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design_pattern_decisions.md

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Design Pattern Decisions

Several contract design patterns are applied.

Using IPFS

Images are stored on IPFS for better decentralization. Read file from input file, and upload it to IPFS.

    captureFile(event) {
      event.preventDefault()
      const file = event.target.files[0]
      const reader = new window.FileReader()
      reader.readAsArrayBuffer(file)
      reader.onloadend = () => {
        this.setState({ isLoading: true })
        this.setState({ buffer: Buffer(reader.result) }, () => {
  
          console.log('buffer', this.state.buffer)
          ipfs.files.add(this.state.buffer, (error, result) => {
            if(error) {
              console.error(error)
              return
            }
            this.setState({ ipfsHash: result[0].hash }, () => {
              this.setState({ imageUrl: `https://ipfs.io/ipfs/${this.state.ipfsHash}`})
              this.setState({ isLoading: false })
              console.log('ipfsHash', this.state.ipfsHash)
            })
            })
        })
      }
    }

Ownership

The Ownable contract from zeppelin (installed with EthPM) restricts access to only the contract owner.

Importing from Zeppelin (from installed_contracts folder generated by EthPM):

import "zeppelin/contracts/ownership/Ownable.sol";

Usage:

function kill() public onlyOwner {

Mortal

The kill() function allows the owner to destroy the contract and send all the Ether to the owner's address.

function kill() public onlyOwner {
        selfdestruct(owner);
    }

Circuit Breaker

This allows the owner to disable the contract.

    /** @dev Initialize CIRCUIT BREAKER */
    constructor() public {
        stopped = false;
    }

    /** @dev CIRCUIT BREAKER: Allows owner to deactivate the contract */
    function toggleContractActive() public onlyOwner {
        stopped = !stopped;
    }

    modifier stopInEmergency() {
        if(!stopped) _;
    }

    modifier onlyInEmergency() {
        if(stopped) _;
    }

Restricting Access

Function modifiers are used to restrict access to some of the contract's functions.

Example:

With the modifier onlyOwner, only the contract owner is allowed to add an address as a store owner.

function addStoreOwner(address _recipient) public onlyOwner stopInEmergency{

...

With the modifier onlyStoreOwner, only store owners are allowed to add new stores or add new items.

function addStore(string _imageUrl, string _title, string _description) public onlyStoreOwner stopInEmergency {

...

function addItem(uint _storeId, string _imageUrl, string _title, string _description, uint _price, uint _quantity) public onlyStoreOwner {

...

Withdrawal Pattern

For buying an item from a store, the withdrawal pattern is used instead of sending Ether directly using send call. That way, if the buy function fails, it will only affect that specific address and will not affect the rest of the contract.

function buy () public {

...

        pendingWithdrawals[storeToStoreOwner[items[_itemId].storeId]] = pendingWithdrawals[storeToStoreOwner[items[_itemId].storeId]]
        .add(items[_itemId].price);

...

    function withdraw() public {
        uint amount = pendingWithdrawals[msg.sender];
        // Prevent re-entrancy attacks
        pendingWithdrawals[msg.sender] = 0;
        msg.sender.transfer(amount);
    }

Fallback Function

The fallback function is executed if the contract receives plain Ether without any data.

function() external payable {

    }