[Major change / Improvement] Change sending flow, UX + allow all paths

[WietseWind]

Suggested new Send flow, to streamline sending any currency to any currency, taking care of paths, rates (DEX), & 'wizard' based sending. Allow to send anything and deliver anything. Allow the user to select what to SEND and what to DELIVER, with:

• If only one option: auto select and don't allow to change (eg. user has no trust lines (currencies), just select XRP. User XRP balance fully reserved but IOU present: select IOU)
• If multiple options available: select (if possible) the same one (eg send Gatehub EUR = deliver Gatehub EUR, unless the recipient doesn't have that currency (trust line) setup), then force picking.

To consider:

• Show live rates, respect ~ for either SENDing or DELIVERing
• Only show/allow selection of liquid pairs (other pairs disabled after the enabled pairs, for the sake of visibility/recognizability)
• Check Rippling possibility, so no paths dry message will be given if wrongly configured line is selected
• If SENDing: just send what user selects. If DELIVERing, add the (IOU) transfer fee if present on selected pairs to the SENDing amount so the recipient will get the full DELIVER amount.
• Ensure full delivery, so if the selected SEND balance is not sufficient, don't allow the user to select (use) that currency.

Idea: to make this less intimidating to users: we could just allow users to enter the sending amount at first, then to just inform them what will be delivered (existing behaviour; xrp<>xrp, same IOU<>same IOU, or sending IOU> XRP. The user could then tap "Change" and go the the more advanced behaviour where they can select SEND/DELIVER and pick currencies.

Use ... to find the available options & rates:

• https://xrpl.org/ripple_path_find.html
• https://xrpl.org/path_find.html

Impacts:
#18 Destination tag handling
#109 IOU DEX exchange notification
#76 Cross IOU sending

❗ When doing this make over, we should implement the same logic for:

• The sending side for Sign Requests (+path finding), like the "Amount to DELIVER" flow for payloads with a predefined amount. If the amount for the Sign Request payload is empty (so the user can decide on the amount) then implement the exact same logic (Amount to [send/deliver] » Amount to [deliver/send]
• IOU Currency Exchange. When tapping an IOU in the home screen, and tapping the "Exchange" button, we can use this exact same flow, except both the Sending and the Receiving account are the same (Prefilled and hidden)

[canny]

This issue has been linked to a Canny post: XRPL PathFinding: Currency conversions. 🎉

[WietseWind]

Note: pretty cool @N3TC4T:

{
  "command": "path_find",
  "subcommand": "create",
  "source_account": "rPdvC6ccq8hCdPKSPJkPmyZ4Mi1oG2FFkT",
  "destination_account": "rwiETSee2wMz3SBnAG8hkMsCgvGy9LWbZ1",
  "destination_amount": {
    "value": "100",
    "currency": "USD",
    "issuer": "rvYAfWj5gh67oV6fW32ZzP3Aw4Eubs59B"
  }
}

»

[WietseWind]

Duplicates:
#129
#76
#108

Will be implemented as xApp (anything to anything)