See NIB identity

1.
When managing nib (.xib) file, we need to enable
View -> Utilities -> identity inspector
2.
Implement button event, we can
1. Layout a button on the NIB file with "Utilities -> attribute inspector"
2. On the button, hold "control" key on the keyboard and use the mouse to drag a "line" into assistant
3. It will appear a small window that helps adding an action