一个关于Media Player 跨iPhone版本的问题

when Run on Device 3.1.2, why it also pass if(NSClassFromString(@"MPMoviePlayerViewController") != nil) and do code of iOS4 then it will crash , how to fix this issues?
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?if(NSClassFromString(@"MPMoviePlayerViewController") != nil) {
??????????????????? // iOS 4 code
??????????????????? NSLog(@"MPMoviePlayerViewController");
??????????????????? MPMoviePlayerViewController *mp = [[MPMoviePlayerViewController alloc] initWithContentURL:[NSURL URLWithString:AppDelegate.PushLink]];
??????????????????? if (mp) {
??????????????????????? // save the movie player object
??????????????????????? self.theMovie4 = mp;
??????????????????????? [mp release];
??????????????????????? //Present???????????????????????
??????????????????????? [self presentMoviePlayerViewControllerAnimated:self.theMovie4];

??????????????????????? // Play the movie!
??????????????????????? self.theMovie4.moviePlayer.movieSourceType = MPMovieSourceTypeStreaming;
??????????????????????? [self.theMovie4.moviePlayer play];
??????????????????? }
??????????????? }
??????????????? else {
??????????????????? //iOS 3 Code
??????????????????? AppDelegate = nil;
??????????????????? AppDelegate = [[UIApplication sharedApplication] delegate];
??????????????????? [AppDelegate ForceHideNavigationBar];
??????????????????? theMovie3 = nil;??????????????????? [[NSNotificationCenter defaultCenter] addObserver:self?
???????????????????????????????????????????????????????????? selector:@selector(moviePreloadDidFinish:)?
???????????????????????????????????????????????????????????????? name:MPMoviePlayerContentPreloadDidFinishNotification?
?????????????????????????????????????????????????????????????? object:theMovie3];??????????????????? [[NSNotificationCenter defaultCenter] addObserver:self?
???????????????????????????????????????????????????????????? selector:@selector(moviePlayBackDidFinish:)?
???????????????????????????????????????????????????????????????? name:MPMoviePlayerPlaybackDidFinishNotification?
?????????????????????????????????????????????????????????????? object:theMovie3];??????????????????? // Register to receive a notification when the movie scaling mode has changed.?
??????????????????? [[NSNotificationCenter defaultCenter] addObserver:self?
???????????????????????????????????????????????????????????? selector:@selector(movieScalingModeDidChange:)?
???????????????????????????????????????????????????????????????? name:MPMoviePlayerScalingModeDidChangeNotification?
?????????????????????????????????????????????????????????????? object:theMovie3];??????????????????? theMovie3 = [[MPMoviePlayerController alloc] initWithContentURL: [NSURL URLWithString:AppDelegate.PushLink]];??????????????????? [theMovie3 play];??????????????? }1 Answer
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- (void) playMovieAtURL: (NSURL*) theURL {
??? NSLog(@"playMovieAtURL");if ([[[UIDevice currentDevice] systemVersion] doubleValue] >= 3.2) {
??????????? NSLog(@"> 3.2");
??????????? MPMoviePlayerViewController *mp = [[MPMoviePlayerViewController alloc] initWithContentURL:theURL];??????????? if (mp)
??????????? {
??????????????? // save the movie player object
??????????????? //self.moviePlayerViewController = mp;
??????????????? //[mp release];
??????????????? [self presentMoviePlayerViewControllerAnimated:mp];
??????????????? mp.moviePlayer.movieSourceType = MPMovieSourceTypeFile;
??????????????? [mp.moviePlayer play];
??????????????? [mp release];
??????????? }??????? }?
??? else if ([[[UIDevice currentDevice] systemVersion] doubleValue] < 3.2) {
??????????? NSLog(@"< 3.2");??????????? MPMoviePlayerController* theMovie = [[MPMoviePlayerController alloc] initWithContentURL: theURL];??????????? theMovie.scalingMode = MPMovieScalingModeAspectFill;??????????? // Register for the playback finished notification
??????????? [[NSNotificationCenter defaultCenter]
???????????? addObserver: self
???????????? selector: @selector(myMovieFinishedCallback:)
???????????? name: MPMoviePlayerPlaybackDidFinishNotification
???????????? object: theMovie];??????????? // Movie playback is asynchronous, so this method returns immediately.
??????????? [theMovie play];??????? }??? }