如果餘額發生變化，我正在嘗試編寫一個 if 條件以給出響應

pragma solidity >0.7.0 <=0.9.0;

contract demoCoin {
   address public Owner;
   mapping(address => uint256) myCoin;

   constructor() {
       Owner = msg.sender;
   }

   function loadCoin(uint256 amount) public {
       require(Owner == msg.sender);
       myCoin[Owner] += amount;
   }

   function sendCoin(address receiver, uint256 amount)
       public
       returns (string memory, uint256)
   {
       require(Owner == msg.sender);
       if (amount > myCoin[msg.sender]) revert("Insufficient balance");
       myCoin[msg.sender] -= amount;
       myCoin[receiver] += amount;

       // I want to return the alert for a successful transaction
       if (myCoin[receiver] += amount) {
           return string = "You have been credited with" + amount + "demoCoin";
       }
   }

   function showBal() public view returns (uint256) {
       return myCoin[msg.sender];
   }
}

您可以使用event註冊智能合約中的特定操作。您必須使用以下語句聲明事件：

event [NameEvent] ([typeof] [nameVariable], [typeof] [nameVariable2]);

並註冊一個事件，你可以使用這個語句：

emit [NameEvent] ([valueVariable], [valueVariable2]);

在您的情況下，您以這種方式聲明一個事件：

...
constructor (){
   Owner = msg.sender;
}

// Declare event
event SuccessfullTransfer(address receiver, uint amount);

function loadCoin(uint amount) public {
   require(Owner == msg.sender);
   myCoin[Owner] += amount;
}
....

並且您必須以這種方式在sendCoin()函式中呼叫它（在這種情況下）：

function sendCoin(address receiver, uint amount) public {
    require(Owner == msg.sender);
    if (amount > myCoin[msg.sender])
    revert ('Insufficient balance');
    myCoin[msg.sender] -= amount;
    myCoin[receiver] += amount;


    // I want to return the alert for a successful transaction 

   if (myCoin[receiver] += amount)
   { 
      emit SuccessfullTransfer(receiver, amount);
   }

當單個使用者與sendCoin()函式互動並將其代幣轉移到智能合約時，您可以在交易中看到該事件。

注意：如果你要使用這種方式，你必須改變sendCoin()函式的返回類型。

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引用自：https://ethereum.stackexchange.com/questions/125212