Precautions for using nsuserdefaults standarduserdefaults

Nsuserdefaults can be used to store nsstring, nsnumber, nsdate, nsarray, and nsdictionary. Custom classes can be stored using nsdata. Of course nscoding Protocol is required. nsobject <nscoding>

1. The system will save it to the/library/preferences/Gongcheng. plist file under the application. Note that if the program unexpectedly exits, nsuserdefaultsstandarduserdefaults data will not be written to this file by the system. Therefore, you must use the [[nsuserdefaultsstandarduserdefaults] synchronize] command to directly synchronize the data to the file, to avoid data loss.

2. All data read from nsdata is unmutable. Therefore, duplcated.