Lazy initialize gradleResolver as long as build target is Android.

Fixed a regression that gradleResolver can be null until Initialize() is called.
This may race with some build process which tries to call PlayServicesResolver.Resolve() in the
next editor frame.
Now it will be lazy initialized as long as current build target is Android.

Change-Id: I25ad9a4d22d2eaf11568be6b9ba0dbd2263acad5

Author: chkuang-g

source/AndroidResolver/src/PlayServicesResolver.cs

@@ -383,8 +383,22 @@ public void Poll(Func<T> getCurrentValue, Changed changed) {
 
         /// <summary>
         /// Resolver that uses Gradle to download libraries and embed them within a Unity project.
+        /// Lazy initialize it only when current build target is Android.
         /// </summary>
-        private static GradleResolver gradleResolver;
+        private static GradleResolver GradleResolverInstance {
+            get {
+                if (gradleResolverInstance == null &&
+                        EditorUserBuildSettings.activeBuildTarget == BuildTarget.Android) {
+                    gradleResolverInstance = new GradleResolver();
+                }
+                return gradleResolverInstance;
+            }
+        }
+
+        /// <summary>
+        /// Instance of GradleResolver.
+        /// </summary>
+        private static GradleResolver gradleResolverInstance = null;
 
         /// <summary>
         /// Resoluton job.
@@ -954,8 +968,6 @@ private static bool Initialize() {
                         "please report to the developer.");
             }
 
-            // Create the resolver.
-            gradleResolver = new GradleResolver();
             // Monitor Android dependency XML files to perform auto-resolution.
             AddAutoResolutionFilePatterns(xmlDependencies.fileRegularExpressions);
 
@@ -1091,7 +1103,7 @@ private static void OnPostprocessAllAssets(string[] importedAssets,
                                                    string[] deletedAssets,
                                                    string[] movedAssets,
                                                    string[] movedFromAssetPaths) {
-            if (gradleResolver != null) {
+            if (GradleResolverInstance != null) {
                 // If the manifest changed, try patching it.
                 var manifestPath = FileUtils.NormalizePathSeparators(
                     SettingsDialogObj.AndroidManifestPath);
@@ -1150,7 +1162,7 @@ private static void OnPostProcessScene() {
             if (UnityEngine.Application.isPlaying) return;
             // If the Android resolver isn't enabled or automatic resolution is disabled,
             // do nothing.
-            if (gradleResolver == null || !SettingsDialogObj.AutoResolveOnBuild) {
+            if (GradleResolverInstance == null || !SettingsDialogObj.AutoResolveOnBuild) {
                 return;
             }
             // If post-processing has already been executed since this module was loaded, don't
@@ -1231,7 +1243,7 @@ private static void AutoResolve(Action resolutionComplete) {
         /// Auto-resolve if any packages need to be resolved.
         /// </summary>
         private static void Reresolve() {
-            if (AutomaticResolutionEnabled && gradleResolver != null) {
+            if (AutomaticResolutionEnabled && GradleResolverInstance != null) {
                 ScheduleAutoResolve();
             }
         }
@@ -1940,15 +1952,23 @@ private static void ResolveUnsafe(Action<bool> resolutionComplete,
                     });
             } else {
                 lastError = "";
-                gradleResolver.DoResolution(
-                    SettingsDialogObj.PackageDir,
-                    closeWindowOnCompletion,
-                    () => {
-                        RunOnMainThread.Run(() => {
-                                finishResolution(String.IsNullOrEmpty(lastError) &&
-                                patchGradleProperties(), lastError);
-                            });
+                if (GradleResolverInstance != null) {
+                    GradleResolverInstance.DoResolution(
+                        SettingsDialogObj.PackageDir,
+                        closeWindowOnCompletion,
+                        () => {
+                            RunOnMainThread.Run(() => {
+                                    finishResolution(String.IsNullOrEmpty(lastError) &&
+                                    patchGradleProperties(), lastError);
+                                });
+                        });
+                } else {
+                    // Fail the resolution if gradleResolver is not initialized.
+                    RunOnMainThread.Run(() => {
+                        finishResolution(false, "GradleResolver is not created. Is your " +
+                                "current build target set to Android?");
                     });
+                }
             }
         }
 
@@ -1986,7 +2006,7 @@ public static void SettingsDialog()
         /// Interactive resolution of dependencies.
         /// </summary>
         private static void ExecuteMenuResolve(bool forceResolution) {
-            if (gradleResolver == null) {
+            if (GradleResolverInstance == null) {
                 NotAvailableDialog();
                 return;
             }
@@ -2428,7 +2448,7 @@ internal static void DeleteLabeledAssets() {
         internal static void OnSettingsChanged() {
             PlayServicesSupport.verboseLogging = SettingsDialogObj.VerboseLogging;
             logger.Verbose = SettingsDialogObj.VerboseLogging;
-            if (gradleResolver != null) {
+            if (GradleResolverInstance != null) {
                 PatchAndroidManifest(GetAndroidApplicationId(), null);
                 Reresolve();
             }