UITextSelectionDidZoom.m

#import "ViewController.h"
#include <objc/message.h>
#include <objc/runtime.h>

@interface MyLabel : UILabel
@end
@implementation MyLabel
- (void)willMoveToWindow:(UIWindow *)newWindow {
    [super willMoveToWindow:newWindow];
    if (newWindow == nil) {
        if (self.window != nil) {
            [NSNotificationCenter.defaultCenter removeObserver:self];
        }
    } else {
        if (self.window == nil) {
            [NSNotificationCenter.defaultCenter addObserver:self selector:@selector(didZoom:) name:@"UITextSelectionDidZoom" object:nil];
        }
    }
}
- (void)didZoom:(NSNotification *)notification {
    UIScrollView * _Nullable scollView = ((id (*)(id, SEL))objc_msgSend)(self, sel_registerName("_scroller"));
    if (scollView == nil) return;
    
    if (![scollView isEqual:notification.object]) return;
    
    [CATransaction setDisableActions:YES];
    self.contentScaleFactor = scollView.zoomScale;
}
@end


@interface ViewController () <UIScrollViewDelegate> {
    MyLabel *_label;
}
@end

@implementation ViewController

- (void)dealloc {
    [_label release];
    [super dealloc];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    MyLabel *label = [[MyLabel alloc] initWithFrame:self.view.bounds];
    label.text = @"A";
    label.font = [UIFont systemFontOfSize:100.];
    [label sizeToFit];
    [_label release];
    _label = label;
    
    UIScrollView *scrollView = [[UIScrollView alloc] initWithFrame:self.view.bounds];
    scrollView.delegate = self;
    scrollView.maximumZoomScale = 100.;
    scrollView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
    scrollView.contentSize = label.bounds.size;
    [self.view addSubview:scrollView];
    [scrollView addSubview:label];
    [scrollView release];
}

- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView {
    return _label;
}

@end