fix: there is a compilation error in Unity 2019.2 or earlier

close #407

Editor/UIParticleEditor.cs

@@ -13,9 +13,7 @@ using Object = UnityEngine.Object;
 #if UNITY_2021_2_OR_NEWER
 using UnityEditor.Overlays;
 #else
-using System;
 using System.Reflection;
-using Object = UnityEngine.Object;
 #endif
 #if UNITY_2021_2_OR_NEWER
 using UnityEditor.SceneManagement;
@@ -207,7 +205,10 @@ namespace Coffee.UIExtensions
             serializedObject.Update();
 
             // Maskable
-            EditorGUILayout.PropertyField(_maskable);
+            if (_maskable != null)
+            {
+                EditorGUILayout.PropertyField(_maskable);
+            }
 
             // Scale
             EditorGUI.BeginDisabledGroup(!_meshSharing.hasMultipleDifferentValues && _meshSharing.intValue == 4);

Runtime/Internal/Extensions/ComponentExtensions.cs

@@ -12,6 +12,33 @@ namespace Coffee.UIParticleInternal
     /// </summary>
     internal static class ComponentExtensions
     {
+#if !UNITY_2019_2_OR_NEWER
+        public static bool TryGetComponent<T>(this GameObject self, out T component)
+            where T : Component
+        {
+            if (self == null)
+            {
+                component = null;
+                return false;
+            }
+
+            component = self.GetComponent<T>();
+            return component != null;
+        }
+
+        public static bool TryGetComponent<T>(this Component self, out T component)
+            where T : Component
+        {
+            if (self == null)
+            {
+                component = null;
+                return false;
+            }
+
+            return self.gameObject.TryGetComponent(out component);
+        }
+#endif
+
         /// <summary>
         /// Get components in children of a specific type in the hierarchy of a GameObject.
         /// </summary>

Runtime/Internal/Utilities/FrameCache.cs

@@ -14,7 +14,7 @@ namespace Coffee.UIParticleInternal
             UIExtraCallbacks.onLateAfterCanvasRebuild += ClearAllCache;
         }
 
-#if UNITY_EDITOR
+#if UNITY_EDITOR && UNITY_2019_3_OR_NEWER
         [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.SubsystemRegistration)]
         private static void Clear()
         {

Runtime/Internal/Utilities/MaterialRepository.cs

@@ -15,7 +15,7 @@ namespace Coffee.UIParticleInternal
 
         public static Func<string, Shader> onShaderFind = Shader.Find;
 
-#if UNITY_EDITOR
+#if UNITY_EDITOR && UNITY_2019_3_OR_NEWER
         [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.SubsystemRegistration)]
         public static void Clear()
         {

Runtime/Internal/Utilities/Misc.cs

@@ -109,7 +109,7 @@ namespace Coffee.UIParticleInternal
         {
             if (Misc.isBatchOrBuilding) return;
 
-            var types = TypeCache.GetTypesWithAttribute<IconAttribute>();
+            var types = TypeCache.GetTypesWithAttribute(typeof(IconAttribute));
             var scripts = MonoImporter.GetAllRuntimeMonoScripts();
             foreach (var type in types)
             {

Runtime/Internal/Utilities/TypeCache.cs

@@ -0,0 +1,88 @@
+#if !UNITY_2019_2_OR_NEWER
+using System;
+using System.Collections.Generic;
+using System.Reflection;
+
+namespace Coffee.UIParticleInternal
+{
+    public static class TypeCache
+    {
+        private static readonly object s_Lock = new object();
+        private static readonly Dictionary<Type, Type[]> s_DerivedTypesCache = new Dictionary<Type, Type[]>();
+        private static readonly Dictionary<Type, Type[]> s_AttributeTypesCache = new Dictionary<Type, Type[]>();
+
+        public static IEnumerable<Type> GetTypesDerivedFrom(Type baseType)
+        {
+            lock (s_Lock)
+            {
+                if (s_DerivedTypesCache.TryGetValue(baseType, out var cached))
+                {
+                    return cached;
+                }
+
+                var types = new List<Type>();
+                foreach (var t in GetAllLoadableTypes())
+                {
+                    if (t != baseType && baseType.IsAssignableFrom(t))
+                    {
+                        types.Add(t);
+                    }
+                }
+
+                return s_DerivedTypesCache[baseType] = types.ToArray();
+            }
+        }
+
+        public static IEnumerable<Type> GetTypesWithAttribute(Type attr)
+        {
+            lock (s_Lock)
+            {
+                if (s_AttributeTypesCache.TryGetValue(attr, out var cached))
+                {
+                    return cached;
+                }
+
+                var types = new List<Type>();
+                foreach (var t in GetAllLoadableTypes())
+                {
+                    if (t.GetCustomAttributes(attr, inherit: true).Length > 0)
+                    {
+                        types.Add(t);
+                    }
+                }
+
+                return s_AttributeTypesCache[attr] = types.ToArray();
+            }
+        }
+
+        private static IEnumerable<Type> GetAllLoadableTypes()
+        {
+            foreach (var assembly in AppDomain.CurrentDomain.GetAssemblies())
+            {
+                Type[] types;
+                try
+                {
+                    types = assembly.GetTypes();
+                }
+                catch (ReflectionTypeLoadException ex)
+                {
+                    types = ex.Types;
+                }
+
+                if (types == null)
+                {
+                    continue;
+                }
+
+                foreach (var t in types)
+                {
+                    if (t != null)
+                    {
+                        yield return t;
+                    }
+                }
+            }
+        }
+    }
+}
+#endif

Runtime/Internal/Utilities/TypeCache.cs.meta

@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: bc1207b657ed74ec19e459664d06925f
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: