CrowdFunding (solidity)

/ SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Crowdfunding {
struct Campaign {
address payable creator;
string title;
string description;
string evidence;
uint fundingGoal;
uint deadline;
uint amountRaised;
bool isCompleted;
mapping(address => uint) contributions;
}

address public owner;
uint public campaignCount;
uint public totalFeesCollected;
mapping(uint => Campaign) public campaigns;

event CampaignCreated(
    uint campaignId,
    address creator,
    string title,
    string evidence,
    uint fundingGoal,
    uint deadline
);
event ContributionMade(uint campaignId, address contributor, uint amount);
event FundsWithdrawn(uint campaignId, uint amountToCreator, uint feeToContract);
event ContractFeeWithdrawn(uint amount);

modifier onlyOwner() {
    require(msg.sender == owner, "Only the owner can perform this action");
    _;
}

modifier isCampaignActive(uint _campaignId) {
    Campaign storage campaign = campaigns[_campaignId];
    require(block.timestamp < campaign.deadline, "Campaign deadline passed");
    require(!campaign.isCompleted, "Campaign is completed");
    _;
}

constructor() {
    owner = msg.sender;
}

function createCampaign(
    string memory _title,
    string memory _description,
    string memory _evidence,
    uint _fundingGoal,
    uint _duration
) external {
    require(_fundingGoal > 0, "Funding goal must be greater than 0");
    require(_duration > 0, "Duration must be greater than 0");

    Campaign storage newCampaign = campaigns[campaignCount];
    newCampaign.creator = payable(msg.sender);
    newCampaign.title = _title;
    newCampaign.description = _description;
    newCampaign.evidence = _evidence;
    newCampaign.fundingGoal = _fundingGoal;
    newCampaign.deadline = block.timestamp + _duration;

    emit CampaignCreated(campaignCount, msg.sender, _title, _evidence, _fundingGoal, newCampaign.deadline);

    campaignCount++;
}

function contribute(uint _campaignId) external payable isCampaignActive(_campaignId) {
    Campaign storage campaign = campaigns[_campaignId];
    require(msg.value > 0, "Contribution must be greater than 0");
    require(campaign.amountRaised < campaign.fundingGoal, "Funding goal already reached");

    campaign.contributions[msg.sender] += msg.value;
    campaign.amountRaised += msg.value;

    emit ContributionMade(_campaignId, msg.sender, msg.value);
}

function withdrawFunds(uint _campaignId) external {
    Campaign storage campaign = campaigns[_campaignId];
    require(msg.sender == campaign.creator, "Only the creator can withdraw funds");
    require(!campaign.isCompleted, "Funds already withdrawn");
    require(
        block.timestamp >= campaign.deadline || campaign.amountRaised >= campaign.fundingGoal,
        "Campaign is still active"
    );

    uint fee = (campaign.amountRaised * 2) / 100; // 2% fee
    uint amountToCreator = campaign.amountRaised - fee;

    campaign.isCompleted = true;
    totalFeesCollected += fee;

    (bool success, ) = campaign.creator.call{value: amountToCreator}("");
    require(success, "Transfer to creator failed");

    emit FundsWithdrawn(_campaignId, amountToCreator, fee);
}

function withdrawContractFees() external onlyOwner {
    require(totalFeesCollected > 0, "No fees available to withdraw");
    uint amount = totalFeesCollected;
    totalFeesCollected = 0;

    (bool success, ) = owner.call{value: amount}("");
    require(success, "Transfer to owner failed");

    emit ContractFeeWithdrawn(amount);
}

function getCampaignDetails(uint _campaignId) external view returns (
    address creator,
    string memory title,
    string memory description,
    string memory evidence,
    uint fundingGoal,
    uint deadline,
    uint amountRaised,
    bool isCompleted
) {
    Campaign storage campaign = campaigns[_campaignId];
    return (
        campaign.creator,
        campaign.title,
        campaign.description,
        campaign.evidence,
        campaign.fundingGoal,
        campaign.deadline,
        campaign.amountRaised,
        campaign.isCompleted
    );
}}

💰 Solidity Smart Contract: Crowdfunding

📍Deployed on Polygon Testnet

This contract enables decentralized crowdfunding with campaign creation, contribution handling, and fee-based fund withdrawals.

---

✅ License & Version

// SPDX-License-Identifier: MIT

• Declares the open-source license (MIT).

pragma solidity ^0.8.0;

• Requires Solidity version 0.8.0 or higher.
• Ensures compatibility and access to the latest features and safety improvements.

---

🏗️ Contract Declaration

contract Crowdfunding {

• Defines a contract named Crowdfunding.

---

🧱 Struct: Campaign

struct Campaign {
    address payable creator;
    string title;
    string description;
    string evidence;
    uint fundingGoal;
    uint deadline;
    uint amountRaised;
    bool isCompleted;
    mapping(address => uint) contributions;
}

Defines the structure for a single campaign:

Field	Type	Description
creator	address payable	Campaign owner, allowed to receive funds.
title	string	Campaign title.
description	string	Description of the campaign.
evidence	string	Proof or evidence (e.g., IPFS link, image URL).
fundingGoal	uint	Funding target in wei.
deadline	uint	Campaign end time (timestamp).
amountRaised	uint	Amount contributed so far.
isCompleted	bool	True once funds have been withdrawn.
contributions	mapping	Tracks how much each address has contributed.

🧠 Note: mapping inside a struct cannot be returned directly — it’s internal storage only.

---

📦 Global Variables

address public owner;
uint public campaignCount;
uint public totalFeesCollected;
mapping(uint => Campaign) public campaigns;

• owner: Contract deployer; can withdraw platform fees.
• campaignCount: Tracks number of campaigns.
• totalFeesCollected: 2% fee from campaigns.
• campaigns: Maps a campaign ID (uint) to its data (Campaign struct).

---

📣 Events

event CampaignCreated(...);
event ContributionMade(...);
event FundsWithdrawn(...);
event ContractFeeWithdrawn(...);

These allow external interfaces (like dApps) to listen for changes:

Event	Description
CampaignCreated	Triggered when a new campaign is made.
ContributionMade	Logs a new contribution.
FundsWithdrawn	Logs fund withdrawal by a creator.
ContractFeeWithdrawn	Logs withdrawal of contract fees by the owner.

---

🔐 Access Control Modifiers

modifier onlyOwner() {
    require(msg.sender == owner, "Only the owner can perform this action");
    _;
}

• Restricts a function to the contract owner.

modifier isCampaignActive(uint _campaignId) {
    Campaign storage campaign = campaigns[_campaignId];
    require(block.timestamp < campaign.deadline, "Campaign deadline passed");
    require(!campaign.isCompleted, "Campaign is completed");
    _;
}

• Ensures a campaign is still running before allowing contributions.

---

🏁 Constructor

constructor() {
    owner = msg.sender;
}

• Sets the contract deployer as owner.

---

🆕 Function: createCampaign(...)

function createCampaign(...) external { ... }

Parameter	Type	Description
_title	string	Campaign name.
_description	string	Campaign details.
_evidence	string	Link or hash to proof (images, documents).
_fundingGoal	uint	Minimum funding target in wei.
_duration	uint	Campaign length in seconds.

Steps:

• Validates goal and duration.
• Initializes a new campaign in the campaigns mapping.
• Emits CampaignCreated.

---

💸 Function: contribute(...)

function contribute(uint _campaignId) external payable isCampaignActive(_campaignId)

• Requires payment (payable) and checks if the campaign is active.
• Verifies non-zero contribution and funding goal isn’t already reached.
• Adds contribution to campaign’s amountRaised.
• Tracks how much each sender contributed.
• Emits ContributionMade.

---

🏦 Function: withdrawFunds(...)

function withdrawFunds(uint _campaignId) external

Only callable by campaign creator if:

• Campaign is complete (deadline passed or goal reached).
• Campaign not already withdrawn.

Fee Structure:

• 2% platform fee taken from total raised.
• Remaining 98% sent to the creator using call{value: ...} (modern recommended pattern).

Updates:

• Marks campaign as isCompleted = true.
• Adds fee to totalFeesCollected.
• Emits FundsWithdrawn.

---

🧾 Function: withdrawContractFees()

function withdrawContractFees() external onlyOwner

• Lets contract owner collect accumulated 2% fees.
• Uses call to transfer fees.
• Emits ContractFeeWithdrawn.

---

🔍 Function: getCampaignDetails(...)

function getCampaignDetails(...) external view returns (...)

Returns read-only information about a campaign, excluding internal mappings:

• creator
• title
• description
• evidence
• fundingGoal
• deadline
• amountRaised
• isCompleted

Useful for front-end UIs.

---

📚 Summary Table

Concept	Description
struct	Used to model a campaign’s data.
mapping	Associates campaign IDs with campaigns, and contributors with amounts.
modifier	Reusable conditions (like access control).
event	Emits logs for external applications to track changes.
call{value: ...}	Secure and flexible Ether transfer method.
block.timestamp	Current time used to compare against deadlines.
payable	Marks functions or addresses that can send/receive Ether.