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];

        }

    }