将ReactiveCocoa中的信号组合到一个新的信号中，当所有信号都发生变化时触发

I'm trying to learn ReactiveCocoa and I'm writing a simple Space Invaders clone, based on a Ray Wenderlich tutorial. Lately during the development, I faced an issue I can't resolve. Basically I've two signals:I'm trying to learn ReactiveCocoa and I'm writi