Question about contract.callStatic.methodName()

[AlexK020908]

I am currently following the video at the 15:40:21 mark ,

It was mentioned that calling
raffle.checkUpkeep() would kick off a transaction, so we need to use callStatic to simulate calling the function without a tx.
Could someone explain why calling checkUpKeep will kick off a tx? and furthermore, why don't we need to use callStatic for other functions in raffle like raffle.EnterRaffle()?

test portion for check up keep function

          describe("check up keep", function () {
              it("returns false if no one is in the game", async function () {
                  //we want everything in check up keep to be true except for has players
                  await network.provider.send("evm_increaseTime", [interval.toNumber() + 1])
                  await network.provider.send("evm_mine", []) //just want to mine one extra block
                  //calling check up keep would send a tx , we do not want that, we can use callstatic
                  const { upkeepNeeded } = await raffle.callStatic.checkUpkeep([])
                  assert(!upkeepNeeded)
              })
          })

code for checkUpKeep

function checkUpkeep(
        bytes memory /* checkData */
    )
        public
        view
        override
        returns (
            bool upkeepNeeded,
            bytes memory /* performData */
        )
    {
        bool isOpen = s_raffleState == RaffleState.OPEN;
        // block.timeStamp gives the current time, in order to get the time passed
        //we could do something like block.timeStamp - prevTimeStamp(we neeod a variable for this )
        bool timepassed = ((block.timestamp - s_lastTimeStamp)) > c_interval;
        bool hasplayers = (s_players.length > 0);
        bool hasBalance = address(this).balance > 0;
        upkeepNeeded = isOpen && timepassed && hasplayers && hasBalance;
    }

[krakxn]

I believe it is so that we do not change the state of our Contract (or, in this case, Raffle), which in this case would be upkeepNeeded. Quoted from docs below which should clarify it further:

contract.callStatic.METHOD_NAME( ...args [ , overrides ] ) ⇒ Promise< any >
Rather than executing the state-change of a transaction, it is possible to ask a node to pretend that a call is not state-changing and return the result.

This does not actually change any state, but is free. This in some cases can be used to determine if a transaction will fail or succeed.

This otherwise functions the same as a Read-Only Method.

Hope this helps!

[ManuWeb3]

Hi @krakxn,
Just went through all the comments in this thread.
I'm wondering, where in checkUpkeep() is the state of Blockchain actually getting chanegd ?
I mean how upkeepNeeded is a state-variable here?

One, all the variables declared are scoped locally and two, it's just the comparison of variables being made in boolean expressions.

[alymurtazamemon]

@AlexK020908 See we want to test the checkUpkeep function but if we do a transaction it will then call the performUpkeep but we do not want that, we only want to test checkUpkeep. Because of that when we add callStatic with it it will execute all of the code but does not call other functions.

[AlexK020908]

To clarify it a bit, this is my understanding
performUpKeep will be triggered once checkUpKeep returns true , since the keepers will call performUpKeep on chain once upkeepNeeded is true , to prevent this, we use callStatic because it will only execute the code in the function we are calling, and it will not trigger any other functions.

And the reason why we do not need callStatic for functions like EnterRaffle() is because calling it does not call other functions, so we are testing that functions only.

The reason why calling checkUpKeep will trigger a transaction is becauseupKeepNeeded(which is a state)will be modified, and whenever we modify state, it is considered a transaction since a new block will be mined

[letallan]

Why will performUpkeep be called automatically in the unit test? It doesn't work that way. We only have an interface implementation for interacting with Chainlink Keeper, but no network itself (or mock)

[silent7-x]

Hello everyone, I got your points but i'm wondering, do we still need to perform the callStatic call as In the recent gh, checkUpkeep is a view function (which isn't at the time of the video). Thanks for your reply.

code for memory:

function checkUpkeep(
        bytes memory /* checkData */
    ) public view override returns (bool upkeepNeeded, bytes memory /* performData */) {
        bool isOpen = RaffleState.OPEN == s_raffleState;
        bool timePassed = ((block.timestamp - s_lastTimeStamp) > i_interval);
        bool hasPlayers = s_players.length > 0;
        bool hasBalance = address(this).balance > 0;
        upkeepNeeded = (timePassed && isOpen && hasBalance && hasPlayers);
        return (upkeepNeeded, "0x0"); 
    }

[letallan]

The reason Patrick uses callStatic is because his checkUpkeep function has no view keyword. As far as I understand, this is done in order to show how callStatic works. We can freely add the view keyword and write the test this way:

it("returns false if people haven't send any ETH", async function () {
                  await network.provider.send("evm_increaseTime", [interval.toNumber() + 1])
                  await network.provider.send("evm_mine", [])
                  const { upkeepNeeded } = await raffle.checkUpkeep([])
                  assert(!upkeepNeeded)
              })

[alymurtazamemon]

@letallan callStatic will execute all of the code and return what you returned but do not create a transaction.

More you can read from solidity docs.

[ManuWeb3]

I have similar concern as @letallan does.
performUpkeep() should Not be called automatically bcz we're just running a Unit test. We haven't yet registered our contract's address (upon depoloyment at testnet / mainnet) with Chainlink Keepers registry.

Secondly, the test still passes even when checkUpkeep() is NOT marked view and neither did I use callStatic in the unit test at the same time.
As far as I understand, this happnes because the state is actually nowhere getting changed in checkUpkeep() even if it's sent as a transaction (no view used). So, there is no point in simulating calling checkUpkeep() (no callStatic used).
Is it that Patrick used callStatic just for demo purpose here ?
Hope I could convey my thoughts.

Please correct me if any of my understanding is wrong here.