why schedule() can't be used within AppDelegate.cpp?

i am new to cocos2d-x,when i was compiling the tutorial “how to move a sprite”,i found there wasn’t any reaction when the function schedule() be excuted in AppDelegate.cpp.Only in HelloWorldScene.cpp it was correctly excuted and show me a instant animation.
who knows why,thanks for reminding me!

the difference is i used a new layer to call schedule() in AppDelegate.cpp;