dispatch_block_t / closures中的变量范围

Question

我正在尝试运行此代码：

for spawn : NSDictionary! in spawns {
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(waitTime * Double(NSEC_PER_SEC))), dispatch_get_main_queue(), {
        self.spawnSkirmisher(spawn)
    })
}

我收到的编译器错误并未指向任何代码行 - 只是构建失败。我可以通过复制spawn变量来使程序正常工作：

for spawn : NSDictionary! in spawns {
    let localSpawn = spawn.copy() as NSDictionary
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(waitTime * Double(NSEC_PER_SEC))), dispatch_get_main_queue(), {
        self.spawnSkirmisher(localSpawn)
    })
}

似乎spawn变量是循环的局部变量，因此它会使闭包/ dispatch_block内的范围失去。

还有另一种处理这种情况的方法吗？好像我正在接近它。谢谢！