If the array’s contents are all property list objects (NSString,
NSData, NSArray, or NSDictionary objects), the file written by this
method can be used to initialize a new array with the class method
arrayWithContentsOfFile: or the instance method
initWithContentsOfFile:. This method recursively validates that all
the contained objects are property list objects before writing out the
file, and returns NO if all the objects are not property list objects,
since the resultant file would not be a valid property list.

自定有类对象实例item不能用这种方法进行存储, 可以通过NSKeyedArchiver转成NSdata，然后才能使用writeToFile:方法。