Writing Your Own Trait

Tact doesn't support classical class inheritance and instead introduces the concept of traits. Traits are similar to simplified base classes that potentially add state variables, receivers, getters or contract methods.

Contracts can rely on multiple traits. Extract logic into a trait if you have multiple contracts that share this logic.

The Trackable trait

This example shows how to write a new trait that adds simple analytics behavior to any contract.

This trait also makes use of the virtual keyword which lets the contract relying on the trait override some of the trait's behaviors. In the example, the default filter behavior ignores messages from owner in the analytics.

The contract relying on the trait can change this default behavior by specifying the override keyword and providing a new implementation to this method. In our case, the custom filter is to have no filters.

All Examples

import "@stdlib/deploy";

message HiFromParent {
    greeting: String;
}

message HiFromChild {
    fromSeqno: Int as uint64;
    greeting: String;
}

// we have multiple instances of the children
contract TodoChild {

    parent: Address; // we added this variable so a child always knows who the parent is
    seqno: Int as uint64;
 
    // when deploying an instance, we must specify its index (sequence number)
    init(parent: Address, seqno: Int) {
        require(sender() == parent, "not the parent");
        self.parent = parent;
        self.seqno = seqno;
    }

    receive(msg: HiFromParent) {
        require(sender() == self.parent, "Access denied");  // only the real parent can get here
       
        dump(self.seqno);
        dump("😃 handling hi from parent");
        self.reply(HiFromChild{fromSeqno: self.seqno, greeting: "sup"}.toCell());
    }
}

// we have one instance of the parent
contract TodoParent with Deployable {
 
    init() {}

    receive("greet 3") {
        let i: Int = 0;
        repeat (3) {
            i = i + 1;
            let init: StateInit = initOf TodoChild(myAddress(), i);
            send(SendParameters{
                to: contractAddress(init),
                body: HiFromParent{ greeting: "darling" }.toCell(),
                value: ton("0.1"),              // pay for message and potential deployment
                mode: SendIgnoreErrors,
                code: init.code,                // if child is not deployed, also deploy it
                data: init.data
            });
        }
    }

    receive(msg: HiFromChild) {
        let expectedAddress: Address = contractAddress(initOf TodoChild(myAddress(), msg.fromSeqno));
        
        require(sender() == expectedAddress, "Access denied");
        // only the real children can get here
        
        dump(msg.fromSeqno);
        dump("😑 handling hi from child");
    }
}