Integrating the 0x API into your contracts
How to automatically get the best prices via 0x
Why you want 0x in your contracts? It's simple:
Okay, but seriously. Let's see why the 0x Swap API is interesting.
What is the 0x Swap API?
Last month the 0x team released the new v1 of the API. First announced in the beginning of the year, it was introduced to provide an easy way to get the best prices for trades without having to worry too much about the trade details. It combines a number of markets which in v1 now include
This video visualizes it pretty well:
What this means for you is getting the best prices available. According to 0x (so take it with a grain of salt) the results outperform any other options out there. If you can believe 0x, the API yields very convincing swaps:
What's new in v1?
With the new API version you now get heavily gas optimized trades. In fact the gas fees can sometimes be cheaper than trading on Uniswap directly. There are also several new liquidity providers added and many more changes, see here.
Alright, you are now interested? Let's take a look on how you could add it in your contracts.
Integrating 0x into your contracts
We are going to add automatic conversion of ETH to DAI to our contracts. It's always best to learn by example, so let's try to do this for a contract. A fully working Remix example can be found at the end.
1. Adding automatic conversions
Assume we have a pay function that users can call to interact with our contract. We want to give them the option to pay in DAI directly or they can automatically trade ETH to DAI using the 0x API.
First the function might look likt this:
function pay(uint256 amount) public payable {
require(
DAI.transferFrom(
msg.sender,
address(this),
amount
),
"DAI transfer failed"
);
// do something with that DAI
[...]
}But now we want to add a second option for the user to pay in ETH. You can see this on the right. In the case of users sending 0 ETH (msg.value == 0), we assume that the user wants to pay in DAI directly. Then we do a transferFrom as before and additionally require all swap related parameters to be empty.
In the case of a user wanting to trade his ETH, we call our convert function _convertEthToDai with all the swap parameters. Let's take a look how the convert function looks like...
function pay(
uint256 paymentAmountInDai,
address spender,
address payable swapTarget,
bytes calldata swapCallData
) public payable {
if (msg.value > 0) {
_convertEthToDai(
paymentAmountInDai,
spender,
swapTarget,
swapCallData
);
} else {
require(
spender == address(0),
"EMPTY_SPENDER_WITHOUT_SWAP"
);
require(
swapTarget == address(0),
"EMPTY_TARGET_WITHOUT_SWAP"
);
require(
swapCallData.length == 0,
"EMPTY_CALLDATA_WITHOUT_SWAP"
);
require(
DAI.transferFrom(msg.sender, address(this), paymentAmountInDai),
"DAI transfer failed"
);
}
// do something with that DAI
[...]
}2. Converting the ETH to DAI
Now we get to the heart of the logic. On a high level we do
- wrap ETH into WETH
- approve WETH for target
- execute 0x API swap
- run refunds
How you would get these parameters for the conversion, you can see in step 3. But essentially what they represent is an optimized swap contract call. swapTarget.call(swapCallData) executes the trade which internally transfers the WETH funds from this contract to the spender address.
Once the swap is finished, we can return any leftover ETH and DAI to the trader. The DAI refund might not be required as the leftover amount in my tests was always almost non existent. But keep it when doing large scale trades to be safe.
function _convertEthToDai(
uint256 paymentAmountInDai,
address spender, // API: "allowanceTarget"
address swapTarget, // API: "to"
bytes calldata swapCallData // API: "data"
) private {
WETH.deposit{value: msg.value}();
uint256 currentDaiBalance = DAI.balanceOf(address(this));
require(
WETH.approve(spender, type(uint256).max),
"approve failed"
);
(bool success, bytes memory res) = swapTarget.call(swapCallData);
require(
success,
string(bytes('SWAP_CALL_FAILED: ').concat(bytes(res.getRevertMsg())))
);
msg.sender.transfer(address(this).balance);
uint256 boughtAmount = DAI.balanceOf(address(this)) - currentDaiBalance;
require(boughtAmount >= paymentAmountInDai, "INVALID_BUY_AMOUNT");
// may not be required?
uint256 daiRefund = boughtAmount - paymentAmountInDai;
DAI.transfer(msg.sender, daiRefund);
}3. Retrieving the API Request Data
Now we have the swap functionality, but the user still needs to know what to send as the swap parameters. For this we add a view function that returns the 0x API Request URL. We use https://kovan.api.0x.org for our tests, for mainnet you of course would use https://api.0x.org/.
For example given we want to buy one DAI, we would pass "1000000000000000000" (1e18) to the function and we get our request url looking like this: https://kovan.api.0x.org/swap/v1/quote?sellToken=0xd0A1E359811322d97991E03f863a0C30C2cF029C&buyToken=0x1528f3fcc26d13f7079325fb78d9442607781c8c&buyAmount=1000000000000000000. You can directly open this in the browser to get the results. Obviously if you have a frontend, you can automate all of this.
Now once you click the link, all you need are three values:
- "allowanceTarget" →
address spender - "to" →
address payable swapTarget - "data" →
bytes calldata swapCallData
string private api0xUrl = 'https://kovan.api.0x.org/swap/v1/quote';
string private wethToDai0xApiRequest = '?sellToken=0xd0A1E359811322d97991E03f863a0C30C2cF029C&buyToken=0x1528f3fcc26d13f7079325fb78d9442607781c8c&buyAmount=';
function get0xApiRequest(uint256 paymentAmountInDai) external view returns(string memory) {
return string(bytes(api0xUrl).concat(bytes(wethToDai0xApiRequest)).concat(paymentAmountInDai.toBytes()));
}Now once you click the link, all you need are three values:
- "allowanceTarget" →
address spender - "to" →
address swapTarget - "data" →
bytes calldata swapCallData
With these variables a user now has all he needs to call myContract.pay(amount, spender, swapTarget, swapCallData).
4. Useful Helper Functions
If you wondered about some of the functions in the code above, don't worry. We've implemented and used a few useful helper functions, namingly concat, toStringBytes and getRevertMsg. Those may be quite useful to you in general, so it's worth taking a closer look.
1. Concat String Bytes
With the new abi.encodePacked function since Solidity v5, concatenating strings is particularly easy. You can use this function for strings like this: concat(bytes(myString1), bytes(myString2)).
function concat(
bytes memory a,
bytes memory b
) internal pure returns (bytes memory) {
return abi.encodePacked(a, b);
}2. Uint256 to String Bytes
Inspired by the Provable code here, this function computes the string representation of a uint256 number returned as bytes array.
Strings in Solidity are UTF-8 encoded. The value 48 implies the character '0'. So to convert a number to the correct string, we essentially compute and store 48 + remainder of modulo 10 for each digit.
function toStringBytes(
uint256 v
) internal pure returns (bytes memory) {
if (v == 0) { return "0"; }
uint256 j = v;
uint256 len;
while (j != 0) {
len++;
j /= 10;
}
bytes memory bstr = new bytes(len);
uint256 k = len - 1;
while (v != 0) {
bstr[k--] = byte(uint8(48 + v % 10));
v /= 10;
}
return bstr;
}3. Get Revert Message for Low-level Call
Lastly we added a function to retrieve the revert message from low-level contract calls. This allows us to give more detailed information about the revert reason. Implementation was taken from Stackexchange, a website you should check out whenever you can.
function getRevertMsg(
bytes memory _returnData
) internal pure returns (string memory) {
if (_returnData.length < 68)
return 'Transaction reverted silently';
assembly {
_returnData := add(_returnData, 0x04)
}
return abi.decode(_returnData, (string));
}Fully working example for Remix
You can find a fully working example for Remix here. To test it make sure to deploy to Kovan, retrieve the API data and then send along enough ETH for the swap (top left 'value' field).
You can also take a look at the 0x Starter Guide. It further includes a direct swap example in case you don't want to deploy your own contract.
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Navigating the pitfalls of securely interacting with ERC20 tokens
Figuring out how to securely interact might be harder than you think
You would think calling a few functions on an ERC-20 token is the simplest thing to do, right? Unfortunately I have some bad news, it's not. There are several things to consider and some errors are still pretty common. Let's start with the easy ones. Let's take a very common token: ... Now to...
Why you should automatically generate interests from user funds
How to integrate Aave and similar systems in your contracts
If you're writing contracts that use, hold or manage user funds, you might want to consider using those funds for generating free extra income. What's the catch? That's right, it's basically free money and leaving funds unused in a contract is wasting a lot of potential. The way these...
How to use Polygon (Matic) in your Dapp
Deploying and onboarding users to Polygon to avoid the high gas costs
Gas costs are exploding again, ETH2.0 is still too far away and people are now looking at layer 2 solutions. Here's a good overview of existing layer 2 projects: https://github.com/Awesome-Layer-2/awesome-layer-2. Today we will take a closer look at Polygon (previously known as Matic) as a...
Migrating from Truffle to Buidler
And why you should probably keep both.
Why Buidler? Proper debugging is a pain with Truffle. Events are way too difficult to use as logging and they don't even work for reverted transactions (when you would need them most). Buidler gives you a console.log for your contracts which is a game changer. And you'll also get stack traces...
Contract factories and clones
How to deploy contracts within contracts as easily and gas-efficient as possible
The factory design pattern is a pretty common pattern used in programming. The idea is simple, instead of creating objects directly, you have an object (the factory) that creates objects for you. In the case of Solidity, an object is a smart contract and so a factory will deploy new contracts for...
How to use IPFS in your Dapp?
Using the interplanetary file system in your frontend and contracts
You may have heard about IPFS before, the Interplanetary File System. The concept has existed for quite some time now, but with IPFS you'll get a more reliable data storage, thanks to their internal use of blockchain technology. Filecoin is a new system that is incentivizing storage for IPFS...
Downsizing contracts to fight the contract size limit
What can you do to prevent your contracts from getting too large?
Why is there a limit? On November 22, 2016 the Spurious Dragon hard-fork introduced EIP-170 which added a smart contract size limit of 24.576 kb. For you as a Solidity developer this means when you add more and more functionality to your contract, at some point you will reach the limit and when...
Using EXTCODEHASH to secure your systems
How to safely integrate anyone's smart contract
What is the EXTCODEHASH? The EVM opcode EXTCODEHASH was added on February 28, 2019 via EIP-1052. Not only does it help to reduce external function calls for compiled Solidity contracts, it also adds additional functionality. It gives you the hash of the code from an address. Since only contract...
Using the new Uniswap v2 in your contracts
What's new in Uniswap v2 and how to integrate Uniswap v2
Note : For Uniswap 3 check out the tutorial here. What is UniSwap? If you're not familiar with Uniswap yet, it's a fully decentralized protocol for automated liquidity provision on Ethereum. An easier-to-understand description would be that it's a decentralized exchange (DEX) relying on external...
Solidity and Truffle Continuous Integration Setup
How to setup Travis or Circle CI for Truffle testing along with useful plugins.
Continuous integration (CI) with Truffle is great for developing once you have a basic set of tests implemented. It allows you to run very long tests, ensure all tests pass before merging a pull request and to keep track of various statistics using additional tools. We will use the Truffle...
Upcoming Devcon 2021 and other events
The Ethereum Foundation just announced the next Devcon in 2021 in Colombia
Biggest virtual hackathon almost finished First of all, the current HackMoney event has come to an end and it has been a massive success. One can only imagine what kind of cool projects people have built in a 30 days hackathon. All final projects can be seen at:...
The Year of the 20: Creating an ERC20 in 2020
How to use the latest and best tools to create an ERC-20 token contract
You know what an ERC-20 is, you probably have created your own versions of it several times (if not, have a look at: ERC-20). But how would you start in 2020 using the latest tools? Let's create a new ERC-2020 token contract with some basic functionality which focuses on simplicity and latest...
How to get a Solidity developer job?
There are many ways to get a Solidity job and it might be easier than you think!
You have mastered the basics of Solidity, created your first few useful projects and now want to get your hands on some real-world projects. Getting a Solidity developer job might be easier than you think. There are generally plenty of options to choose from and often times not a lot of...
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...
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...