限制AlertView输入框输入的字数

UIAlertView_input.m

// Show a secure text entry alert with two custom buttons.
- (void)showSecureTextEntryAlert {
    NSString *title = NSLocalizedString(@"A Short Title Is Best", nil);
    NSString *message = NSLocalizedString(@"A message should be a short, complete sentence.", nil);
    NSString *cancelButtonTitle = NSLocalizedString(@"Cancel", nil);
    NSString *otherButtonTitle = NSLocalizedString(@"OK", nil);

	UIAlertView *alert = [[UIAlertView alloc] initWithTitle:title message:message delegate:self cancelButtonTitle:cancelButtonTitle otherButtonTitles:otherButtonTitle, nil];

	alert.alertViewStyle = UIAlertViewStyleSecureTextInput;
    
    [alert show];
}


#pragma mark - UIAlertViewDelegate

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex {
    if (alertView.cancelButtonIndex == buttonIndex) {
        NSLog(@"Alert view clicked with the cancel button index.");
    }
    else {
        NSLog(@"Alert view clicked with button at index %ld.", (long)buttonIndex);
    }
}

- (BOOL)alertViewShouldEnableFirstOtherButton:(UIAlertView *)alertView {
    // Enforce a minimum length of >= 5 characters for secure text alert views.
    if (alertView.alertViewStyle == UIAlertViewStyleSecureTextInput) {
        return [[alertView textFieldAtIndex:0].text length] >= 5;
    }

    return YES;
}