Lazy Form Creation Design Pattern

Our application is now ready. We can click on "Run" green triangle button, in order build, deploy and launch the application on a mobile device of our choice: Android or iOS.

That's a great start!

Click on the "Project" menu and select "View Source". You should see the main program file of the app. This is the place where the execution of the app begins.

program LazyApp;



uses

  System.StartUpCopy,

  FMX.MobilePreview,

  FMX.Forms,

  uFormSplash in 'uFormSplash.pas' {FormSplash},

  uFormMain in 'uFormMain.pas' {FormMain}


{$R *.res}



begin

  Application.Initialize;

  Application.CreateForm(TFormSplash, FormSplash);

  Application.CreateForm(TFormMain, FormMain);

  Application.Run;

end

Notice that before the "Application.Run" method is called, which basically starts the main application event loop – and the app starts to wait for user input – both forms of our application are created first. It would be faster to create just the splash form, that is going to be displayed first, and to create the second – main – form when it is needed. If we would have a big number of forms in our project that could significantly slow down our app start up time.

We can either just delete the line of code where "FormMain" is created or more elegantly we can go to the "Project" menu, select "Options" and then select "Forms" in the tree at the left side of the dialog. There are two lists of forms: those that are "auto created" and those that are just "available". Click on the "FormMain" and move it to the list of available forms. Click on "OK".

If we would run our application "as is" right now, we would get an error when trying to call the "Show" method on the "FormMain" object, because it does yet exist. Before we can call any of its methods, we need to create it and we just deleted the line of code that did this.

In most cases at this moment we would just write a line of code, just before the "FormMain.Show" method call, that creates the main form first. Our application would run again correctly. But this is not elegant approach.

Here comes the "Lazy Form Creation Design Pattern"!

Open the "uFormMain" unit in the editor . Move the global variable "var FormMain: TFormMain;" to the beginning of the "implementation" section of the unit. Change "FormMain" to "AFormMain". Now we are going to define in the same place where our global variable was, a global function "function FormMain: TFormMain" that returns an instance of the TFormMain class. In this way we do not need to modify the code in any other unit that accesses any of the methods of the TFormMain class because the name of the function is the same as the name of the global variable was. The last task is to implement the function so it checks if the "AFormMain" variable is not "nil", if it is "nil", creates the form, and returns the resulting form.

unit uFormMain;



interface



uses

  System.SysUtils, System.Types, System.UITypes, System.Classes, System.Variants,

  FMX.Types, FMX.Controls, FMX.Forms, FMX.Graphics, FMX.Dialogs;



type

  TFormMain = class(TForm)

  private

    { Private declarations }

  public

    { Public declarations }

  end


function FormMain: TFormMain;



implementation



{$R *.fmx}



var

  AFormMain: TFormMain;



function FormMain: TFormMain;

begin

  if AFormMain = nil then

    AFormMain := TFormMain.Create(Application);



  Result := AFormMain;

end


end.