WeakObjectReferences

A Delphi Unit that implements weak references for objects. It is compatible with Delphi 2009-XE4 Win32/Win64.

The unit should be placed in the Project's uses list directly after any Memory Manager unit because it hooks some TObject/TMonitor RTL functions. Adding the unit to a DLLs/BPLs can cause problems because the hooking can't be undone, objects are still be alive. So unloading the DLL/BPL is a no-go.

Usage

var
  MyWeakRef, YourWeakRef, HisWeakRef: TWeakReference<TObject>;
  IntfWeakRef: IWeakReference<TObject>;
  MyObject: TObject;
begin
  MyObject := TObject.Create;
  try
    MyWeakRef := MyObject; // implicitly create wrapped weak reference
    YourWeakRef := TWeakReference<TObject>.Create(MyObject); // explicity create wrapped weak reference
    HisWeakRef := YourWeakRef; // copy wrapped weak reference
    IntfWeakRef := TWeakReference<TObject>.New(MyObject); // create weak reference

    Assert(MyWeakRef = YourWeakRef);
    Assert(MyWeakRef.Target <> nil);
    Assert(MyWeakRef = MyObject);
    Assert(MyWeakRef.IsAlive);
    Assert(IntfWeakRef.Target <> nil);
  finally
    MyObject.Free;
  end;
  Assert(MyWeakRef.Target = nil);
  Assert(MyWeakRef = nil);
  Assert(not MyWeakRef.IsAlive);
  Assert(YourWeakRef.Target = nil);
  Assert(HisWeakRef.Target = nil);
  Assert(IntfWeakRef.Target = nil);
end;

Limitation

It is not thread-safe to destroy an object in thread A and access the WeakReference.Target in thread B. For this to work the object would have to be reference counted (like in ARC) and a locking mechanism for the _ObjectRelease and IWeakReference.Target would be needed.

How it works

The first time a WeakReference is created, a 32-36 bytes large record is allocated for the object and stored into the TObject.Monitor field. This record implements the IWeakReference interface with a RefCount and a pointer to the referenced object. All WeakReferences for one object point to this record. The record stays alive until the last IWeakReference is released. The object itself holds one reference to the IWeakReference interface and when the object is destroyed it sets the object reference in the record to nil, nil-ing out every IWeakReference for this object and the object's reference to the record is released, what let the IWeakReference._Release method clean up after the last IWeakReference interface gets out of scope.

Additional data storage

It is possible add additional data to an object through the TInternalWeakReferenceHelper.RegisterAdditionalData method and then query it by using TInternalWeakReferenceHelper.GetAdditionalData. This can be used to implement an alternative TMonitor construct.

What happens to the TObject.Monitor field

The original hidden TObject.Monitor field is moved into the IWeakReference record and all calls from the TMonitor to get the value of the field are redirected to the record's Monitor field.

License

The project is subject to the Mozilla Public License Version 1.1 (the "License");
you may not use this project except in compliance with the License. You may obtain a copy of the License at http://www.mozilla.org/MPL/

Software distributed under the License is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License for the specific language governing rights and limitations under the License.

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ahausladen / WeakObjectReferences