先来一个分类

#import <UIKit/UIKit.h>

NS_ASSUME_NONNULL_BEGIN

@interface UITextField (ExtentRange)

- (NSRange)selectedRange;
 
- (void)setSelectedRange:(NSRange)range;


@end

NS_ASSUME_NONNULL_END

#import "UITextField+ExtentRange.h"


@implementation UITextField (ExtentRange)

- (NSRange) selectedRange
{
    UITextPosition* beginning = self.beginningOfDocument;
    
    UITextRange* selectedRange = self.selectedTextRange;
    UITextPosition* selectionStart = selectedRange.start;
    UITextPosition* selectionEnd = selectedRange.end;
    
    const NSInteger location = [self offsetFromPosition:beginning toPosition:selectionStart];
    const NSInteger length = [self offsetFromPosition:selectionStart toPosition:selectionEnd];
    
    return NSMakeRange(location, length);
}
 
- (void) setSelectedRange:(NSRange) range
{
    UITextPosition* beginning = self.beginningOfDocument;
    
    UITextPosition* startPosition = [self positionFromPosition:beginning offset:range.location];
    UITextPosition* endPosition = [self positionFromPosition:beginning offset:range.location + range.length];
    UITextRange* selectionRange = [self textRangeFromPosition:startPosition toPosition:endPosition];
    
    [self setSelectedTextRange:selectionRange];
}

@end

使用和监听

#import <UIKit/UIKit.h>
@interface ViewController : UIViewController

@end

#import "ViewController.h"
#import "UITextField+ExtentRange.h"

@interface ViewController ()<UITextFieldDelegate>

@property (nonatomic,strong) UITextField *textFiled;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.textFiled = [[UITextField alloc] initWithFrame:CGRectMake(10, 100, 200, 100)];
    self.textFiled.backgroundColor = [UIColor redColor];
    self.textFiled.delegate = self;
    [self.view addSubview:self.textFiled];
}

- (void)textFieldDidChangeSelection:(UITextField *)textField{
    NSLog(@"位置%lu长度%lu",(unsigned long)textField.selectedRange.location,(unsigned long)textField.selectedRange.length);
}

@end