May I ask how the selected string is inserted in an expression corresponding to the current cursor position in UITextFeild?

As the title
Reply:
Their first top down
Reply:
@ Interface NSMutableString (NSMutableStringExtensionMethods)

- (Void) insertString: (NSString *) aString atIndex: (NSUInteger) loc;
Reply:
The expression NSString added to the original text, to get the cursor position, according to the position of the string into your face