不知这个句子的出处为何？是native说的句子吗？有点语法游戏的感觉。假设这是一个native写的句子，我不妨作如下分析：我们知道 be going to可以表示现在打算在将来做某事。例如：I‘m going to finish the job next week. 我打算下周完成工作。不定式改为完成体：I’m going to have finished the job by the time they arrive. 我打算在他们到达以前完成工作。再把be going改为现在完成进行时：I‘ve been going to have finished the job by the time they arrived. 我一直打算在他们到达前完成工作。