UNITY_2020_1 SerializableCallback generic serialization

Changed SerializableCallback  classes for UNITY_2020_1 generic serialization feature. 
Like:
[SerializeField]
 SerializableCallback<bool> myField;

(Siccity#25 fork source pull request)

Author: maxencedcx

Runtime/SerializableCallback.cs

@@ -3,7 +3,13 @@
 using System.Reflection;
 using UnityEngine;
 
-public abstract class SerializableCallback<TReturn> : SerializableCallbackBase<TReturn> {
+#if UNITY_2020_1_OR_NEWER
+[System.Serializable]
+public class SerializableCallback<TReturn> : SerializableCallbackBase<TReturn>
+#else
+public class abstract SerializableCallback<TReturn> : SerializableCallbackBase<TReturn>
+#endif
+{
 	public TReturn Invoke() {
 		if (func == null) Cache();
 		if (_dynamic) {
@@ -27,7 +33,13 @@ protected override void Cache() {
 	}
 }
 
-public abstract class SerializableCallback<T0, TReturn> : SerializableCallbackBase<TReturn> {
+#if UNITY_2020_1_OR_NEWER
+[System.Serializable]
+public class SerializableCallback<T0, TReturn> : SerializableCallbackBase<TReturn>
+#else
+public abstract class SerializableCallback<T0, TReturn> : SerializableCallbackBase<TReturn> 
+#endif
+{
 	public TReturn Invoke(T0 arg0) {
 		if (func == null) Cache();
 		if (_dynamic) {
@@ -51,7 +63,13 @@ protected override void Cache() {
 	}
 }
 
+#if UNITY_2020_1_OR_NEWER
+[System.Serializable]
+public class SerializableCallback<T0, T1, TReturn> : SerializableCallbackBase<TReturn>
+#else
 public abstract class SerializableCallback<T0, T1, TReturn> : SerializableCallbackBase<TReturn> {
+#endif
+{
 	public TReturn Invoke(T0 arg0, T1 arg1) {
 		if (func == null) Cache();
 		if (_dynamic) {
@@ -75,7 +93,13 @@ protected override void Cache() {
 	}
 }
 
-public abstract class SerializableCallback<T0, T1, T2, TReturn> : SerializableCallbackBase<TReturn> {
+#if UNITY_2020_1_OR_NEWER
+[System.Serializable]
+public class SerializableCallback<T0, T1, T2, TReturn> : SerializableCallbackBase<TReturn>
+#else
+public abstract class SerializableCallback<T0, T1, T2, TReturn> : SerializableCallbackBase<TReturn> 
+#endif
+{
 	public TReturn Invoke(T0 arg0, T1 arg1, T2 arg2) {
 		if (func == null) Cache();
 		if (_dynamic) {
@@ -99,7 +123,13 @@ protected override void Cache() {
 	}
 }
 
-public abstract class SerializableCallback<T0, T1, T2, T3, TReturn> : SerializableCallbackBase<TReturn> {
+#if UNITY_2020_1_OR_NEWER
+[System.Serializable]
+public class SerializableCallback<T0, T1, T2, T3, TReturn> : SerializableCallbackBase<TReturn>
+#else
+public abstract class SerializableCallback<T0, T1, T2, T3, TReturn> : SerializableCallbackBase<TReturn> 
+#endif
+{
 	public TReturn Invoke(T0 arg0, T1 arg1, T2 arg2, T3 arg3) {
 		if (func == null) Cache();
 		if (_dynamic) {
@@ -121,4 +151,4 @@ protected override void Cache() {
 			}
 		}
 	}
-}
+}