[fix] fix loop variable names, add span fallback for older Unity versions

Author: claasic

Packages/src/Runtime/Internal/Utilities/ShaderVariantRegistry.cs

@@ -88,9 +88,9 @@ public Shader FindOptionalShader(Shader shader,
             // Find optional shader.
             Shader optionalShader;
             var count = m_OptionalShaders.Count;
-            for (var index = 0; index < count; index++)
+            for (var i = 0; i < count; i++)
             {
-                var pair = m_OptionalShaders[index];
+                var pair = m_OptionalShaders[i];
                 if (pair.key != shaderName) continue;
                 optionalShader = Shader.Find(pair.value);
                 if (!optionalShader) continue;

Packages/src/Runtime/UIEffectContext.cs

@@ -691,10 +691,14 @@ private static float GetMultiplier(float deg)
             return Mathf.Max(Mathf.Abs(cos - sin), Mathf.Abs(cos + sin));
         }
 
+        #if UNITY_2021_2_OR_NEWER
         private static void SetKeyword(Material material, Span<string> keywords, int index)
+        #else
+        private static void SetKeyword(Material material, string[] keywords, int index)
+        #endif
         {
             var length = keywords.Length;
-            if (index < keywords.Length && !string.IsNullOrEmpty(keywords[index]))
+            if (index < length && !string.IsNullOrEmpty(keywords[index]))
             {
                 material.EnableKeyword(keywords[index]);
             }

Packages/src/Runtime/Utilities/GraphicProxy.cs

@@ -16,9 +16,9 @@ public static void Register(GraphicProxy proxy)
         {
             // Register only once.
             var count = s_Proxies.Count;
-            for (var index = 0; index < count; index++)
+            for (var i = 0; i < count; i++)
             {
-                var p = s_Proxies[index];
+                var p = s_Proxies[i];
                 if (p.GetType() == proxy.GetType()) return;
             }