perf: return the contents of the writer as an array segment (#916)

* Return the contents of the writer as an array segment

* Use a safer ToArraySegment method and add some comments

* refactor: for readability and debuggability

* Test that ToArraySegment works

* Updated with master

* Update NetworkWriter.cs

Assets/Mirror/Runtime/NetworkWriter.cs

@@ -34,6 +34,21 @@ public byte[] ToArray()
             return stream.ToArray();
         }
 
+        // Gets the serialized data in an ArraySegment<byte>
+        // this is similar to ToArray(),  but it gets the data in O(1)
+        // and without allocations.
+        // Do not write anything else or modify the NetworkWriter
+        // while you are using the ArraySegment
+        public ArraySegment<byte> ToArraySegment()
+        {
+            stream.Flush();
+            if (stream.TryGetBuffer(out ArraySegment<byte> data))
+            { 
+                return data;
+            }
+            throw new Exception("Cannot expose contents of memory stream. Make sure that MemoryStream buffer is publicly visible (see MemoryStream source code).");
+        }
+
         // reset both the position and length of the stream,  but leaves the capacity the same
         // so that we can reuse this writer without extra allocations
         public void SetLength(long value)

Assets/Mirror/Tests/NetworkWriterTest.cs

@@ -555,6 +555,18 @@ public void TestToArray()
             Assert.That(writer.ToArray().Length, Is.EqualTo(2));
         }
 
+        [Test]
+        public void TestToArraySegment()
+        {
+            NetworkWriter writer = new NetworkWriter();
+            writer.Write("hello");
+            writer.Write("world");
+
+            NetworkReader reader = new NetworkReader(writer.ToArraySegment());
+            Assert.That(reader.ReadString(), Is.EqualTo("hello"));
+            Assert.That(reader.ReadString(), Is.EqualTo("world"));
+        }
+
         [Test]
         public void TestChar()
         {