Question

In iOS, I use Objective C to create my own native module.

RCT_EXPORT_METHOD(startPay:(NSString*)tn :(NSString*)mode)
{
    UIWindow *window = [UIApplication sharedApplication].keyWindow;
    UIViewController *rootViewController = window.rootViewController;
    [[UPPaymentControl defaultControl] startPay:tn
                                     fromScheme:@"UPPayDemo" mode:mode
                                 viewController:rootViewController];
}

When calling this method in Javascript module, Xcode made an error that it must be used from main thread only.

-[UIApplication keyWindow] must be used from main thread only
-[UIWindow rootViewController] must be used from main thread only

Solution

Call it from the main thread using dispatch_async(dispatch_get_main_queue(), ^{}) .

RCT_EXPORT_METHOD(startPay:(NSString*)tn :(NSString*)mode)
{
    dispatch_async(dispatch_get_main_queue(), ^(void) {
        UIWindow *window = [UIApplication sharedApplication].keyWindow;
        UIViewController *rootViewController = window.rootViewController;
        [[UPPaymentControl defaultControl] startPay:tn
                                         fromScheme:@"UPPayDemo" mode:mode
                                     viewController:rootViewController];
    });
}