是否可以获取从另一个合约外部函数获取数据的外部函数的返回值?

Question

注意：这只是从web3js调用外部视图函数时的问题，因为它不会创建事务

---

我有 2 个合约，A和B
(一个是存储合约，另一个更像是逻辑合约(

如果A具有如下函数：

function getValueFor(uint8 index) external view returns(uint8) {
return someMapping[index];
}

B有一个这样的功能：

function isValid() external view returns(bool) {
return instanceOfA.getValueFor(1) == 1;
}

从 Mist 或 web3js 中，来自A的返回值按预期工作，并且在调用B时始终返回默认值.
(在本例中为0(

您不能从 web3js 的合约>外部/常量函数调用中获取返回值吗？这需要交易吗？

更新

请参阅ethereum.stackexchange上的答案。

在交易过程中，我的工作大多不正确。它似乎与我的特定测试功能一起工作，尽管大多数其他功能都失败了(我只是对我使用的一个测试功能感到幸运/不走运(。

问题的根源只是没有在我的 Geth 节点上启用新的拜占庭(启用新的 EVM 功能(。

Answer 1

你肯定可以。在 Truffle 控制台中快速运行将显示它：

pragma solidity >0.4.23 <0.5.0;
contract A {
uint256 value;
constructor(uint256 v) public {
value = v;
}
function getValue() external view returns (uint256) {
return value;
}
}
contract B {
A other;
constructor(address a) public {
other = A(a);
}
function getValueFromA() external view returns (uint256) {
return other.getValue();
}
}

执行：

$ truffle console
truffle(development)> var aContract = web3.eth.contract([{"constant":true,"inputs":[],"name":"getValue","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"inputs":[{"name":"v","type":"uint256"}],"payable":false,"stateMutability":"nonpayable","type":"constructor"}]);
truffle(development)> var transObj = { from: web3.eth.accounts[0], data: '0x608060405234801561001057600080fd5b506040516020806100ea83398101806040528101908080519060200190929190505050806000819055505060a1806100496000396000f300608060405260043610603f576000357c0100000000000000000000000000000000000000000000000000000000900463ffffffff16806320965255146044575b600080fd5b348015604f57600080fd5b506056606c565b6040518082815260200191505060405180910390f35b600080549050905600a165627a7a72305820dd332a08d090a6766cf7c2e86f41abc58a31e4d1afe7cb635a0a1b755a9b81cd0029', gas: '4700000'};
truffle(development)> var a = aContract.new(35, transObj);
truffle(development)> a.getValue();
BigNumber { s: 1, e: 1, c: [ 35 ] }
truffle(development)> var bContract = web3.eth.contract([{"constant":true,"inputs":[],"name":"getValueFromA","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"inputs":[{"name":"a","type":"address"}],"payable":false,"stateMutability":"nonpayable","type":"constructor"}]);
truffle(development)> transObj.data = '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';
truffle(development)> b = bContract.new(a.address, transObj);
truffle(development)> b.getValueFromA();
BigNumber { s: 1, e: 1, c: [ 35 ] }

Answer 2

这是一个变通方法/黑客，可以让它在事务之外工作(使用web3js调用外部视图函数(

pragma solidity >0.4.23 <0.5.0;
contract Parent {
int value;
function setValue(int v) public {
value = v;
}
function getValue() external view returns (int) {
return value;
}
}
contract Child {
Parent parent;
function setParent(address a) public {
parent = Parent(a);
}
function getValue() external view returns (int value) {
// -- Should work
//return parent.getValue();
// -- Make it work
address addr = address(parent);
bytes4 sig = bytes4(keccak256("getValue()"));
assembly {
let o := mload(0x40) // Empty storage pointer
mstore(o,sig)        // Push function signature to memory (function signature is 4 bytes/0x04)
let success := call(
15000,           // Gas limit
addr,            // To address
0,               // No ether to transfer
o,               // Input location ptr
0x04,            // Input size (0)
o,               // Store oputput over input
0x20)            // Output size (32 bytes)
value := mload(o)
mstore(0x40,add(o,0x04))
}
}
}

这似乎仍然很愚蠢，这不仅仅适用于常规函数调用。
(也许这是一个错误？