[[AgustinOrtu]]

Hola el-mono

No es perfecto pero creo que es un comienzo y luego podes personalizarlo mas a tu manera

Código Delphi [-]
unit Unit1;

interface

uses
  System.Classes,
  System.SysUtils,
  FMX.Types,
  FMX.Controls,
  FMX.Controls.Model,
  FMX.Forms,
  FMX.Graphics,
  FMX.Controls.Presentation,
  FMX.Edit,
  FMX.StdCtrls;

type
  TCurrencyEdit = class(FMX.Edit.TEdit)
  strict private
    function GetValue: Currency;
    function GetDefaultValue: Currency;
    function GetFormatSettings: TFormatSettings;

    procedure SetValue(const Value: Currency);
    procedure SetDefaultValue(const Value: Currency);
    procedure SetFormatSettings(const Value: TFormatSettings);
  strict protected
    procedure SetText(const Value: string); override;
    function DefineModelClass: TDataModelClass; override;
  public
    property FormatSettings: TFormatSettings read GetFormatSettings write SetFormatSettings;
  published
    property DefaultValue: Currency read GetDefaultValue write SetDefaultValue;
    property Value: Currency read GetValue write SetValue;
  end;

  TCurrencyEditModel = class(TCustomEditModel)
  private
    FDefaultValue: Currency;
    FFormatSettings: TFormatSettings;

    procedure SetDefaultValue(const Value: Currency);
    procedure SetFormatSettings(const Value: TFormatSettings);
  protected
    function DoValidate(const Value: string): string; override;
  public
    constructor Create(const AOwner: TComponent); override;
    property DefaultValue: Currency read FDefaultValue write SetDefaultValue;
    property FormatSettings: TFormatSettings read FFormatSettings write SetFormatSettings;
  end;

  TEdit = class(TCurrencyEdit);


  TForm1 = class(TForm)
    Edit1: TEdit;
    Button1: TButton;
    Button2: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.fmx}

{$REGION 'TCurrencyEdit'}

function TCurrencyEdit.DefineModelClass: TDataModelClass;
begin
  Result := TCurrencyEditModel;
end;

function TCurrencyEdit.GetDefaultValue: Currency;
begin
  Result := GetModel< TCurrencyEditModel >.DefaultValue;
end;

function TCurrencyEdit.GetFormatSettings: TFormatSettings;
begin
  Result := GetModel< TCurrencyEditModel >.FormatSettings;
end;

function TCurrencyEdit.GetValue: Currency;
begin
  Result := StrToCurrDef(Text, DefaultValue, FormatSettings);
end;

procedure TCurrencyEdit.SetDefaultValue(const Value: Currency);
begin
  GetModel< TCurrencyEditModel >.DefaultValue := Value;
end;

procedure TCurrencyEdit.SetFormatSettings(const Value: TFormatSettings);
begin
  GetModel< TCurrencyEditModel >.FormatSettings := Value;
end;

procedure TCurrencyEdit.SetText(const Value: string);
begin
  SetValue(StrToCurrDef(Value, DefaultValue, FormatSettings));
end;

procedure TCurrencyEdit.SetValue(const Value: Currency);
begin
  inherited SetText(CurrToStr(Value, FormatSettings));
end;

procedure TCurrencyEditModel.SetDefaultValue(const Value: Currency);
begin
  FDefaultValue := Value;
end;

procedure TCurrencyEditModel.SetFormatSettings(const Value: TFormatSettings);
begin
  FFormatSettings := Value;
end;

{$ENDREGION}

{$REGION 'TCurrencyEditModel'}

constructor TCurrencyEditModel.Create(const AOwner: TComponent);
begin
  inherited Create(AOwner);
  FDefaultValue := 0;
  FFormatSettings := TFormatSettings.Create;
end;

function TCurrencyEditModel.DoValidate(const Value: string): string;
var
  TempValue: Currency;
begin
  if not TryStrToCurr(Value, TempValue, FormatSettings) then
    TempValue := DefaultValue;

  Result := CurrToStrF(TempValue, ffCurrency, FormatSettings.CurrencyDecimals, FormatSettings);
end;

{$ENDREGION}

{$REGION 'Ejemplo'}

procedure TForm1.Button1Click(Sender: TObject);
begin
  Edit1.Text := 'asdasd';
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  Edit1.Value := 123.45;
end;

{$ENDREGION}

end.

En este ejemplo hago uso del truco de la clase interpuesta para ahorrarme el problema de instalar y registrar el componente (de esto podes encontrar muchisimos ejemplos en la web de como hacerlo) de este modo en mi form todos los TEdit en realidad son instancias de TCurrencyEdit (es un engaño al framework de persistencia ocultando el tipo original TEdit)

En mi caso yo utilizo una variable privada TFormatSettings para definir el formato. La clase en su constructor inicializa este registro usando el locale por defecto. Si necesitas personalizar el formato a tu gusto podes crear tu propio registro TFormatSettings y luego asignarselo al TCurrencyEdit

Código Delphi [-]
procedure TForm1.OnCreate(Sender: TObject);
var
  MyFormatSettings: TFormatSettings;
begin
  MyFormatSettings := TFormatSettings.Create; // hay mas constructores, revisa la documentacion
  MyFormatSettings.CurrencyString := '$'; // cambiar las propiedades deseadas
  // cambiar mas propiedades
  // asignar 
  Edit1.FormatSettings := MyFormatSettings;
end;

Otra alternativa seria descartar el FormatSettings y usar un mecanismo de callback (un evento) en el que el programador pueda formatear el string como quiera:

El codigo modificado (obviando algun que otro getter/setter trivial)

Código Delphi [-]
type
  TFormatTextEvent = procedure(Sender: TObject; const Value: Currency; var Text: string) of object;

  TCurrencyEdit = class(FMX.Edit.TEdit)
  strict private
    function GetOnFormatText: TFormatTextEvent;
    procedure SetOnFormatText(const Value: TFormatTextEvent);
    ...
  published
    property OnFormatText: TFormatTextEvent read GetOnFormatText write SetOnFormatText;
    ...
  end;

  TCurrencyEditModel = class(TCustomEditModel)
  strict private
    FOnFormatText: TFormatTextEvent;
    ...
    procedure SetOnFormatText(const Value: TFormatTextEvent);
  strict protected
    function DoFormatText(const TempValue: Currency): string; virtual;
  public
    ...
    property OnFormatText: TFormatTextEvent read FOnFormatText write SetOnFormatText;
  end;

function TCurrencyEdit.GetOnFormatText: TFormatTextEvent;
begin
  Result := GetModel< TCurrencyEditModel >.OnFormatText;
end;

procedure TCurrencyEdit.SetOnFormatText(const Value: TFormatTextEvent);
begin
  GetModel< TCurrencyEditModel >.OnFormatText := Value;
end;

function TCurrencyEditModel.DoFormatText(const TempValue: Currency): string;
begin
  if Assigned(FOnFormatText) then
    FOnFormatText(Self, TempValue, Result)
  else
    Result := CurrToStrF(TempValue, ffCurrency, FormatSettings.CurrencyDecimals, FormatSettings);
end;

function TCurrencyEditModel.DoValidate(const Value: string): string;
var
  TempValue: Currency;
begin
  if not TryStrToCurr(Value, TempValue, FormatSettings) then
    TempValue := DefaultValue;

  Result := DoFormatText(TempValue);
end;

Uso:

Código Delphi [-]
  TForm1 = class(TForm)
    ...
    procedure Edit1FormatText(Sender: TObject; const Value: Currency; var Text: string);
  end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Edit1.OnFormatText := Edit1FormatText;
end;

procedure TForm1.Edit1FormatText(Sender: TObject; const Value: Currency; var Text: string);
begin
  Text := CurrToStrF(Value, ffCurrency, 3);
end;