unit Unit2;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, System.Net.URLClient, System.Net.HttpClient,
  System.Net.HttpClientComponent, System.JSON, System.Net.Mime, Vcl.StdCtrls;

type
  TForm2 = class(TForm)
    NetHTTPClient1: TNetHTTPClient;
    ButtonAuth: TButton;
    Login: TEdit;
    Pass: TEdit;
    GetDialogs: TButton;
    procedure ButtonAuthClick(Sender: TObject);
    procedure GetDialogsClick(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    Access_token, Ver: string;
    function AuthVK(const Login, Pass, Client_id, Client_secret, Scope, V: string): string;
    function Call(const Method: string; Data: TMultipartFormData): TJSONValue;
  public
    { Public declarations }
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}

function TForm2.AuthVK(const Login, Pass, Client_id, Client_secret, Scope, V: string): string;
var
  Resp: IHTTPResponse;
  JsonValue: TJSONObject;
  Val: TJSONValue;
  Str: string;
begin
  Result := '';
  Resp := NetHTTPClient1.Get('https://oauth.vk.com/token?username=' + Login + '&password=' + Pass +
    '&client_id=' + Client_id + '&client_secret=' + Client_secret + '&grant_type=password&scope=' +
    Scope + '&v=' + V);

  JsonValue := TJSonObject.ParseJSONValue(Resp.ContentAsString()) as TJSONObject;
  if JsonValue = nil then
    Exit;

  Val := JsonValue.Values['access_token'];
  if Val = nil then
    Application.MessageBox(PChar(JsonValue.Values['error_description'].Value),
      PChar(JsonValue.Values['error'].Value), MB_OK or MB_ICONINFORMATION)
  else
    Result := Val.Value;

  Ver := V;
end;

function TForm2.Call(const Method: string; Data: TMultipartFormData): TJSONValue;
var
  Val: TJSONValue;
begin
  try
    Result := nil;
    if Access_token <> '' then
    begin
      if Data = nil then
      begin
        Application.MessageBox(PChar('Empty Data in call method to ' + Method), 'Error',
          MB_OK or MB_ICONINFORMATION);
        Exit;
      end;

      Data.AddField('access_token', Access_token);
      Data.AddField('v', Ver);

      Result := TJSonObject.ParseJSONValue(NetHTTPClient1.Post('https://api.vk.com/method/' +
        Method, Data).ContentAsString()) as TJSONObject;
      if Result = nil then
        Exit;

      Val := Result.GetValue<TJSONValue>('error', nil);
      if Val <> nil then
        Application.MessageBox(PChar(Val.ToJSON), PChar(Val.GetValue<string>('error_msg')),
          MB_OK or MB_ICONINFORMATION)
      else
        Result := Result.GetValue<TJSONValue>('response', nil);
    end
  except

  end;
end;

procedure TForm2.FormCreate(Sender: TObject);
begin
  NetHTTPClient1.UserAgent :=
    'VKAndroidApp/4.38-816 (Android 6.0; SDK 23; x86; Google Nexus 5X; ru)';
end;

procedure TForm2.ButtonAuthClick(Sender: TObject);
begin
  Access_token := AuthVK(Login.Text, Pass.Text, '2274003', 'hHbZxrka2uZ6jB1inYsH',
    '244236287', '5.95');
  if Access_token <> '' then
    ShowMessage('Удачно');
end;

procedure TForm2.GetDialogsClick(Sender: TObject);
var
  P: TMultipartFormData;
  JsonValue, Tmp: TJSONValue;
  Items, Items2: TJSONArray;
  Count, I: Integer;
  Vv: TJSONValue;
  Ss: string;
begin
  if Access_token <> '' then
  begin
    P := TMultipartFormData.Create();

    P.AddField('count', '5'); // 5 записей

    JsonValue := Call('messages.getDialogs', P);
    if JsonValue = nil then
      Exit;
    Items := JsonValue.GetValue<TJSONArray>('items', nil);
    if Items = nil then
      Exit;

    For I := 0 to Items.Count - 1 do
    begin
      Tmp := Items.Items[I].GetValue<TJSONValue>('message', nil);
      ShowMessage('user_id: ' + Tmp.GetValue<string>('user_id', '') + #13 + 'Message:' +
        Tmp.GetValue<string>('body', ''));
    end;
  end;
end;

end.