Design Pattern Solidity: Mock contracts for testing
Why you should make fun of your contracts
Mock objects are a common design pattern in object-oriented programming. Coming from the old French word 'mocquer' with the meaning of 'making fun of', it evolved to 'imitating something real' which is actually what we are doing in programming. Please only make fun of your smart contracts if you want to, but mock them whenever you can. It makes your life easier.
Unit-testing contracts with mocks
Mocking a contract essentially means creating a second version of that contract which behaves very similar to the original one, but in a way that can be easily controlled by the developer. You often end up with complex contracts where you only want to unit-test small parts of the contract. The problem is what if testing this small part requires a very specific contract state that is difficult to end up in?
You could write complex test setup logic everytime that brings in the contract in the required state or you write a mock. Mocking a contract is easy with inheritance. Simply create a second mock contract that inherits from the original one. Now you can override functions to your mock. Let us see it with an example.
Example: Private ERC20
We use an example ERC-20 contract that has an initial private time. The owner can manage private users and only those will be allowed to receive tokens at the beginning. Once a certain time has passed, everyone will be allowed to use the tokens. If you are curious, we are using the _beforeTokenTransfer hook from the new OpenZeppelin contracts v3.
pragma solidity ^0.6.0;
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
contract PrivateERC20 is ERC20, Ownable {
mapping (address => bool) public isPrivateUser;
uint256 private publicAfterTime;
constructor(uint256 privateERC20timeInSec) ERC20("PrivateERC20", "PRIV") public {
publicAfterTime = now + privateERC20timeInSec;
}
function addUser(address user) external onlyOwner {
isPrivateUser[user] = true;
}
function isPublic() public view returns (bool) {
return now >= publicAfterTime;
}
function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual override {
super._beforeTokenTransfer(from, to, amount);
require(_validRecipient(to), "PrivateERC20: invalid recipient");
}
function _validRecipient(address to) private view returns (bool) {
if (isPublic()) {
return true;
}
return isPrivateUser[to];
}
}And now let's mock it.
pragma solidity ^0.6.0;
import "../PrivateERC20.sol";
contract PrivateERC20Mock is PrivateERC20 {
bool isPublicConfig;
constructor() public PrivateERC20(0) {}
function setIsPublic(bool isPublic) external {
isPublicConfig = isPublic;
}
function isPublic() public view returns (bool) {
return isPublicConfig;
}
}You will get one of the following error messages:
PrivateERC20Mock.sol: TypeError: Overriding function is missing "override" specifier.-
PrivateERC20.sol: TypeError: Trying to override non-virtual function. Did you forget to add "virtual"?.
Since we are using the new 0.6 Solidity version, we have to add the virtual keyword for functions that can be overriden and override for the overriding function. So let us add those to both isPublic functions.
Now in your unit tests, you can use PrivateERC20Mock instead. When you want to test the behaviour during the private usage time, use setIsPublic(false) and likewise setIsPublic(true) for testing the public usage time. Of course in our example, we could just use time helpers to change the times accordingly as well. But the idea of mocking should be clear now and you can imagine scenarios where it is not as easy as simply advancing the time.
Mocking many contracts
It can become messy if you have to create another contract for every single mock. If this bothers you, you can take a look at the MockContract library. It allows you to override and change behaviours of contracts on-the-fly. However, it works only for mocking calls to another contract, so it would not work for our example.
Mocking can be even more powerful
The powers of mocking do not end there.
- Adding functions: Not only overriding a specific function is useful, but also just adding additional functions. A good example for tokens is just having an additional
mintfunction to allow any user to get new tokens for free. - Usage in testnets: When you deploy and test your contracts on testnets together with your Dapp, consider using a mocked version. Avoid overriding functions unless you really have to. You want to test the real logic after all. But adding for example a
resetfunction can be useful that simply resets the contract state to the beginning, no new deployment required. Obviously you would not want to have that in a mainnet contract.
Solidity Developer
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Kickstart your Dapp frontend development with create-eth-app
An overview on how to use the app and its features
Last time we looked at the big picture of Solidity and already mentioned the create-eth-app. Now you will find out how to use it, what features are integrated and additional ideas on how to expand on it. Started by Paul Razvan Berg, the founder of sablier, this app will kickstart your frontend...
The big picture of Solidity and Blockchain development in 2020
Overview of the most important technologies, services and tools that you need to know
Now, I do not know about you, but I remember when I first started with Solidity development being very confused by all the tools and services and how they work in connection with one another. If you are like me, this overview will help you understand the big picture of Solidity development. As I...
Design Pattern Solidity: Free up unused storage
Why you should clean up after yourself
You may or may not be used to a garbage collectors in your previous programming language. There is no such thing in Solidity and even if there was a similar concept, you would still be better off managing state data yourself. Only you as a programmer can know exactly which data will not be used...
How to setup Solidity Developer Environment on Windows
What you need to know about developing on Windows
Using Windows for development, especially for Solidity development, can be a pain sometimes, but it does not have to be. Once you have configured your environment properly, it can actually be extremely efficient and Windows is a very, very stable OS, so your overall experience can be amazing. The...
Avoiding out of gas for Truffle tests
How you do not have to worry about gas in tests anymore
You have probably seen this error message a lot of times: Error: VM Exception while processing transaction: out of gas Disclaimer : Unfortunately, this does not always actually mean what it is saying when using Truffle , especially for older versions. It can occur for various reasons and might be...
Design Pattern Solidity: Stages
How you can design stages in your contract
Closely related to the concept of finite-state machines, this pattern will help you restrict functions in your contract. You will find a lot of situations where it might be useful. Any time a contract should allow function calls only in certain stages. Let's look at an example: contract Pool {...
Web3 1.2.5: Revert reason strings
How to use the new feature
A new Web3 version was just released and it comes with a new feature that should make your life easier. With the latest version 1.2.5, you can now see the the revert reason if you use the new handleRevert option. You can activate it easily by using web3.eth.handleRevert = true . Now when you use...
Gaining back control of the internet
How Ocelot is decentralizing cloud computing
I recently came across an ambitious company that will completely redefine the way we are using the internet. Or rather, the way we are using its underlying infrastructure which ultimately is the internet. While looking at their offering, I also learned how to get anonymous cloud machines, you...
Devcon 5 - Review
Impressions from the conference
I had a lot to catch up on after Devcon. Also things didn't go quite as planned, so please excuse my delayed review! This year's Devcon was certainly stormy with a big typhoon warning already on day 1. Luckily (for us, not the people in Tokyo), it went right past Osaka. Nevertheless, a lot of...
Devcon 5 - Information, Events, Links, Telegram
What you need to know
Devcon 5 is coming up soon and there are already lots of events available, information about Osaka and more. Here is a short overview: Events Events Calendar Events Google Docs Events Kickback Most events are in all three, but if you really want to see all, you will have to look at all three...
Design Pattern Solidity: Off-chain beats on-chain
Why you should do as much as possible off-chain
As you might have realized, Ethereum transactions are anything but cheap. In particular, if you are computing complex things or storing a lot of data. That means sometimes we cannot put all logic inside Solidity. Instead, we can utilize off-chain computations to help us. A very simple example...
Design Pattern Solidity: Initialize Contract after Deployment
How to use the Initializable pattern
There are a few reasons why you might want to initialize a contract after deployment and not directly by passing constructor arguments. But first let's look at an example: contract MyCrowdsale { uint256 rate; function initialize(uint256 _rate) public { rate = _rate; } } What's the advantage over...
Consensys Blockchain Jobs Report
What the current blockchain job market looks like
Consensys published their blockchain jobs report which you can checkout in their Blockchain Developer Job Kit. The most interesting aspects are Blockchain developer jobs have been growing at a rate of 33x of the previous year according to LinkedIns jobs report Typical salary is about...
Provable — Randomness Oracle
How the Oraclize random number generator works
One particularly interesting approach by Provable is the usage of a hardware security device, namely the Ledger Nano S. It uses a trusted execution environment to generate random numbers and provides a Provable Connector Contract as interface. How to use the Provable Randomness Oracle? Use the...
Solidity Design Patterns: Multiply before Dividing
Why the correct order matters!
There has been a lot of progress since the beginning of Ethereum about best practices in Solidity. Unfortunately, I have the feeling that most of the knowledge is within the circle of experienced people and there aren’t that many online resources about it. That is why I would like to start this...
Devcon 5 Applications closing in one week
Devcon 5 Applications closing
Watch out for the Devcon 5 applications. You only have one week left to apply either as Buidler Student Scholarship Press Devcon is by far the biggest and most impressive Ethereum conference in the world. And it's full of developers! I am especially excited about the cool location this year in...
Randomness and the Blockchain
How to achieve secure randomness for Solidity smart contracts?
Update 2023 : Ethereum transitioned to Proof of Stake! If you are interested in the randomness there, you can now use the updated info over at https://soliditydeveloper.com/prevrandao. When we talk about randomness and blockchain, these are really two problems: How to generate randomness in smart...