MycLib/Src/Myc.Core.Notifier.pas

unit Myc.Core.Notifier;

interface

uses
    System.SysUtils,
    System.SyncObjs;

type
    // A thread-safe, low-level list for managing multicast event sinks of type IInterface.
    // It uses a doubly linked list and a spinlock for synchronization.
    // `Advise` registers a sink and returns a tag for constant-time removal via `Unadvise`.
    TMycNotifyList<T: IInterface> = record
        type
            // Opaque tag used to identify a registered receiver for unsubscription.
            TTag = Pointer;

            // Pointer to an internal list item.
            PItem = ^TItem;
            // Internal structure for storing a receiver and list linkage.
            TItem = record
                Next, Prev: PItem; // Pointers to the next and previous items in the doubly linked list.
                Receiver: T; // The registered interface instance (the event sink).
            end;

            // Notify function. If this results false, it will be removed from the notify list
            TNotifyProc = reference to function(const Obj: T): Boolean;

    strict private
        // Head of the linked list. The pointer value itself is repurposed for a spinlock.
        // Bit 0 of the address stores the lock state (0 = locked, 1 = unlocked).
        [volatile]
        FList: PItem;

        // Allocates memory for a new TItem.
        class function AllocItem: PItem; static; inline;
        // Frees memory previously allocated for a TItem.
        class procedure FreeItem(Item: PItem); static; inline;

    private
        function GetFirst: PItem;

    public
        // Initializes the list in an unlocked state.
        class operator Initialize(out Dest: TMycNotifyList<T>);
        // Safely clears all registered receivers and cleans up resources.
        procedure Finalize;
        // Registers a receiver interface and returns an opaque tag for later unsubscription.
        function Advise(const Receiver: T): TTag;
        // Unregisters a single receiver using its subscription tag.
        procedure Unadvise(Tag: TTag);
        // Unregisters all currently advised receivers.
        procedure UnadviseAll;
        // Acquires an exclusive lock for thread-safe operations on the list.
        procedure Lock; inline;
        // Releases the previously acquired exclusive lock.
        procedure Release; inline;
        // Checks if the list is currently locked by any thread.
        function IsLocked: Boolean; inline;
        // Invokes a predicate for each registered receiver.
        // If the predicate returns false, the receiver is detached from further notifications
        // by setting its interface reference to nil. The list item itself is not freed here.
        procedure Notify(const Func: TNotifyProc);
        property First: PItem read GetFirst;
    end;

implementation

class operator TMycNotifyList<T>.Initialize(out Dest: TMycNotifyList<T>);
begin
    NativeUInt(Dest.FList) := 1;
end;

procedure TMycNotifyList<T>.Finalize;
begin
    Lock;
    UnadviseAll;
end;

function TMycNotifyList<T>.Advise(const Receiver: T): TTag;
var
    Item: PItem;
begin
    Assert(IsLocked);

    if not Assigned(Receiver) then
        exit(nil);

    Item := AllocItem;
    Item.Receiver := Receiver;
    Item.Prev := nil;
    Item.Next := FList;
    if Item.Next <> nil then
        Item.Next.Prev := Item;

    FList := Item;

    exit(Item);
end;

procedure TMycNotifyList<T>.Lock;
begin
    while not TInterlocked.BitTestAndClear(NativeUint(FList), 0) do
        YieldProcessor;

    Assert(IsLocked, 'Locking failed');
end;

class function TMycNotifyList<T>.AllocItem: PItem;
begin
    Result := AllocMem(sizeof(TItem));
end;

class procedure TMycNotifyList<T>.FreeItem(Item: PItem);
begin
    FreeMem(Item, sizeof(TItem));
end;

function TMycNotifyList<T>.GetFirst: PItem;
begin
    Assert(IsLocked);
    Result := FList;
end;

function TMycNotifyList<T>.IsLocked: Boolean;
begin
    Result := NativeUInt(FList) and 1 = 0;
end;

procedure TMycNotifyList<T>.Notify(const Func: TNotifyProc);
var
    Item: PItem;
begin
    Assert(IsLocked);

    var rev: PItem := nil;
    var tmp: PItem := nil;

    while (FList <> nil) and Assigned(FList.Receiver) do
    begin
        Item := FList;
        FList := Item.Next;

        if Func(Item.Receiver) then
        begin
            Item.Next := rev;
            rev := Item;
        end
        else
        begin
            // release the receiver
            Item.Receiver := nil;

            Item.Next := tmp;
            tmp := Item;
        end;
    end;

    while tmp <> nil do
    begin
        Item := tmp;
        tmp := Item.Next;

        Item.Prev := nil;
        Item.Next := FList;
        FList := Item;
        if Item.Next <> nil then
            Item.Next.Prev := Item;
    end;

    while rev <> nil do
    begin
        Item := rev;
        rev := Item.Next;

        Item.Prev := nil;
        Item.Next := FList;
        FList := Item;
        if Item.Next <> nil then
            Item.Next.Prev := Item;
    end;
    {
    tmp := nil;
    Last := nil;
    Item := FList;
    while (Item <> nil) and Assigned(Item.Receiver) do
    begin
        if not Func(Item.Receiver) then
        begin
            // Receiver wants no more notifications, detach it
            if Item = FList then
                FList := Item.Next;

            if Item.Prev <> nil then
                Item.Prev.Next := Item.Next;
            if Item.Next <> nil then
                Item.Next.Prev := Item.Prev;

            // release the receiver
            Item.Receiver := nil;

            // and save the list item in a tmp list for later use
            var nxt := Item.Next;
            Item.Next := tmp;
            tmp := Item;
            Item := nxt;
        end
        else
        begin
            Last := Item;
            Item := Last.Next;
        end;
    end;

    // Append all detached items at end of the list. We can't simply free them, because the subscription is owned by the client.
    while tmp <> nil do
    begin
        Item := tmp;
        tmp := Item.Next;

        if Last = nil then
        begin
            Item.Prev := nil;
            Item.Next := FList;
            FList := Item;
        end
        else
        begin
            Item.Prev := Last;
            Item.Next := Last.Next;
            if Item.Prev <> nil then
                Item.Prev.Next := Item;
        end;
        if Item.Next <> nil then
            Item.Next.Prev := Item;
    end;
    }
end;

procedure TMycNotifyList<T>.Release;
begin
    Assert(IsLocked);
    TInterlocked.BitTestAndSet(NativeUint(FList), 0);
end;

procedure TMycNotifyList<T>.Unadvise(Tag: TTag);
var
    Item: PItem;
begin
    Assert(IsLocked);

    if (Tag = nil) or (FList = nil) then
        exit;

{$ifdef DEBUG}
    Item := FList;
    while Item <> nil do
    begin
        if Item = Tag then
            break;
        Item := Item.Next;
    end;
    Assert(Item <> nil, 'Receiver not advised');
{$endif}

    if FList = nil then
        exit;

    Item := PItem(Tag);
    Item.Receiver := nil;

    if Item = FList then
        FList := Item.Next;

    if Item.Prev <> nil then
        Item.Prev.Next := Item.Next;
    if Item.Next <> nil then
        Item.Next.Prev := Item.Prev;

    FreeItem(Item);
end;

procedure TMycNotifyList<T>.UnadviseAll;
begin
    Assert(IsLocked);
    while FList <> nil do
        Unadvise(TTag(FList));
end;

end.