add Codecs for FluidVariant and ResourceAmount<FluidVariant>

fabric-transfer-api-v1/src/main/java/net/fabricmc/fabric/api/transfer/v1/fluid/FluidVariant.java

@@ -16,6 +16,14 @@
 
 package net.fabricmc.fabric.api.transfer.v1.fluid;
 
+import java.util.Optional;
+
+import com.mojang.serialization.Codec;
+import com.mojang.serialization.codecs.RecordCodecBuilder;
+
+import net.fabricmc.fabric.api.transfer.v1.storage.base.ResourceAmount;
+import net.minecraft.registry.Registries;
+
 import org.jetbrains.annotations.ApiStatus;
 import org.jetbrains.annotations.Nullable;
 
@@ -43,7 +51,19 @@
 @ApiStatus.Experimental
 @ApiStatus.NonExtendable
 public interface FluidVariant extends TransferVariant<Fluid> {
-	/**
+	Codec<FluidVariant> CODEC = RecordCodecBuilder.create(instance ->
+			instance.group(
+					Registries.FLUID.getCodec().fieldOf("fluid").forGetter(FluidVariant::getFluid),
+					NbtCompound.CODEC.optionalFieldOf("nbt").forGetter(variant -> Optional.ofNullable(variant.getNbt()))
+			).apply(instance, (fluid, optionalNbt) -> FluidVariant.of(fluid, optionalNbt.orElse(null))));
+
+	Codec<ResourceAmount<FluidVariant>> AMOUNT_CODEC = RecordCodecBuilder.create(instance ->
+			instance.group(
+					CODEC.fieldOf("variant").forGetter(ResourceAmount::resource),
+					Codec.LONG.fieldOf("amount").forGetter(ResourceAmount::amount)
+			).apply(instance, ResourceAmount::new));
+
+    /**
 	 * Retrieve a blank FluidVariant.
 	 */
 	static FluidVariant blank() {

fabric-transfer-api-v1/src/testmod/java/net/fabricmc/fabric/test/transfer/gametests/CodecsTest.java

@@ -0,0 +1,46 @@
+package net.fabricmc.fabric.test.transfer.gametests;
+
+import com.google.gson.JsonElement;
+import com.google.gson.JsonParser;
+import com.mojang.datafixers.util.Pair;
+import com.mojang.serialization.DataResult;
+import com.mojang.serialization.JsonOps;
+
+import net.fabricmc.fabric.api.transfer.v1.storage.base.ResourceAmount;
+
+import net.minecraft.fluid.Fluids;
+import net.minecraft.test.GameTest;
+import net.minecraft.test.TestContext;
+
+import net.fabricmc.fabric.api.gametest.v1.FabricGameTest;
+import net.fabricmc.fabric.api.transfer.v1.fluid.FluidVariant;
+
+public class CodecsTest {
+	@GameTest(templateName = FabricGameTest.EMPTY_STRUCTURE)
+	public void testDecode(TestContext context) {
+		String input = """
+				{
+					"variant": {
+						"fluid": "minecraft:water",
+						"nbt": {
+							"test": 42
+						}
+					},
+					"amount": 81000
+				}
+				""";
+
+		JsonElement json = JsonParser.parseString(input);
+
+		DataResult<Pair<ResourceAmount<FluidVariant>, JsonElement>> result = FluidVariant.AMOUNT_CODEC.decode(JsonOps.INSTANCE, json);
+		context.assertTrue(result.result().isPresent(), "Couldn't decode JSON");
+
+		ResourceAmount<FluidVariant> decoded = result.result().get().getFirst();
+		context.assertTrue(decoded.resource().getFluid() == Fluids.WATER, "Fluid was not water");
+		context.assertTrue(decoded.resource().getNbt() != null, "NBT was null");
+		context.assertTrue(!decoded.resource().getNbt().isEmpty(), "NBT was empty");
+		context.assertTrue(decoded.amount() == 81000, "Amount was not 81000");
+
+		context.complete();
+	}
+}

fabric-transfer-api-v1/src/testmod/resources/fabric.mod.json

@@ -18,7 +18,8 @@
     ],
     "fabric-gametest": [
       "net.fabricmc.fabric.test.transfer.gametests.VanillaStorageTests",
-      "net.fabricmc.fabric.test.transfer.gametests.WorldDependentAttributesTest"
+      "net.fabricmc.fabric.test.transfer.gametests.WorldDependentAttributesTest",
+      "net.fabricmc.fabric.test.transfer.gametests.CodecsTest"
     ]
   },
   "mixins": [