Started work on plugin messaging API.

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelIdentifier.java

@@ -0,0 +1,7 @@
+package com.velocitypowered.api.proxy.messages;
+
+/**
+ * Represents a kind of channel identifier.
+ */
+public interface ChannelIdentifier {
+}

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelRegistrar.java

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+package com.velocitypowered.api.proxy.messages;
+
+public interface ChannelRegistrar {
+}

api/src/main/java/com/velocitypowered/api/proxy/messages/LegacyChannelIdentifier.java

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+package com.velocitypowered.api.proxy.messages;
+
+import com.google.common.base.Preconditions;
+import com.google.common.base.Strings;
+
+import java.util.Objects;
+
+/**
+ * Reperesents a legacy channel identifier (for Minecraft 1.12 and below). For modern 1.13 plugin messages, please see
+ * {@link MinecraftChannelIdentifier}.
+ */
+public class LegacyChannelIdentifier implements ChannelIdentifier {
+    private final String name;
+
+    public LegacyChannelIdentifier(String name) {
+        Preconditions.checkArgument(!Strings.isNullOrEmpty(name), "provided name is empty");
+        this.name = name;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    @Override
+    public String toString() {
+        return "LegacyChannelIdentifier{" +
+                "name='" + name + '\'' +
+                '}';
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        LegacyChannelIdentifier that = (LegacyChannelIdentifier) o;
+        return Objects.equals(name, that.name);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(name);
+    }
+}

api/src/main/java/com/velocitypowered/api/proxy/messages/MinecraftChannelIdentifier.java

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+package com.velocitypowered.api.proxy.messages;
+
+import com.google.common.base.Preconditions;
+import com.google.common.base.Strings;
+
+import java.util.Objects;
+import java.util.regex.Pattern;
+
+/**
+ * Represents a Minecraft 1.13+ channel identifier.
+ */
+public class MinecraftChannelIdentifier implements ChannelIdentifier {
+    private static final Pattern VALID_IDENTIFIER_REGEX = Pattern.compile("[a-z0-9\\-_]+");
+
+    private final String namespace;
+    private final String name;
+
+    private MinecraftChannelIdentifier(String namespace, String name) {
+        this.namespace = namespace;
+        this.name = name;
+    }
+
+    public static MinecraftChannelIdentifier forDefaultNamespace(String name) {
+        return new MinecraftChannelIdentifier("minecraft", name);
+    }
+
+    public static MinecraftChannelIdentifier create(String namespace, String name) {
+        Preconditions.checkArgument(!Strings.isNullOrEmpty(namespace), "namespace is null or empty");
+        Preconditions.checkArgument(!Strings.isNullOrEmpty(name), "namespace is null or empty");
+        Preconditions.checkArgument(VALID_IDENTIFIER_REGEX.matcher(namespace).matches(), "namespace is not valid");
+        Preconditions.checkArgument(VALID_IDENTIFIER_REGEX.matcher(name).matches(), "name is not valid");
+        return new MinecraftChannelIdentifier(namespace, name);
+    }
+
+    public String getNamespace() {
+        return namespace;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    @Override
+    public String toString() {
+        return "MinecraftChannelIdentifier{" +
+                "namespace='" + namespace + '\'' +
+                ", name='" + name + '\'' +
+                '}';
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        MinecraftChannelIdentifier that = (MinecraftChannelIdentifier) o;
+        return Objects.equals(namespace, that.namespace) &&
+                Objects.equals(name, that.name);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(namespace, name);
+    }
+}