Replay system using Unity

Unity is an incredible tool for making quality games at a blazing fast pace. However, like all closed systems there are some limitations to how you can extend the engine and one such limitation is developing a good replay system for a game. I will talk about two possible approaches and how to solve other issues along the way. The system was devised for a Match 3 prototype, but can be applied to any project. There are commercial solutions available, but this post is intended for coders.

If the game has been designed with a deterministic outcome in mind, the most essential parts of recording are input events, delta times (optional) and random seeds. What this means is the only input available to the game will be the players’ actions, the rest should be simulated properly to arrive at the same outcome. Since storing random seeds and loading as appropriate is more or less straightforward, we will focus on the input.

1) The first issue is how to capture and replay input in the least disturbing way possible. If you have used Unity’s Input class, Input.GetMouseButton(i) should look familiar. The replay system was added after developing the main mechanics, and we didn’t want to go back and rewrite how the game worked. Such a system should ideally work for future or existing games, and Unity already provides a nice interface that other programmers use. Furthermore, plugins use this interface, and not sticking to it can severely limit your ability to record games.

The solution we arrived at was shadowing Unity’s Input class by creating a new class with the same name and accessing it through the UnityEngine namespace inside of the new class. This allows for conditional routing of Unity’s input, therefore passing recorded values into the Input.GetMouseButtonX functions, and essentially ‘tricking’ the game into thinking it is playing real player input. You can do the same with keys.

There are many functions and properties to override, it can take time and care to get it all working properly. Once you have this new layer you can create a RecordManager class and start creating methods that connect with the new Input class.

2) The second issue is trickier to get properly working, due to common misconceptions (myself included) about how Unity’s Update loops work. Unity has two different Update loops that serve different purposes, Update and FixedUpdate. Update runs at every frame, whereas FixedUpdate updates at a fixed, specified time interval. FixedUpdate has absolutely nothing to do with Update. No rule says that for every Update there should be a FixedUpdate, or that there should be no more than one for every Update.

Let’s explain it with two use cases. For both, the FixedUpdate interval is 0,017 s (~60 fps).

a) Update runs at 60 fps (same as FixedUpdate). The order of updates would be:

b) Update runs faster (120 fps). I have chosen this number because it is exactly double that of FixedUpdate. In this case, the order of updates would be as follows:
There is one FixedUpdate every two Update.

c) Update runs slower (30 fps). Same rule as above, but 30 = 60/2

Since FixedUpdate can’t keep up with Update, it updates twice to compensate.

This brings up the following question: where should I record input events, and where should I replay them? How can I replay something I recorded on one computer on another, and have the same output?

The answer to the first question is record in Update. It is guaranteed to run in every Unity tick, and doing so in FixedUpdate will cause you to miss input events and mess up your recording. The answer to the second question is a little more open, and depends on how you recorded your data.

One approach is to record the deltaTime in Update for every Update, and shadow Unity’s Time class the same way we did with Input to be able to read a recorded Time.deltaTime property wherever it’s used. This has two possible issues, namely precision (of the deltaTime) and storage.

The second approach is to save events and link them to their corresponding FixedUpdate tick, that way you can associate many events to a single tick (if Update goes too fast) or none (if Update goes too slow). With this approach you can only execute your code in FixedUpdate, and execute as many times as recorded Updates there are. It’s also important to save the average Update time of the original recording and set it as the FixedUpdate interval. The simulation will not be 100% accurate in that Update times won’t fluctuate as they did in the original recording session, but it is guaranteed to execute the same code.


There is one last setting that’s needed to properly record events, which is set the RecordManager to record all input at the beginning of every frame. Unity has a Script Execution Order option under Project Settings where you can set the RecordManager to run before any other script. That way recording and replaying are guaranteed to run in the correct order.

Java and Vector Math

Some time ago, I had to develop a 3D vector utility class for Java. Because the Android platform only uses Java, this is a must if you’re developing for it (unless you’re directly using the NDK or some middleware such as Unity. I’ll get back to this later).

Java, like all programming languages, has its virtues and weaknesses. I found trying to develop a solid Vector3 class to be one such weakness, because Java lacks two main features that I consider core to what I was trying to achieve: stack-allocated objects and operator overloading. These missing features make operating with vectors in Java an annoyance beyond measure.

As much as some people seem to dislike operator overloading, vector/matrix math is one domain where I consider they excel, and the lack of it is going to force me to always go through functions for even the simplest of operations, such as adding/subtracting two vectors or multiplying/dividing by a scalar.

Compare the following lines of code:

Doesn’t seem too bad, does it? Let’s try something different, like obtaining a direction from two points, normalizing, scaling by a factor, and adding it to a point (a relatively frequent operation)

It’s either that or separating into several lines so it becomes clearer and a bit more readable. This is clearly an undesirable way of working with vectors, but the only at our disposal when using Java.

There’s another caveat, though, one that is implicit in the way we have used the equal operator until now. We have been assigning the Vector3 by reference all this time, invalidating the erroneous assumption that we get a new Vector3 out of the operation. What we want is a copy of the resulting Vector3, which means create a new Vector3 using the new operator, and copy the values into the new Vector3. Therefore, line [2] would become something along the lines of:


In any case it is a very confusing way of working with vectors.

There is yet another annoyance to be wary of when working with instantiations on the heap in a garbage collected language such as Java which is, precisely, the dreaded Garbage Collector. Vector3 operations typically go inside long loops where interesting calculations take place for many objects in the virtual world, and creating all those new objects in the heap is asking for trouble when the GC comes to inspect your pile of vector rubbish and tries to clean up the mess. This is due to the fact that there are no stack-allocated objects in Java, leaving us with one option – creating temporary Vector3’s and reusing them. This has its fair share of problems too – mainly readability, the ability to use several temporary vectors for intermediate calculations, and having to pass the Vector3 reference as a parameter instead of returning it as a normal function return value. Let’s go back to our example.

Definitely not ideal. In contrast, and for all its similarity with Java, C# has both these features, which makes it a language of choice for these kinds of applications, and I wonder if it was one among the multiple reasons why Unity chose it as their main development language.