Solidity 限制(restricted)访问
对合约进行访问限制，是一种常见做法。默认情况下合约是只读的，除非将合约状态指定为public。
使用限制访问修饰符，我们可以限制谁能修改合约状态，或者调用合约函数等操作。
下面示例中，创建了多个修饰符：
onlyBy 限制可以调用该函数的调用者（根据地址）。
onlyAfter 限制该函数只能在特定的时间段之后调用。
costs 调用方只能在提供特定值的情况下调用此函数。。
示例
pragma solidity ^0.5.0;
contract Test {
address public owner = msg.sender;
uint public creationTime = now;
modifier onlyBy(address _account) {
require(
msg.sender == _account,
"Sender not authorized."
);
_;
}
function changeOwner(address _newOwner) public onlyBy(owner) {
owner = _newOwner;
}
modifier onlyAfter(uint _time) {
require(
now >= _time,
"Function called too early."
);
_;
}
function disown() public onlyBy(owner) onlyAfter(creationTime + 6 weeks) {
delete owner;
}
modifier costs(uint _amount) {
require(
msg.value >= _amount,
"Not enough Ether provided."
);
_;
if (msg.value > _amount)
msg.sender.transfer(msg.value-_amount);
}
function forceOwnerChange(address _newOwner) public payable costs(200 ether) {
owner = _newOwner;
if (uint(owner) & 0 == 1) return;
}
}
