如果调用新代码,运行较低版本IOS的用户会怎样？

Question

我是iOS开发的新手,我一直想知道在iOS 4上运行我的应用程序的用户是否尝试运行此代码:

//POST TWEET//
- (void)showTweetSheet
{
    TWTweetComposeViewController *tweetSheet =
    [[TWTweetComposeViewController alloc] init];
    tweetSheet.completionHandler = ^(TWTweetComposeViewControllerResult result) {
        switch(result) {
            case TWTweetComposeViewControllerResultCancelled:
                break;
            case TWTweetComposeViewControllerResultDone:
                break;
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            [self dismissViewControllerAnimated:YES completion:^{
                NSLog(@"Tweet Sheet has been dismissed.");
            }];
        });
    };
    [tweetSheet setInitialText:@"Check out this cool picture I found on @Pickr_"];
    //  Add an URL to the Tweet.  You can add multiple URLs.
    if (![tweetSheet addURL:[NSURL URLWithString:ImageHost]]){
        NSLog(@"Unable to add the URL!");
    }
    [self presentViewController:tweetSheet animated:YES completion:^{
        NSLog(@"Tweet sheet has been presented.");
    }];
}

会发生什么？应用程序是否会因错误而终止,或者代码是否会运行？我如何正确实现特定于操作系统的功能？我会用这样的东西:

NSString *DeviceVersion = [[UIDevice currentDevice] systemVersion];
int DeviceVersionInt = [DeviceVersion intValue];
if (DeviceVersionInt > 5)
{
    //do something.
}
else
{
    //don't do a thing.
}

Answer 1

如果您编写iOS5功能而不检查它们是否可用,它将在iOS 4上崩溃.尝试像这样实现Twitter

Class twClass = NSClassFromString(@"TWTweetComposeViewController");
if (!twClass) // Framework not available, older iOS
{
    //use iOS4 SDK to implement Twitter framework
}
else {
    //use Apple provided default Twitter framework

}

确保您添加了具有弱链接的Twitter框架.