A demonstration of how UITableView caches the results of `[self.dataSource respondsToSelector:]` and `self.delegate respondsToSelector:]`

uitableview_responds_to_selector_cache.m

@import UIKit;
@import ObjectiveC.runtime;

@interface MyDataSource: NSObject <UITableViewDataSource>
@end

@implementation MyDataSource

 /**
 Do not implement `numberOfSectionsInTableView:` to add it dynamically in runtime
 */

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    return 1;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    return nil;
}

@end

// IMP to be added in runtime as `-[UITableViewDataSource numberOfSectionsInTableView:]`
static NSInteger NumberOfSectionsInTableView(MyDataSource *self, SEL sel, UITableView *tableView) {
    return 10;
}

static void Test() {
    UITableView *tableView = [[UITableView alloc] initWithFrame:CGRectZero style:UITableViewStylePlain];
    MyDataSource *dataSource = [[MyDataSource alloc] init];
    tableView.dataSource = dataSource;

    [tableView reloadData];
    NSLog(@"%d", (int)[tableView numberOfSections]);
    // Output: 1

    // Dynamically add an implementation for `-[MyDataSource numberOfSectionsInTableView:]`
    char sig[16];
    snprintf(sig, 16, "%s%s%s%s", @encode(NSInteger), @encode(id), @encode(SEL), @encode(id));
    class_addMethod([MyDataSource class],
                    @selector(numberOfSectionsInTableView:),
                    (IMP)NumberOfSectionsInTableView,
                    sig);

    [tableView reloadData];
    NSLog(@"%d", (int)[tableView numberOfSections]);
    // Output: 1, because UITableView cached the result of
    // [tableView.dataSource respondsToSelector:@selector(numberOfSectionsInTableView:)]

    /**
     See https://github.com/nst/iOS-Runtime-Headers/blob/fbb634c78269b0169efdead80955ba64eaaa2f21/PrivateFrameworks/UIKitCore.framework/UITableView.h
     struct {
         unsigned int dataSourceNumberOfRowsInSection : 1;
         unsigned int dataSourceCellForRow : 1;
         unsigned int dataSourceNumberOfSectionsInTableView : 1;
         unsigned int dataSourceTitleForHeaderInSection : 1;
         unsigned int dataSourceTitleForFooterInSection : 1;
         ....
     } _tableFlags;
     */
    Ivar ivar = class_getInstanceVariable([UITableView class], "_tableFlags");
    char* _tableFlags = (char*)(__bridge void*)tableView + ivar_getOffset(ivar);
    char firstFlags = *_tableFlags;
    firstFlags = firstFlags | 0b100; // set dataSourceNumberOfSectionsInTableView bit
    *_tableFlags = firstFlags;

    [tableView reloadData];
    NSLog(@"%d", (int)[tableView numberOfSections]);
    // Output: 10
}