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Writing Unpackers

To write an OFRAK Unpacker, an OFRAK contributor needs to:

  1. Create a class that inherits from ofrak.component.component_unpacker.Unpacker with a defined component config (ofrak.model.component_model.CC);
  2. Implement the targets to indicate what resource tags the unpacker targets (if necessary, register a new identifier pattern);
  3. Implement children to indicate what resource tags are valid children that the unpacker can create;
  4. Implement the unpack method such that it unpacks the resource into children.

The following is an example of a fully-implemented OFRAK unpacker:

from dataclasses import dataclass

from ofrak.component.unpacker import Unpacker
from ofrak.service.resource_service_i import ResourceFilter
from ofrak.core.binary import GenericBinary
from ofrak.resource import Resource
from ofrak_type.range import Range

[ ... ]

class UImage(GenericBinary):
    async def get_header(self) -> UImageHeader:
        return await self.resource.get_only_child_as_view(
            UImageHeader, ResourceFilter.with_tags(UImageHeader)

    async def get_body(self) -> UImageBody:
        return await self.resource.get_only_child_as_view(
            UImageBody, ResourceFilter.with_tags(UImageBody),

[ ... ]

class UImageUnpacker(Unpacker[None]):
    UImage unpacker.

    It separates the UImage resource into its 2 parts: the header, and the body.
    id = b"UImageUnpacker"
    targets = (UImage,)
    children = (UImageHeader, UImageBody)

    async def unpack(self, resource: Resource, config=None):
        uimage_header_r = await resource.create_child(
            tags=(UImageHeader,), data_range=Range(0, UIMAGE_HEADER_LEN)
        uimage_header = await uimage_header_r.view_as(UImageHeader)
        await resource.create_child(
            data_range=Range.from_size(UIMAGE_HEADER_LEN, uimage_header.ih_size),

This unpacker targets a UImage and unpacks two possible children: a UImageHeader and UImageBody.

Handling External Dependencies

If the Unpacker makes use of tools that are not packaged in modules installable via pip from PyPI (commonly command-line tools), these dependencies must be explicitly declared as part of the unpacker's class declaration. See the Components Using External Tools doc for information on how to do that.