@polkadot-api/metadata-builders

This package has multiple functions that read a metadata object, denormalizes it, and builds other structures needed for different processes

## getLookupFn

1interface MetadataLookup {
2  (id: number): LookupEntry
3  metadata: V14 | V15
4}
5
6function getLookupFn(metadata: Metadata): MetadataLookup

Given the a metadata, returns a function that will give the LookupEntry for an id along. The function also has access to the original metadata, as it's usually needed to work with the actual lookup.

The LookupEntry is a denormalized data structure for one entry in the metadata. It also "shortcuts" type references when those are pointers (composites or tuples of length 1). Essentially, it's a union of each of the different types that can be found in the lookup, mostly equivalent to something like:

1type TerminalVar =
2  | PrimitiveVar // u8, str, char, i128, etc.
3  | CompactVar
4  | BitSequenceVar
5  | AccountId32
6
7type ComposedVar =
8  | TupleVar
9  | StructVar
10  | SequenceVar
11  | ArrayVar
12  | OptionVar
13  | ResultVar
14  | EnumVar
15
16type LookupEntry = TerminalVar | ComposedVar

Where, for instance, a StructVar is of the shape

1type StructVar = {
2  type: "struct"
3  value: Record<string, LookupEntry>
4}

It's useful to get types referenced by storage calls, etc.

getDynamicBuilder

1function getDynamicBuilder(metadataLookup: MetadataLookup): {
2  buildDefinition: (id: number) => Codec
3  buildConstant: (pallet: string, name: string) => Codec
4  buildEvent: (pallet: string, name: string) => VariantEntry
5  buildError: (pallet: string, name: string) => VariantEntry
6  buildCall: (pallet: string, name: string) => VariantEntry
7  buildStorage: (pallet: string, entry: string) => StorageEntry
8  buildRuntimeCall: (api: string, method: string) => RuntimeEntry
9}

Generates all the codecs needed to SCALE encode or decode the data for any interaction with the chain.

buildDefinition returns the codec for the type identified by the parameter id

buildConstant returns the codec for the requested constant (equivalent as calling buildDefinition with the type id of that constant)

buildEvent, buildError and buildCall return an object with the codec, and the indices of the pallet and entry within the metadata:

1interface VariantEntry {
2  location: [number, number] // [palletIdx, entryIdx],
3  codec: Codec
4}

buildStorage creates all the encoders/decoders needed to encode a storage call and decode its result:

1interface StorageEntry {
2  // Encodes the arguments of the storage call.
3  enc: (...args: any[]) => string
4  // Decodes the result from the storage call.
5  dec: (value: string) => any
6  // Decodes the arguments of the storage call
7  keyDecoder: (value: string) => any[]
8  // Expected number of arguments
9  len: number
10  // Decoded fallback value as defined in the metadata entry
11  fallback: unknown
12}

Similarly, buildRuntimeCall returns the codecs for both encoding the arguments of the runtime call, and the codec for decoding the result

1interface RuntimeEntry {
2  args: Codec<any[]>
3  value: Codec<any>
4}

getChecksumBuilder

1function getChecksumBuilder(metadataLookup: MetadataLookup): {
2  buildDefinition: (id: number) => string | null
3  buildRuntimeCall: (api: string, method: string) => string | null
4  buildStorage: (pallet: string, entry: string) => string | null
5  buildCall: (pallet: string, name: string) => string | null
6  buildEvent: (pallet: string, name: string) => string | null
7  buildError: (pallet: string, name: string) => string | null
8  buildConstant: (pallet: string, constantName: string) => string | null
9}

Generates the checksums for the different components defined in the metadata.

buildDefinition builds the checksum of one of the types in the lookup. The rest of the methods build the checksum for each of the interfaces of the chain.