Swift重构自定义空等运算符“？？=”实例

var a1: UIView?;
let b1 = a1 ??= UITableView();
print("a1: \(a1)");//a1: Optional(<UITableView: 0x7feda0830600;
print("b1: \(b1)");//b1: <UITableView: 0x7feda0830600

var a1: UIView?;
if(a1 == nil) {
    a1 = UITableView();
}
let b1 = a1;

precedencegroup NilEqualPrecedence {
  /// 优先从左向右， left, right or none
  associativity: left
  higherThan: AssignmentPrecedence//优先级，比等号运算符高
  // lowerThan: AdditionPrecedence // 优先级, 比...低
  assignment: true // 是否是赋值运算
}
/// 空等运算符声明
infix operator ??=: NilEqualPrecedence
/// 空等运算符实现
func ??= <T>(lhs: inout T?, rhs: T) -> T {
    if lhs == nil {
        lhs = rhs;
    }
    return lhs!;
 }