Solidity Examples

This section focuses on the Solidity code for important interactions with the Optimistic Oracle. We recommend starting with the Getting Started tutorial that walks developers through setting up the developer quickstart repo and working with UMA contracts.
UMA Contract Lifecycle
UMA's Optimistic Oracle allows contracts to quickly request and receive price information. A request is made when a contract submits the following parameters with a request to the Optimistic Oracle contract:
identifier: price identifier being requested.
timestamp: timestamp of the price being requested.
ancillaryData: additional arguments passed with the price request.
currency: ERC20 token used for payment of rewards and fees. Must be approved for use with the DVM.
reward: reward offered to a successful proposer. Will be paid by the caller. Note: this can be 0.
Proposers respond to price requests by referencing off-chain price feeds to submit the price of an asset. In return for their work they will receive a pre-defined proposal reward set by the Requestor. To propose prices, the Proposer is required to stake a proposal bond. In the event that the price information they proposed is disputed and deemed incorrect, the Proposer will lose their bond.
Disputers can refute a price submitted by a Proposer within the proposal liveness period by referencing their own off-chain price feeds. The proposal liveness period is a pre-defined amount of time a proposal can be disputed before the Requestor receives the price of the asset.
f Disputers do not refute the price submitted by the Proposer within the proposal liveness period, the price is sent to the Requestor. If a proposal is disputed, the price will be submitted to UMA’s DVM and resolved after a 48-96 hour voting period.
Integrating with the Optimistic Oracle
We continue to use the OptimisticDepositBox contract as an example. The OptimisticDepositBox is a minimal financial contract that allows a user to deposit collateral into a contract and later withdraw their collateral corresponding to a desired USD amount. When the user wants to make a withdrawal, a price request is made to the Optimistic Oracle.
Let's first take a look at the OptimisticDepositBox constructor. Optimistic Oracle price requests require the use of a whitelisted identifier and collateral. The OptimisticDepositBox uses the protocol Finder to discover UMA protocol contracts and call the isOnWhitelist method on the AddressWhitelist and the isIdentifierSupported method on the IdentifierWhitelist contract to confirm both are whitelisted before deployment.
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constructor(
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    address _collateralAddress,
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    address _finderAddress,
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    bytes32 _priceIdentifier,
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    address _timerAddress
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) nonReentrant() Testable(_timerAddress) {
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    finder = FinderInterface(_finderAddress);
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    require(_getIdentifierWhitelist().isIdentifierSupported(_priceIdentifier), "Unsupported price identifier");
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    require(_getAddressWhitelist().isOnWhitelist(_collateralAddress), "Unsupported collateral type");
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    collateralCurrency = IERC20(_collateralAddress);
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    priceIdentifier = _priceIdentifier;
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}
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​
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...
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​
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function _getIdentifierWhitelist() internal view returns (IdentifierWhitelistInterface) {
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    return IdentifierWhitelistInterface(finder.getImplementationAddress(OracleInterfaces.IdentifierWhitelist));
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}
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​
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function _getAddressWhitelist() internal view returns (AddressWhitelist) {
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    return AddressWhitelist(finder.getImplementationAddress(OracleInterfaces.CollateralWhitelist));
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}
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Example arguments used to deploy the contract can be found and tested in the OptimisticDepositBox fixture. A whitelisted USDC contract is used for collateral, the Finder and Timer addresses use existing UMA ecosystem contracts that are deployed, and a price identifier that has been whitelisted with the IdentifierWhitelist contract.
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// Deploy the OptimisticDepositBox contract.
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  const optimisticDepositBox = await (
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    await getContractFactory("OptimisticDepositBox", deployer)
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  ).deploy(usdc.address, parentFixture.finder.address, identifier, parentFixture.timer.address);
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Requesting a Price
A price request is made when the requestWithdrawal method is called. A depositor submits the denominatedCollateralAmount to withdrawal and a requestPrice call is constructed with the following arguments shown above and in the requestPrice interface:
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function requestWithdrawal(uint256 denominatedCollateralAmount)
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    public
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    noPendingWithdrawal(msg.sender)
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    nonReentrant
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{
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    OptimisticDepositBoxData storage depositBoxData = depositBoxes[msg.sender];
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    require(denominatedCollateralAmount > 0, "Invalid collateral amount");
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​
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    // Update the position data for the user.
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    depositBoxData.withdrawalRequestAmount = denominatedCollateralAmount;
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    depositBoxData.withdrawalRequestTimestamp = getCurrentTime();
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​
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    emit RequestWithdrawal(msg.sender, denominatedCollateralAmount, depositBoxData.withdrawalRequestTimestamp);
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​
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    // A price request is sent for the current timestamp.
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    _requestOraclePrice(depositBoxData.withdrawalRequestTimestamp);
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}
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​
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.....
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​
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// Requests a price for `priceIdentifier` at `requestedTime` from the Optimistic Oracle.
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function _requestOraclePrice(uint256 requestedTime) internal {
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    OptimisticOracleInterface oracle = _getOptimisticOracle();
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    // For other use cases, you may need ancillary data or a reward. Here, they are both zero.
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    oracle.requestPrice(priceIdentifier, requestedTime, "", IERC20(collateralCurrency), 0);
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}
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After price request functionality has been implemented into your contracts, you can test your changes by creating a test in the developer quickstart repo similar to the OptimisticDepositBox test below:
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await expect(optimisticeDepositBox.connect(depositor).requestWithdrawal(amountToWithdraw))
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  .to.emit(optimisticDepositBox, "RequestWithdrawal")
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  .withArgs(depositor.address, amountToWithdraw, requestTimestamp);
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Price Proposals
The code snippets above represent the core functionality for deploying a minimal contract and requesting a price. The next step in the contract lifecycle after a request has been made is a price can be proposed. 
​Here is the proposePriceFor function from the Optimistic Oracle contract that is used to propose a price for requests. The price proposal will revert if the parameters do not match an existing price request. For reference, the developer quickstart repo demonstrates how to call the proposePriceFor method.
Liveness
An important aspect to consider when using the Optimistic Oracle is the liveness period which is the number of seconds a proposal must wait before a price can be resolved and contracts can be settled. A defaultLiveness value is set (currently 7,200) or developers can set a customLiveness value.
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request.expirationTime = getCurrentTime().add(
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    request.customLiveness != 0 ? request.customLiveness : defaultLiveness
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);
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To set a customLiveness value with our OptimisticDepositBox example above, we could add line 5 to change the liveness period from 7,200 seconds to 3,600:
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function _requestOraclePrice(uint256 requestedTime) internal {
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    OptimisticOracleInterface oracle = _getOptimisticOracle();
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    // For other use cases, you may need ancillary data or a reward. Here, they are both zero.
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    oracle.requestPrice(priceIdentifier, requestedTime, "", IERC20(collateralCurrency), 0);
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    oracle.setCustomLiveness(priceIdentifier, requestedTime, "", 3600);
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}
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Proposal Bond
A proposal bond is a preloaded reward to incentivize price proposals. Similar to the liveness period, a custom bond amount can be set for a price request. 
Two important aspects to remember when setting a proposal bond is:
When making a price request, the caller must approve the contract to transfer the value of the proposer reward for the collateral being used.
The value corresponds to the collateral decimal value. So setting the reward to 1 for USDC that uses 6 decimals would be 1000000 while an 18 decimal token would be 1. 
The below shows an example of setting the proposer bond to 50 USDC (6 decimals):
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function _requestOraclePrice(uint256 requestedTime) internal {
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    OptimisticOracleInterface oracle = _getOptimisticOracle();
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    // For other use cases, you may need ancillary data. Here, the ancillary data is set to zero.
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    oracle.requestPrice(priceIdentifier, requestedTime, "", IERC20(collateralCurrency), 0);
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    oracle.setBond(priceIdentifier, requestedTime, "", 50000000);
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}
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Settlement
The OptimisticDepositBox contract uses the executeWithdrawal method to settle contracts. It first checks if a price has been resolved by calling the hasPrice method on the Optimistic Oracle which reverts if the request is not settled or settleable. If a price has been resolved, settleAndGetPrice is called.
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function _getOraclePrice(uint256 withdrawalRequestTimestamp) internal returns (uint256) {
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    OptimisticOracleInterface oracle = _getOptimisticOracle();
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    require(
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        oracle.hasPrice(address(this), priceIdentifier, withdrawalRequestTimestamp, ""),
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        "Unresolved oracle price"
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    );
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    int256 oraclePrice = oracle.settleAndGetPrice(priceIdentifier, withdrawalRequestTimestamp, "");
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​
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    // For simplicity we don't want to deal with negative prices.
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    if (oraclePrice < 0) {
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        oraclePrice = 0;
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    }
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    return uint256(oraclePrice);
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}
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