AVAssetWriterInput does not currently support AVVi

I'm attempting to use the AVAssetWriterInput to crop a video that I read in a screencast of my application. Here is my current configuration.

NSDictionary *videoCleanApertureSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                            [NSNumber numberWithInt:320], AVVideoCleanApertureWidthKey,
                                            [NSNumber numberWithInt:480], AVVideoCleanApertureHeightKey,
                                            [NSNumber numberWithInt:10], AVVideoCleanApertureHorizontalOffsetKey,
                                            [NSNumber numberWithInt:10], AVVideoCleanApertureVerticalOffsetKey,
                                            nil];


NSDictionary *videoAspectRatioSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                          [NSNumber numberWithInt:3], AVVideoPixelAspectRatioHorizontalSpacingKey,
                                          [NSNumber numberWithInt:3],AVVideoPixelAspectRatioVerticalSpacingKey,
                                          nil];

NSDictionary *codecSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                               [NSNumber numberWithInt:960000], AVVideoAverageBitRateKey,
                               [NSNumber numberWithInt:1],AVVideoMaxKeyFrameIntervalKey,
                               videoCleanApertureSettings, AVVideoCleanApertureKey,
                               videoAspectRatioSettings, AVVideoPixelAspectRatioKey,
                               AVVideoProfileLevelH264BaselineAutoLevel, AVVideoProfileLevelKey,
                               nil];

NSDictionary *videoSettings = @{AVVideoCodecKey:AVVideoCodecH264,
                                AVVideoCompressionPropertiesKey:codecSettings,
                                AVVideoScalingModeKey:AVVideoScalingModeResizeAspectFill,
                                AVVideoWidthKey:[NSNumber numberWithInt:320],
                                AVVideoHeightKey:[NSNumber numberWithInt:480]};

_videoWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:videoSettings];

I'm receiving the following error: "AVAssetWriterInput does not currently support AVVideoScalingModeFit"

This is a common error for anyone using this library, but I can't find the actual solution to it. I just see people saying: "I figured it out eventually" without explaining it. The problem is definitely related to this line: "AVVideoScalingModeKey:AVVideoScalingModeResizeAspectFill," which tells the AVAssetWriter to crop the video and maintain the aspect ratio. Anyone know the solution to this?

There is no "solution to it" per se. It's simply unsupported. You'll need to scale the video frames yourself using Core Image or a VTPixelTransferSession or whatever is appropriate for your pipeline.