45. Building a simple staking contract

A staking contract tracks each user's deposited amount and the time (or block) since their last update, then computes rewards proportionally when they claim or withdraw.

staked: public(HashMap[address, uint256])
reward_rate: public(uint256)          # reward units per second per token
last_update: public(HashMap[address, uint256])

@external
def stake(amount: uint256):
    self._update_rewards(msg.sender)
    self.staked[msg.sender] += amount
    ERC20(self.token).transferFrom(msg.sender, self, amount)

@internal
def _update_rewards(user: address):
    elapsed: uint256 = block.timestamp - self.last_update[user]
    self.rewards[user] += self.staked[user] * self.reward_rate * elapsed
    self.last_update[user] = block.timestamp

Always update rewards *before* changing a user's staked balance, otherwise you'll compute rewards using the wrong principal amount for part of the elapsed period.

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