1. Describe what an event is, and why it might be useful to a client.
An event is a smart contract's way of communicating to the outside world that something happened, like if an NFT was minted. Events could be useful to clients for two reasons:
- So clients can update thier code accordingly, like the live claim feed on floats.city that shows every time a FLOAT is minted and claimed by an address
- So clients can avoid constantly checking the smart contract to check if an event occurred, which is inefficient and annoying
2. Deploy a contract with an event in it, and emit the event somewhere else in the contract indicating that it happened.
pub contract YouMintedAnNFT {
pub event MintedNFT(id: UInt64)
pub resource NFT {
pub let id: UInt64
init() {
self.id = self.uuid
emit MintedNFT(id: self.id)
}
}
}3. Using the contract in step 2), add some pre conditions and post conditions to your contract to get used to writing them out.
pub contract YouMintedAnNFT {
pub fun logMoney(money: String) {
pre {
money.length > 5: "You don't have enough FLOW to mint this NFT."
}
log(money)
}
pub fun updateMoney() {
post {
before(self.number) ==self.number - 1
}
self.number = self.number + 1
pub event MintedNFT(id: UInt64)
pub resource NFT {
pub let id: UInt64
init() {
self.id = self.uuid
emit MintedNFT(id: self.id)
}
}
}4. For each of the functions below (numberOne, numberTwo, numberThree), follow the instructions.
pub contract Test {
// TODO
// Tell me whether or not this function will log the name.
// name: 'Jacob'
pub fun numberOne(name: String) {
pre {
name.length == 5: "This name is not cool enough."
}
log(name)
}
// TODO
// Tell me whether or not this function will return a value.
// name: 'Jacob'
pub fun numberTwo(name: String): String {
pre {
name.length >= 0: "You must input a valid name."
}
post {
result == "Jacob Tucker"
}
return name.concat("Tucker")
}
pub resource TestResource {
pub var number: Int
// TODO
// Tell me whether or not this function will log the updated number.
// Also, tell me the value of `self.number` after it's run.
pub fun numberThree(): Int {
post {
before(self.number) == result + 1
}
self.number = self.number + 1
return self.number
}
init() {
self.number = 0
}
}
}THE NEW ANSWER: numberOne: Yes, the function will log the name "Jacob" (I didn't know what == meant before, so that's why I got confused.)
numberTwo: Yes, the function will return a value.
numberThree: No, the function won't run because the pre-condition of 5 isn't met.
THE OLD ANSWER:
numberOne: No, the function will not log the name "Jacob"
numberTwo: Yes, the function will return a value.
numberThree: Yes, the function will log the updated number. After it's run, the value of self.number will be 2.