Promise Implementation in Delphi

Overview

This Delphi library implements promises, enabling asynchronous programming by facilitating the handling of operations that may require time to complete. A promise represents a guarantee for an eventual result, streamlining the way asynchronous operations are managed in your applications. Additionally, this implementation embraces monadic principles, offering a structured approach to chaining computations and handling their outcomes.

Features

Promise Chaining: Easily chain multiple asynchronous operations, passing the result of one as the input to the next.
Exception handling: The promise is side-affect free, so it manages exceptions inside the chain and you are able to recover from them.
Type Transformation: Transform the resolved value's type through the .Op method, enabling flexible data handling.
Memory Management: Control the lifecycle of promise-resolved values with dvKeep and dvFree directives, ensuring efficient resource utilization.
Starts immediately: The promise execution starts immediately, you do not have to call Await to execute the chain.

Getting Started

To integrate this promise library into your Delphi projects, include the necessary unit in your project source and follow the examples provided below.

Here is a simple example to see how you could perform a background operation and wait (blocking) for it to complete.

uses Next.Core.Promises;

// Create and resolve a simple promise
var
  value: String;
begin
  value := Promise.Resolve<String>(function: String
    begin
      Result := 'Hello, World!';
    end).Await;
 
  // Outputs: Hello, World!
  WriteLn(value);
end;

Chaining promises

On of the key features of promises is the ability to chain them. You chain promises to perform a sequence of operations where each step depends on the outcome of the previous one. During these steps the result types of the promise can change as you can see in the following example.

uses Next.Core.Promises;

// Chain promises to multiply an integer and convert it to a string
// Ensure UI update is on the main thread
begin
  Promise.Resolve<Integer>(function: Integer
    begin
      Result := 10;
    end)
  .ThenBy<Integer>(function(const value: Integer): Integer
    begin
      Result := value * 2; // Process in background thread
    end)
  .Op.ThenBy<String>(function(const value: Integer): String
    begin
      Result := IntToStr(value); // Process in background thread
    end)
  .Main.ThenBy<TVoid>(function(const value: String): TVoid
    begin
      WriteLn(value); // Synchronized to the main thread
      Result := Void;
    end);
end;

Type Transformation with `.Op`

Use the .Op method to transform the type of the resolved value of a promise.

uses Next.Core.Promises;

// Transform the resolved value type from Integer to String
var promise: IPromise<String>;
begin
  promise := Promise.Resolve<Integer>(function: Integer
    begin
      Result := 10;
    end)
  .Op.ThenBy<String>(function(const value: Integer): String
    begin
      Result := 'Transformed Value: ' + IntToStr(value); // Process in background thread
    end);

  WriteLn(promise.Await); // Outputs: Transformed Value: 10
end;

Exception handling

If any exceptions occurs, the chain will be interrupted until the first .Catch in the chain. If there is no catch, the promise will be rejected. Calling .Await on a rejected promise will raise the exception that was caught inside the promise in the caller context.

Recover from exceptions

Use the .Catch method to recover from any exception in the chain.

uses Next.Core.Promises;

// Handling exceptions and synchronizing error handling to the main thread
begin
  Promise.Resolve<Boolean>(function: Boolean
    begin
      raise Exception.Create('Simulated error');
    end)
  .Catch<Boolean>(function(E: Exception): Boolean
    begin
      if E.Message = 'Simulated error' then
        Result := False // Recover from the error condition
      else
        raise; // Re-throw for other exceptions
    end)
  .Main.ThenBy<TVoid>(function(const value: Boolean): TVoid
    begin
      if not value then
        WriteLn('Error handled, alternative value provided.'); // UI handling in the main thread
      Result := Void; // Setting result to Void correctly
    end);
end;

Waiting for multiple promises to resolve

If you perform multiple background operations you might want to wait until all of them are completed and then perform some action on it. You can use Promise.All for this. This waits until all promises are fulfilled. If one of the promises rejects (raises an exception), it will immediately go to the first .Catch in the chain without waiting for the rest of the promises to be fulfilled.

var LWidth := Promise.Resolve<Integer>(function: Integer
  begin
    //Heavy operation
    Result := 10;
  end);

var LHeight := Promise.Resolve<Integer>(function: Integer
  begin
    //Heavy operation
    Result := 10;
  end);

var LDepth := Promise.Resolve<Integer>(function: Integer
  begin
    //Heavy operation
    Result := 10;
  end);

//Now all three operations are running simultaneously, as soon as all are finished the next method in the chain will be called to calculate the volume 
Promise.All<Integer>([LWidth, LHeight, LDepth])
  .Op.ThenBy<Integer>(function(const AResults: TArray<Integer>): Integer
    begin
      Result := AResults[0] * AResults[1] * AResults[2];
    end)
  .Main.ThenBy(procedure(const AVolume: Integer)
    begin
      WriteLn(AVolume.ToString()); // Synchronized to the main thread
    end)
  .Main.Catch(procedure(E: Exception)
    begin
      WriteLn('Calculation failed: ' + E.Message); // Synchronized to the main thread
    end)

//Here you can continue your codeflow, for example show a waiting indicator

UI interaction

VCL operations are only allowed in main thread. Therefore this promise supports synchronisation natively using the .Main directive. This instructs the promise to execute the anonymous method in the main thread context.

Simple UI example

Because the promise starts executing immediately (there is no need for .Await), you can create a promise and continue your codeflow. In the following example we perform a simple background operation and show a messagebox on completion while keeping the UI responsive.

uses Next.Core.Promises;

// Create and resolve a simple promise
procedure DoHeavyOperationOnButtonClick()
begin
  Promise.Resolve<String>(function: String
    begin
      //Do some heavy operation
      Result := 'Heavy operation completed!';
    end)
  .Main.ThenBy(procedure(const value: String)
    begin
      ShowMessage(value);
    end);

  //Continue your flow, the messagebox will popup when the operation is completed. You might consider showing a indicator that shows that the operation is going on.
end;

Interacting with your forms

The previous example is simple and has no interaction with any form. However, in practice this will not always be the case. In that situation it is important to make sure your promises does not interact with your form-objects after a form is disposed.

You can solve this by instructing the form using the Windows message queue with PostMessage. Make sure that you capture the forms Handle and that you do not use Self.Handle, because that could also be invalid at that point.

uses Next.Core.Promises;

class
  TMyForm = class(TForm)
    procedure DoSomething(var AMessage: TMessage); message WM_MY_MESSAGE;
  end;

procedure TMyForm.DoHeavyOperationOnButtonClick()
begin
  var LHandle := Self.Handle;

  Promise.Resolve<String>(function: String
    begin
      //Do some heavy operation
      Result := 'Heavy operation completed!';
    end)
  .ThenBy(procedure(const value: String)
    begin
      PostMessage(LHandle, WM_MY_MESSAGE, 0, 0);
    end);

  //Continue your flow, the procedure DoSomething will be called when the heavy operation is completed.
end;

Memory Management

By default, the promise is responsible for handling the memory of everything that is returned by any of the anonymous methods. That means that if you create an object in a resolver function, that object is disposed when the promise is disposed. The resolved value will stay "in" the promise and can be used multiples times by calling .Await. This works for all managed types, but not for objects.

Use Await to transfer ownership of an object

With .Await you can retrieve the value inside the promise and use it. After calling .Await the ownership of the value is transfered to the caller, so -if it is an object- the promise will no longer dispose it.

The following example makes clear what the effect would be if the promise would keep ownership of the object. In the following situation the promise can already be destroyed before the last code is executed. That would mean that LObject points to an already disposed object.

var LObject := Promise.Resolve<TMyObject>(function: TMyObject
  begin
    Result := TMyObject.Create('test');
  end).Await;

LObject.DoSomething(); //here LObject can already be disposed by the promise

Chaining (ThenBy) disposes the argument if it is different from the return value

With ThenBy you have the option to return a different type or different instance of the same type. If the returned instance points to the same object that object will not be disposed. If the returned instance is another object, the argument passed to ThenBy (eg. this is the value of the previous resolved promise) will be disposed.

var LPromise1 := Promise.Resolve<TMyObject>(function: TMyObject
  begin
    Result := TMyObject.Create('test');
  end)
  .ThenBy<TMyObject>(function (const AValue: TObject): TMyObject
  begin
    Result := AValue;
  end); //Here AValue will not be disposed

var LPromise2 := Promise.Resolve<TMyObject>(function: TMyObject
  begin
    Result := TMyObject.Create('test');
  end)
  .ThenBy<TMyObject>(function (const AValue: TObject): TMyObject
  begin
    Result := TMyObject.Create('test2');
  end); //Here AValue will be disposed

var LPromise2 := Promise.Resolve<TMyObject>(function: TMyObject
  begin
    Result := TMyObject.Create('test');
  end)
  .Op.ThenBy<String>(function (const AValue: TObject): String
  begin
    Result := 'test2;'
  end); //Here AValue will be disposed

From TObject to TInterface

An interesting case is when you return an interface that points to the object that was passed as the argument to ThenBy (the value of the previous resolved promise). Although the return type differs from the argument, the argument (object) will not always be disposed. If the returned interface points to the object passed in the argument, this object will not be disposed.

var LPromise1 := Promise.Resolve<TMyObject>(function: TMyObject
  begin
    Result := TMyObject.Create('test');
  end)
  .Op.ThenBy<IMyObject>(function (const AValue: TObject): IMyObject
  begin
    Result := AValue;
  end); //Here AValue will not be disposed

var LPromise2 := Promise.Resolve<TMyObject>(function: TMyObject
  begin
    Result := TMyObject.Create('test');
  end)
  .Op.ThenBy<IMyObject>(function (const AValue: TObject): IMyObject
  begin
    Result := TMyObject.Create('test2');
  end); //Here AValue will be disposed

Changing the default behavior

You can interfer with the default memory management of resolved values within promises using two directives:

dvKeep: Indicates that the promise should transfer ownership of the resolved value and that it is not disposed at the end of the promise' lifecycle. This can be necessary to move objects in another object (for example putting the result of Promise.All in a TObjectList).
dvFree: Specifies that the promise should take responsibility for freeing the resolved value, suitable for managing the lifecycle of dynamically created objects within asynchronous operations. This is the default behavior.

Example of Memory Management

In the following example we instruct the promise to not dispose the object after adding it to the newly created TObjectList.

var
  LObjectList: TObjectList<TMyObject>;
begin
  LObjectList := Promise.Resolve<TMyObject>(function: TMyObject
      begin
        Result := TMyObject.Create('test');
      end)
    .Op.ThenBy<TObjectList<TMyObject>>(function(const o: TMyObject): TObjectList<TMyObject>
      begin
        Result := TObjectList<TMyObject>.Create();
        Result.Add(o)
      end, TDisposeValue.dvKeep)    // Do not dispose the argument 'o' 
    .Catch(function(e: Exception): TObjectList<TMyObject>
      begin
        Result := TObjectList<TMyObject>.Create();
      end)
    .Await;

Extended example: using Promises in Asynchronous Methods

Promises can be elegantly integrated into methods to encapsulate asynchronous operations, offering a streamlined approach to handling such tasks. This section demonstrates how to implement a method that performs an asynchronous operation (like fetching data) and returns a IPromise<T> where T is the type of the data being fetched.

In this example, we define a method FetchUserData within a TUserRepository class that simulates fetching user data asynchronously and returns a promise of TUserData.

Defining the Data Structure

First, define a data structure TUserData to hold user information:

type
  TUserData = record
    UserID: Integer;
    UserName: String;
    Email: String;
  end;

Implementing the Asynchronous Method

Next, implement the FetchUserData method in the TUserRepository class:

uses
  Next.Core.Promises;

type
  TUserRepository = class
  public
    function FetchUserData(const UserID: Integer): IPromise<TUserData>;
  end;

function TUserRepository.FetchUserData(const UserID: Integer): IPromise<TUserData>;
begin
  // Return a promise that resolves with the user data
  Result := Promise.Resolve<TUserData>(function: TUserData
    begin
      // Simulate an asynchronous data fetching operation
      Sleep(1000); // Simulate delay
      Result.UserID := UserID;
      Result.UserName := 'John Doe';
      Result.Email := '[email protected]';
    end);
end;

This method returns a IPromise<TUserData>, encapsulating the asynchronous fetching operation.

Using the Asynchronous Method

To use FetchUserData, call the method and handle the result, either by awaiting the promise or by chaining additional operations:

var
  UserRepository: TUserRepository;
  UserData: TUserData;
begin
  UserRepository := TUserRepository.Create;
  try
    // Fetch user data and await the promise
    UserData := UserRepository.FetchUserData(123).Await;

    // Use the fetched data, ensuring any UI updates are synchronized with the main thread
    WriteLn('User ID: ' + IntToStr(UserData.UserID));
    WriteLn('User Name: ' + UserData.UserName);
    WriteLn('Email: ' + UserData.Email);
  finally
    UserRepository.Free;
  end;
end;

This approach simplifies managing asynchronous operations by encapsulating them within methods that return promises. It demonstrates how to perform asynchronous operations, await their completion, and safely update the UI with the results.

License

This project is licensed under the MIT License - see the LICENSE file for details.

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