[iOS] 视频压缩

- (void) lowQuailtyWithInputURL:(NSURL*)inputURL
outputURL:(NSURL*)outputURL
blockHandler:(void (^)(AVAssetExportSession*))handler
{
AVURLAsset *asset = [AVURLAsset URLAssetWithURL:inputURL options:nil];
AVAssetExportSession *session = [[AVAssetExportSession alloc] initWithAsset:asset     presetName:AVAssetExportPresetMediumQuality];
session.outputURL = outputURL;
session.outputFileType = AVFileTypeQuickTimeMovie;
[session exportAsynchronouslyWithCompletionHandler:^(void)
{
handler(session);
}];
}

[self lowQuailtyWithInputURL:video outputURL:output blockHandler:^(AVAssetExportSession *session)
{
if (session.status == AVAssetExportSessionStatusCompleted)
{

}
else
{

}
}];

在block里面检测成功，失败，或者是取消，然后释放session.

期间可以通过不断的查看session的progress属性来获取转换的进度。

可以设置这些压缩质量

AVF_EXPORT NSString *const AVAssetExportPresetLowQuality        NS_AVAILABLE_IOS(4_0);
AVF_EXPORT NSString *const AVAssetExportPresetMediumQuality     NS_AVAILABLE_IOS(4_0);
AVF_EXPORT NSString *const AVAssetExportPresetHighestQuality    NS_AVAILABLE_IOS(4_0);