Okay, this PR is quite big.

Description

Both CurrencyEntryBox and DualCurrencyEntryBox have been completely rewritten.

The following changes have been made:

• a CurrencyFormat model has been introduced, which Represents a specific Currency's Parsing and Formatting rules.

  • This also defines aspects such as CurrencyCode, MaxFractionalDigits, MaxIntegralDigits, etc.
  • there are 3 CurrencyFormats right now: CurrencyFormat.Btc, CurrencyFormat.Usd and CurrencyFormat.SatvByte
  • More could be introduced in the future to support other currencies.

• CurrencyEntryBox now delegates all of the above aspects to the model, and deals only with user input, caret positioning, etc. It only deals with text and decimal Value property. it is up to the ViewModel to convert this numeric data into whatever it needs.

• Both CurrencyEntryBox and DualCurrencyEntryBox are completely agnostic to aspects such as conversion rates, currencies, etc.

• The clipboard handling logic has been moved to CurrencyEntryBoxClipboardBehavior. Once again this only deals with input and delegates logic to the CurrencyEntryBox.CurrencyFormat for parsing, etc.

  • The max amount logic depends on CurrencyEntryBox.MaxValue which gets updated (for instance in the Send View) accoding to the available balance and exchange rate. 

• ClipboardObserver class has been removed.

• DualCurrencyEntryBox no longer has a readonly state (and corresponding template). For cases where IsFixedAmount=true in the send dialog, the entry box gets replaced by a simple AmountControl which shows the amount as readonly text.

• 2 new ViewModels have been introduced in the send screen:

  • CurrencyViewModel deals with a single currency input and its corresponding MaxValue (both in decimal format)
  • CurrencyConversionViewModel deals with BTC <-> USD conversion and also with left/right switching.

Results

• All the input bugs and usability problems that appeared after Avalonia V11 migration are gone.

  • Fixes [UI] Entering amount to send bug #11745
  • Fixes [VDG] CurrencyEntryBox is not consistent #10877:
    • When you enter an amount and move the focus, the amount doesn't get formatted: Fixed.
    • When you paste an amount it gets formatted: this is the correct behavior. Formatting happens after pasting.
    • Weird behavior: [VDG] CurrencyEntryBox - do not allow space #10875 (comment): Fixed. Whatever numerical string with at most 8 integral and 8 fractional digits can be pasted in the BTC box, regardless of spaces
    • Copying an amount from the field also copies the format: Fixed. See CurrencyLocalization.LocalizedFormat() for details.
  • Fixes [VDG] Half selected amount #11414:

• The general design is much cleaner. Spaghetti has been removed everywhere, and now each layer (UI, Models, ViewModel) deals only with its corresponding responsibility.

• The above makes the design of DualCurrencyEntryBox extremely simple. It is just a container for the 2 currency inputs and the toggle button. It has only 76 lines of code now (down from 471 on master). The switching from left to right happens in the data, not the UI, this can be done because the UI elements are currency-agnostic.

• Groundwork has been laid to enable supporting multiple currencies (other than USD) and also localization-sensitive aspects have been moved to CurrencyLocalization class.

• ViewModels no longer need to deal with money amounts as strings but rather as decimal amounts, regardless of currency. This also helped clean up some logic in the VMs such as format validation, etc.

• SendViewModel no longer deals with clipboard contents

• DualCurrencyEntryBox now has only 6 lines of logic, dealing with focus.

• CurrencyEntryBox no longer needs a specialized version of the paste command. Simple and standard copy and paste just works.

• Most of the code has been thoroughly documented.

Testing

• The following manual tests have been made:
  • text input, including selection
  • typing "." with an empty box results in "0."
  • caret positioning affected by formatting rules
  • not being able to enter more than the max number of integral digits and fractional digits (defined by CurrencyFormat)
  • clipboard logic
    • paste valid values for both currencies
    • paste invalid values for both currencies
    • paste non numerical text
    • paste using Ctrl+V and right click context menu option
  • clipboard auto suggestion logic:
    • for valid numerical values
    • for invalid numerical values
    • for valid amounts within the range defined by MaxValue property (bound to the available balance in the Send view)
    • for invalid amounts as per the above
  • left/right switching and its effects on keyboard focus
  • readonly amount in the send dialog
  • BTC only input in the send dialog when exchange rate is not available
  • send full balance by clicking in the balance text in send view
  • standalone uses of CurrencyEntryBox:
    • CustomFeeRateDialog
    • PlebStopThreshold
    • DustThreshold

Problems

While writing this PR, I found and reported AvaloniaUI/Avalonia#13648. There may be other issues (mostly related to caret positioning) related to this.

There are also stylistic things in the code, such as CurrencyConversionViewModel relying too much on the _isUpdating flag. Maybe @SuperJMN can come up with a cleaner way to handle this using Rx.

Next steps

• Further testing
• Headless Avalonia based testing
• Accept comma in CurrencyEntryBox and handle validation using CurrencyInput #11310: Localized input support for Currencies requires some further discussion, simply supporting comma instead of the period symbol will not cover all cases (for instance, 12,355.88 is not covered and will not be parseable using the current logic)