XCM Fungible Asset Implementation Design

[xlc]

See #385

• orml-xcm-support
  • Types
    • trait OnUnknownAssetDeposit
      • fn on_unknown_asset_deposit(id: &MultiLocation, val: u128)
    • trait AssetIdConversion
      • type Error
      • fn from_multi_asset(asset: &MultiAsset) -> Result<Self, Error>
      • fn to_multi_asset(self: &Self) -> Result<MultiAsset, Error>
    • struct MultiCurrencyAdapter<MultCurrency, LocationConversion, AssetId: AssetIdConversion, OnUnknownAssetDeposit>
      • impl TransactAsset
        • use LocationConversion to get origin
        • ues AssetIdConversion to get CurrencyId
        • use MultiCurrency to deposit/withdraw
    • other helper types as needed
• orml-unknown-tokens
  • Notes
    • handle unknown tokens
    • allow to query balance
    • allow transfer to other chain (i.e. send money back)
  • Storages
    • map AccountId, MultiLocation => u128
  • Calls
    • transfer(origin, asset: MultiLocation, amount: u128, dest: MultiLocation)
  • Module
    • impl OnUnknownAssetDeposit
    • impl
      • fn burn(who: AccountId, asset: MultiLocation, amount: u128)
        • useful to migrate Unknown asset to known one
• orml-xtokens
  • Config
    • XcmExecutor
    • CurrencyId
    • Balance
  • Calls
    • transfer(origin, currency_id: CurrencyId, amount: Balance, dest: MultiLocation)
    • transfer_xcm(origin, asset: MultiAsset, dest: MultiLocation)

[shaunxw]

Additional implementation notes:

• In MultiCurrencyAdapter deposit asset impl, if the CurrencyId conversion failed, consider the asset as unknown asset, and deposit it via OnUnknownAssetDeposit trait.
• While integrating the XCM related pallets, use orml-unknown-tokens pallet as OnUnknownAssetDeposit impl in MultiCurrencyAdapter.