Function get_boxes_with_registry 

pub fn get_boxes_with_registry<R: Read + Seek>(
    r: &mut R,
    size: u64,
    decode: bool,
    reg: &Registry,
) -> Result<Vec<Box>>

Like get_boxes, but decodes with a caller-supplied Registry instead of the default one.

To extend the default registry with custom decoders (rather than replace it), build on default_registry:

use mp4box::{BoxKey, FourCC, get_boxes_with_registry};
use mp4box::registry::{BoxDecoder, BoxValue, default_registry};
use std::fs::File;
use std::io::Read;

struct MyDecoder;
impl BoxDecoder for MyDecoder {
    fn decode(
        &self,
        r: &mut dyn Read,
        _hdr: &mp4box::BoxHeader,
        _version: Option<u8>,
        _flags: Option<u32>,
    ) -> anyhow::Result<BoxValue> {
        let mut buf = Vec::new();
        r.read_to_end(&mut buf)?;
        Ok(BoxValue::Text(format!("{} payload bytes", buf.len())))
    }
}

let reg = default_registry().with_decoder(
    BoxKey::FourCC(FourCC(*b"xyz ")),
    "xyz ",
    Box::new(MyDecoder),
);

let mut file = File::open("video.mp4")?;
let size = file.metadata()?.len();
let boxes = get_boxes_with_registry(&mut file, size, true, &reg)?;