Prevent incomplete iOS export after build target switch. (juicycleff#838)

* Avoid code being skipped after changing buildtarget to iOS.

* Prevent unreachable code warnings from early return.

Author: timbotimbo

example/unity/DemoApp/Assets/FlutterUnityIntegration/Editor/Build.cs

@@ -356,8 +356,35 @@ private static void ModifyAndroidGradle(bool isPlugin)
 
         private static void BuildIOS(String path, bool isReleaseBuild)
         {
-            // Switch to ios standalone build.
-            EditorUserBuildSettings.SwitchActiveBuildTarget(BuildTargetGroup.iOS, BuildTarget.iOS);
+            bool abortBuild = false;
+
+            // abort iOS export if #UNITY_IOS is false.
+            // Even after SwitchActiveBuildTarget() it will still be false as the code isn't recompiled yet.
+            // As a workaround, make the user trigger an export again after the switch.
+
+#if !UNITY_IOS
+            abortBuild = true;
+            if (Application.isBatchMode)
+            {
+                Debug.LogError("Incorrect iOS buildtarget, use the -buildTarget argument to set iOS");
+            }
+            else
+            {
+                bool dialogResult = EditorUtility.DisplayDialog(
+                    "Switch build target to iOS?",
+                    "Exporting to iOS first requires a build target switch.\nClick 'Export iOS' again after all importing has finished.",
+                    "Switch to iOS",
+                    "Cancel"
+                );
+                if (dialogResult)
+                {
+                    EditorUserBuildSettings.SwitchActiveBuildTarget(BuildTargetGroup.iOS, BuildTarget.iOS);
+                }
+            } 
+#endif
+            //don't return within #if !UNITY_IOS as that results in unreachable code warnings.
+            if (abortBuild)
+                return;
 
             if (Directory.Exists(path))
                 Directory.Delete(path, true);
@@ -396,15 +423,25 @@ private static void BuildIOS(String path, bool isReleaseBuild)
             if (report.summary.result != BuildResult.Succeeded)
                 throw new Exception("Build failed");
 
-            //trigger postbuild script manually
+            // log an error if this code is skipped. (might happen when buildtarget is switched from code)
+            bool postBuildExecuted = false;
 #if UNITY_IOS
             XcodePostBuild.PostBuild(BuildTarget.iOS, report.summary.outputPath);
+            postBuildExecuted = true;
 #endif
-
-            if (isReleaseBuild) {
-                Debug.Log("-- iOS Release Build: SUCCESSFUL --");
-            } else {
-                Debug.Log("-- iOS Debug Build: SUCCESSFUL --");
+            if (postBuildExecuted)
+            {
+                if (isReleaseBuild)
+                {
+                    Debug.Log("-- iOS Release Build: SUCCESSFUL --");
+                }
+                else
+                {
+                    Debug.Log("-- iOS Debug Build: SUCCESSFUL --");
+                }
+            } else
+            {
+                Debug.LogError("iOS export failed. Failed to modify Unity's Xcode project.");
             }
         }