how to write test code for delegator contract

[ghost]

Questions
when I want to test delegator contract function, I got error, when I use mockall, also got compile error

test code is here

#![cfg_attr(not(feature = "std"), no_std)]

use ink_lang as ink;

#[ink::contract]
mod delegator {
    use accumulator::Accumulator;
    use adder::Adder;
    use ink_storage::{
        traits::{
            PackedLayout,
            SpreadLayout,
        },
        Lazy,
    };
    use subber::Subber;

    /// Specifies the state of the delegator.
    ///
    /// In `Adder` state the delegator will delegate to the `Adder` contract
    /// and in `Subber` state will delegate to the `Subber` contract.
    ///
    /// The initial state is `Adder`.
    #[derive(
        Debug,
        Copy,
        Clone,
        PartialEq,
        Eq,
        scale::Encode,
        scale::Decode,
        SpreadLayout,
        PackedLayout,
    )]
    #[cfg_attr(
        feature = "std",
        derive(::scale_info::TypeInfo, ::ink_storage::traits::StorageLayout)
    )]
    pub enum Which {
        Adder,
        Subber,
    }

    /// Delegates calls to an adder or subber contract to mutate
    /// a value in an accumulator contract.
    ///
    /// In order to deploy the delegator smart contract we first
    /// have to manually put the code of the accumulator, adder
    /// and subber smart contracts, receive their code hashes from
    /// the signalled events and put their code hash into our
    /// delegator smart contract.
    #[ink(storage)]
    pub struct Delegator {
        /// Says which of adder or subber is currently in use.
        which: Which,
        /// The accumulator smart contract.
        accumulator: Lazy<Accumulator>,
        /// The adder smart contract.
        adder: Lazy<Adder>,
        /// The subber smart contract.
        subber: Lazy<Subber>,
    }

    impl Delegator {
        /// Instantiate a delegator with the given sub-contract codes.
        #[ink(constructor)]
        pub fn new(
            init_value: i32,
            accumulator_code_hash: Hash,
            adder_code_hash: Hash,
            subber_code_hash: Hash,
        ) -> Self {
            let total_balance = Self::env().balance();
            let accumulator = Accumulator::new(init_value)
                .endowment(total_balance / 4)
                .code_hash(accumulator_code_hash)
                .instantiate()
                .expect("failed at instantiating the `Accumulator` contract");
            let adder = Adder::new(accumulator.clone())
                .endowment(total_balance / 4)
                .code_hash(adder_code_hash)
                .instantiate()
                .expect("failed at instantiating the `Adder` contract");
            let subber = Subber::new(accumulator.clone())
                .endowment(total_balance / 4)
                .code_hash(subber_code_hash)
                .instantiate()
                .expect("failed at instantiating the `Subber` contract");
            Self {
                which: Which::Adder,
                accumulator: Lazy::new(accumulator),
                adder: Lazy::new(adder),
                subber: Lazy::new(subber),
            }
        }

        /// Returns the accumulator's value.
        #[ink(message)]
        pub fn get(&self) -> i32 {
            self.accumulator.get()
        }

        /// Delegates the call to either `Adder` or `Subber`.
        #[ink(message)]
        pub fn change(&mut self, by: i32) {
            match self.which {
                Which::Adder => self.adder.inc(by),
                Which::Subber => self.subber.dec(by),
            }
        }

        /// Switches the delegator.
        #[ink(message)]
        pub fn switch(&mut self) {
            match self.which {
                Which::Adder => {
                    self.which = Which::Subber;
                }
                Which::Subber => {
                    self.which = Which::Adder;
                }
            }
        }

    }


    #[cfg(test)]
    mod tests {
        use super::*;
        use ink_lang as ink;

        #[ink::test]
        pub fn default_works() {
            let mut delegator = Delegator::new(1, Hash::from([0x99; 32]),
                                           Hash::from([0x99; 32]), Hash::from([0x99; 32]));
            delegator.change(2);
        }

    }
}

then run cargo test, got error

running 1 test
test delegator::tests::default_works ... FAILED

failures:

---- delegator::tests::default_works stdout ----
thread 'delegator::tests::default_works' panicked at 'not implemented: off-chain environment does not support contract instantiation', /Users/jacksenguo/RustProjects/apron-contracts/ink/crates/env/src/engine/off_chain/impls.rs:373:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    delegator::tests::default_works

[HCastano]

@sharkygg This is a limitation of the current testing environment. However, there is a new off-chain environment in the works which will allow for this. See #788.

I was wrong about the new off-chain environement, see the comment below.

[Robbepop]

For off-chain testing purposes it is better to have a constructor like the following:

pub fn new(
            init_value: i32,
            accumulator: Accumulator,
            adder: Adder,
            subber: Subber,
        ) -> Self { ... }

So basically replacing hashes with their concrete contract instances.
This way you do part of the job of the original constructor but have full control over the process in the off-chain testing.

Note that the new off-chain environment won't really fix the previous problems either.