Delay Android Resolver initialization

Delay Android Resolver initialization until
1. Current active build target is Android
2. When the editor is NOT in play mode

This is to prevent unnecessary initialization when the play mode start
and AppDomain reload.

Improves #639

Bug: 154730431
Change-Id: Id63883478f3096cc20453e35ce831deb7afeb614

Author: chkuang-g

source/AndroidResolver/src/PlayServicesResolver.cs

@@ -901,35 +901,74 @@ public static bool AutomaticResolutionEnabled {
         /// Initializes the <see cref="GooglePlayServices.PlayServicesResolver"/> class.
         /// </summary>
         static PlayServicesResolver() {
-            // Create the resolver.
-            if (EditorUserBuildSettings.activeBuildTarget == BuildTarget.Android) {
-                gradleResolver = new GradleResolver();
-                // Monitor Android dependency XML files to perform auto-resolution.
-                AddAutoResolutionFilePatterns(xmlDependencies.fileRegularExpressions);
+            // Cache the flag to prevent string comparison in every frame during
+            // PollOnUpdateUntilComplete()
+            bool isExecuteMethodEnabled =  ExecutionEnvironment.ExecuteMethodEnabled;
+
+            // Delay initialization until the build target is iOS and the editor is not in play
+            // mode.
+            RunOnMainThread.PollOnUpdateUntilComplete(() => {
+                if (EditorUserBuildSettings.activeBuildTarget != BuildTarget.Android ||
+                    EditorApplication.isPlayingOrWillChangePlaymode) {
+                    // If Unity is launched with -executeMethod, in some Unity versions, editor
+                    // update will never be called. As a result, PollOnUpdateUntilComplete() will
+                    // attempt to call this poll function repeating on current thread until it
+                    // returns true.  Therefore, return true immediately and stop the polling in
+                    // executeMethod mode.
+                    return isExecuteMethodEnabled;
+                }
+                Initialize();
+                return true;
+            });
+
+        }
 
-                svcSupport = PlayServicesSupport.CreateInstance(
-                    "PlayServicesResolver",
-                    AndroidSdkRoot,
-                    "ProjectSettings",
-                    logMessageWithLevel: LogDelegate);
+        /// <summary>
+        /// Whether Android Resolver have been initialized.
+        /// </summary>
+        private static bool isInitialized = false;
+
+        /// <summary>
+        /// Initialize the module. This should be called on the main thread only if
+        /// current active build target is Android and not in play mode.
+        /// </summary>
+        private static void Initialize() {
+            if (isInitialized) return;
+
+            if ( EditorUserBuildSettings.activeBuildTarget != BuildTarget.Android ) {
+                throw new Exception("PlayServiceResolver.Initialize() is called when active " +
+                        "build target is not Android. This should never happen. If it does, " +
+                        "please report to the developer.");
             }
+
+            // Create the resolver.
+            gradleResolver = new GradleResolver();
+            // Monitor Android dependency XML files to perform auto-resolution.
+            AddAutoResolutionFilePatterns(xmlDependencies.fileRegularExpressions);
+
+            svcSupport = PlayServicesSupport.CreateInstance(
+                "PlayServicesResolver",
+                AndroidSdkRoot,
+                "ProjectSettings",
+                logMessageWithLevel: LogDelegate);
+
             RunOnMainThread.OnUpdate -= PollBundleId;
             RunOnMainThread.OnUpdate += PollBundleId;
 
             // Initialize settings and resolve if required.
             OnSettingsChanged();
 
             // Setup events for auto resolution.
-            if (EditorUserBuildSettings.activeBuildTarget == BuildTarget.Android) {
-                BundleIdChanged += ResolveOnBundleIdChanged;
-                AndroidBuildSystemChanged += ResolveOnBuildSystemChanged;
-                AndroidAbisChanged += ResolveOnAndroidAbisChanged;
-                AndroidSdkRootChanged += ResolveOnAndroidSdkRootChange;
-                Reresolve();
+            BundleIdChanged += ResolveOnBundleIdChanged;
+            AndroidBuildSystemChanged += ResolveOnBuildSystemChanged;
+            AndroidAbisChanged += ResolveOnAndroidAbisChanged;
+            AndroidSdkRootChanged += ResolveOnAndroidSdkRootChange;
+            Reresolve();
 
-                if (SettingsDialogObj.EnableAutoResolution) LinkAutoResolution();
-            }
+            if (SettingsDialogObj.EnableAutoResolution) LinkAutoResolution();
 
+            isInitialized = true;
+            Log("Android Resolver Initialized", level: LogLevel.Verbose);
         }
 
         // Unregister events to monitor build system changes for the Android Resolver and other