问iOS 4和iOS 5向后兼容

EN

Class twRequestClass = NSClassFromString(@"TWRequest");
if (twRequestClass == nil) {
    // TWRequest does not exist on this device (running iOS version < 5.0)
    // ... do something appropriate ...
} else {
    TWRequest *twRequest = [[twRequestClass alloc] init];
    // ^ I didn't look up the proper initializer, so you should change that line if necessary
    // ...
}

// not supported on iOS4
UINavigationBar *navBar = [myNavController navigationBar];
if ([navBar respondsToSelector:@selector(setBackgroundImage:forBarMetrics:)])
{
    [navBar setBackgroundImage:[UIImage imageNamed:@"bg.jpg"] forBarMetrics:UIBarMetricsDefault];
} 

if(NSClassFromString(@"UIPopoverController")) {
// Do something
}

float version = [[[UIDevice currentDevice] systemVersion] floatValue];