: String, value: String) {
         guard !value.isEmpty else { return }
-
-        let existingValue = (buildSettings[name] as? String) ?? "$(inherited)"
-        let newValue = [existingValue, value].joined(separator: " ")
-
-        // Remove duplicates
-        let newValueComponents = Set(newValue.split(separator: " "))
-
-        buildSettings[name] = newValueComponents.joined(separator: " ")
+
+        let existing: Any = buildSettings[name] ?? "$(inherited)"
+
+        switch existing {
+        case let string as String:
+            let newValue = [string, value].joined(separator: " ")
+            buildSettings[name] = newValue
+        case let array as [String]:
+            var newValue = array
+            newValue.append(value)
+            buildSettings[name] = newValue.uniqued()
+        default:
+            break
+        }
     }
 }
 

Tests/xcodeprojTests/Objects/Configuration/XCBuildConfigurationTests.swift

@@ -44,19 +44,32 @@ final class XCBuildConfigurationTests: XCTestCase {
         XCTAssertEqual(subject.buildSettings["OTHER_LDFLAGS"] as? String, "flag1 flag2")
     }
 
-    func test_append_removesDuplicates() {
+    func test_append_removesDuplicates_when_theSettingIsAnArray() {
         // Given
         let subject = XCBuildConfiguration(name: "Debug",
                                            baseConfiguration: nil,
                                            buildSettings: [
-                                               "PRODUCT_NAME": "$(TARGET_NAME:c99extidentifier)",
-                                           ])
-
+                                            "OTHER_LDFLAGS": ["flag1", "flag2"],
+            ])
+        
         // When
-        subject.append(setting: "PRODUCT_NAME", value: "$(TARGET_NAME:c99extidentifier)")
-
+        subject.append(setting: "OTHER_LDFLAGS", value: "flag1")
+
+        // Then
+        XCTAssertEqual(subject.buildSettings["OTHER_LDFLAGS"] as? [String], ["flag1", "flag2"])
+    }
+
+    func test_append_when_theSettingExistsAsAnArray() {
+        // Given
+        let subject = XCBuildConfiguration(name: "Debug",
+                                           baseConfiguration: nil,
+                                           buildSettings: ["OTHER_LDFLAGS": ["flag1", "flag2"]])
+
+        // When
+        subject.append(setting: "OTHER_LDFLAGS", value: "flag3")
+
         // Then
-        XCTAssertEqual(subject.buildSettings["PRODUCT_NAME"] as? String, "$(TARGET_NAME:c99extidentifier)")
+        XCTAssertEqual(subject.buildSettings["OTHER_LDFLAGS"] as? [String], ["flag1", "flag2", "flag3"])
     }
 
     private func testDictionary() -> [String: Any] {