Interfaces SOLID Mock Stub IoC Delphi OpenSource mORMot

1. Software Architecture & Design
 Architecture
 From n-Tier to SOA
 From SOAP to REST
 Technical Debt
 Design
 From SQL to ORM, NoSQL and ODM
 From RAD to MVC
 SOLID principles
 Domain Driven Design (DDD)
Applying patterns on Delphi code using mORMot
Software Architecture & Design

2. From Interfaces to SOLID

3. From Interfaces to SOLID
 Delphi and interfaces
 SOLID design principles
 Dependency Injection, stubs and mocks
 Using mORMot features
 Delphi and Weak pointers
From Interfaces to SOLID

4. Delphi and interfaces
 In Delphi OOP model
 An interface defines a type
that comprises abstract virtual methods
 It is a declaration of functionality
without an implementation of that functionality
 It defines "what" is available,
not "how" it is made available
From Interfaces to SOLID

5. Delphi and interfaces
 Declaring an interface
 Naming convention: ICalculator
 No visibility attribute: all published
 No fields, just methods and properties
 Unique identifier by GUID (Ctrl Shift G)
From Interfaces to SOLID
type
ICalculator = interface(IInvokable)
['{9A60C8ED-CEB2-4E09-87D4-4A16F496E5FE}']
/// add two signed 32 bit integers
function Add(n1,n2: integer): integer;
end;

6. Delphi and interfaces
 Implementing an interface
 ICalculator added to the “inheritance chain”
 TCalculator implements a behavior
From Interfaces to SOLID
type
TServiceCalculator = class(TInterfacedObject, ICalculator)
protected
fBulk: string;
public
function Add(n1,n2: integer): integer;
procedure SetBulk(const aValue: string);
end;
function TServiceCalculator.Add(n1, n2: integer): integer;
begin
result := n1+n2;
end;
procedure TServiceCalculator.SetBulk(const aValue: string);
begin
fBulk := aValue;
end;

7. Delphi and interfaces
 Using an interface
 Strong typing
 The variable defines a behavior (contract)
 Path to abstraction
From Interfaces to SOLID
function MyAdd(a,b: integer): integer;
var Calculator: ICalculator;
begin
Calculator := TServiceCalculator.Create;
result := Calculator.Add(a,b);
end;

8. Delphi and interfaces
 Using an interface
 Strong typing
 The variable defines a behavior (contract)
 Path to abstraction
 Automatic try..finally
From Interfaces to SOLID
function MyAdd(a,b: integer): integer;
var Calculator: TServiceCalculator;
begin
Calculator := TServiceCalculator.Create;
try
result := Calculator.Add(a,b);
finally
Calculator.Free;
end;
end;
function MyAdd(a,b: integer): integer;
var Calculator: ICalculator;
begin
Calculator := TServiceCalculator.Create;
result := Calculator.Add(a,b);
end;

9. Delphi and interfaces
 Automatic try..finally
 Compiler generates
some hidden code…
 Behavior inherited from TInterfacedObject
 Similar to COM / ActiveX
From Interfaces to SOLID
function MyAdd(a,b: integer): integer;
var Calculator: ICalculator;
begin
Calculator := TServiceCalculator.Create;
result := Calculator.Add(a,b);
end;
function MyAdd(a,b: integer): integer;
var Calculator: TServiceCalculator;
begin
Calculator := nil;
Calculator := TServiceCalculator.Create;
try
Calculator.FRefCount := 1;
result := Calculator.Add(a,b);
finally
dec(Calculator.FRefCount);
if Calculator.FRefCount=0 then
Calculator.Free;
end;
end;

10. Delphi and interfaces
 Interfaces are orthogonal to implementation
 There is more than one way to do it
From Interfaces to SOLID
type
TOtherServiceCalculator = class(TInterfacedObject, ICalculator)
protected
function Add(n1,n2: integer): integer;
end;
function TOtherServiceCalculator.Add(n1, n2: integer): integer;
begin
result := n2+n1;
end;
function MyOtherAdd(a,b: integer): integer;
var Calculator: ICalculator;
begin
ICalculator := TOtherServiceCalculator.Create;
result := Calculator.Add(a,b);
end;

11. SOLID design principles
 Single responsibility principle
 Open/closed principle
 Liskov substitution principle (design by contract)
 Interface segregation principle
 Dependency inversion principle
From Interfaces to SOLID

12. SOLID design principles
 Single responsibility
 Object should have only a single responsibility
 Open/closed
 Entities should be open for extension,
but closed for modification
 Liskov substitution (design by contract)
 Objects should be replaceable with instances of their
subtypes without altering the correctness of that program
 Interface segregation
 Many specific interfaces are better than one
 Dependency inversion
 Depend upon abstractions, not depend upon concretions
From Interfaces to SOLID

13. SOLID design principles
 Help to fight well-known weaknesses
 Rigidity
 Hard to change something because every change
affects too many other parts of the system
 Fragility
 When you make a change, unexpected parts of the
system break
 Immobility
 Hard to reuse in another application because it cannot
be disentangled from the current application
From Interfaces to SOLID

14. SOLID design principles
 Single Responsibility
 When you define a class, it shall be designed to
implement only one feature
 The so-called feature can be seen as
an "axis of change" or a "a reason for change"
From Interfaces to SOLID

15. SOLID design principles
 Single Responsibility
 One class shall have only one reason
that justifies changing its implementation
 Classes shall have few dependencies
on other classes
 Classes shall be abstracted
from the particular layer they are running
From Interfaces to SOLID

16. SOLID design principles
 Single Responsibility
 Splitting classes
 Imagine a TBarCodeScanner class
to handle a serial bar-code scanner
 Later on, we want to implement USB support
 First idea may be to inherit
From Interfaces to SOLID

17. SOLID design principles
 Single Responsibility
 Splitting classes
 Imagine a TBarCodeScanner class
to handle a serial bar-code scanner
 Later on, we want to implement USB support
 But we would rather split the class hierarchy
From Interfaces to SOLID

18. SOLID design principles
 Single Responsibility
 Splitting classes
 Later on, the manufacturer updates its protocol
To add new features, e.g. 3D scanning or coffee making
 How do we implement this?
 We have two “axis of change”
so we would define two classes
From Interfaces to SOLID

19. SOLID design principles
 Single Responsibility
 Splitting classes
 Later on, the manufacturer updates its protocol
To add new features, e.g. 3D scanning or coffee making
 We defined two classes, which will be joined/composed
in actual barcode scanner classes
From Interfaces to SOLID

20. SOLID design principles
 Single Responsibility
 Splitting classes
TAbstractBarcodeScanner = class(TComponent)
protected
fProtocol: TAbstractBarcodeProtocol;
fConnection: AbstractBarcodeConnection;
...
constructor TSerialBarCodeScanner.Create(
const aComPort: string; aBitRate: integer);
begin
fConnection := TSerialBarcodeConnection(aComPort,aBitRate);
fProtocol := TBCP1BarcodeProtocol.Create(fConnection);
end;
From Interfaces to SOLID

21. SOLID design principles
 Single Responsibility
 Another example
from SynDB class definitions:
 TSQLDBConnectionProperties
 TSQLDBConnection
 TSQLDBStatement
 Each class has its own purpose,
and axis of change
 And could be implemented via ODBC, OleDB,
direct client access…
From Interfaces to SOLID

22. SOLID design principles
 Single Responsibility
 Do not mix GUI and logic
 Do not mix logic and database
 Do not couple your code to an OS
 Check your uses statement
 There should be no reference to the UI
(e.g. Dialogs) in your business class
 No dependency to DB libraries (this is a hard one)
 No reference to WinAPI.Windows
From Interfaces to SOLID

23. SOLID design principles
 Single Responsibility
 Code smells
 When abusing of:
FreeAndNil()
{$ifdef} … {$endif}
uses unit1, unit2, unit3, … unit1000;
 When you can’t find the right method in a class
 (mORMot may need some refactoring here) 
 When unitary tests are hard to write
From Interfaces to SOLID

24. SOLID design principles
 Open / Close principle
 When you define a class or an interface
 it shall be open for extension
 but closed for modification
From Interfaces to SOLID

25. SOLID design principles
 Open / Closed principle
 Open for extension
 Abstract class is overridden by implementations
 No singleton nor global variable – ever
 Rely on abstraction
 if aObject is aClass then … it stinks!
 Closed for modification
 E.g. via explicitly protected or private members
 RTTI is dangerous: it may open the closed door
From Interfaces to SOLID

26. SOLID design principles
 Open / Closed principle
 In practice
 Define meaningful interface types
 Following the Design By Contract principle
 Define Value objects (DTOs) to transfer the data
 With almost no methods, but for initialization
 With public members, to access the value
 Define Process objects to modify the data
 With public methods, abstracted in the interface
 With mostly private members
From Interfaces to SOLID

27. SOLID design principles
 Open / Close principle
type
TAbstractBarcodeScanner = class(TComponent)
protected
fProtocol: TAbstractBarcodeProtocol;
fConnection: AbstractBarcodeConnection;
public
property Protocol: TAbstractBarcodeProtocol read fProtocol;
property Connection: AbstractBarcodeConnection read fConnection;
...
 Protocol and Connection are read/only
 They are injected by the overridden class constructor
 So it will be Open for extension
From Interfaces to SOLID

28. SOLID design principles
 Open / Close principle
type
TAbstractBarcodeScanner = class(TComponent)
protected
fProtocol: TAbstractBarcodeProtocol;
fConnection: AbstractBarcodeConnection;
public
property Protocol: TAbstractBarcodeProtocol read fProtocol;
property Connection: AbstractBarcodeConnection read fConnection;
...
 Protocol and Connection are read/only
 You could not change the behavior of a class
 It is Closed for modification
From Interfaces to SOLID

29. SOLID design principles
 Liskov substitution principle
From Interfaces to SOLID

30. SOLID design principles
 Liskov substitution principle
From Interfaces to SOLID

31. SOLID design principles
 Liskov substitution principle
 If TChild is a subtype of TParent
 then objects of type TParent
may be replaced with objects of type TChild
 without altering any of the desirable properties
of that program (correctness, task performed, etc.)
From Interfaces to SOLID

32. SOLID design principles
 Liskov substitution principle
 Tied to the Open / Closed principle
 If you define a child, you should not modify the parent
 Code-reusability of the parent implementation
 You will be able
to stub or mock an interface or a class
 Allow correct testing of a whole system:
even if all single unit tests did pass,
real system may not work if this principle was broken
From Interfaces to SOLID

33. SOLID design principles
 Liskov substitution patterns
 Design by contract
 Meyer's (Eiffel language) rule:
“when redefining a routine [in a derivative],
you may only replace its precondition by a weaker
one, and its postcondition by a stronger one”
 Define assertions at method level:
 What does it expect, guarantee, and maintain?
 About input/output values or types, side effects,
invariants, errors/exceptions, performance…
From Interfaces to SOLID

34. SOLID design principles
 Liskov substitution patterns
 Write your code using abstract variables
 Rely on parents methods and properties
 Use interface variables
instead of class implementation
 Uncouple dependencies via class composition
From Interfaces to SOLID

35. SOLID design principles
 Liskov substitution patterns
 Factory pattern
 In strongly typed languages (Java, C#, Delphi),
the factory is the class/object type and its constructor
 Repository pattern
 Allow persistence agnosticism (e.g. via ORM)
 Service locator pattern
 Get a class instance implementing a given interface
 According to the given context (no global/singleton)
From Interfaces to SOLID

36. SOLID design principles
 Liskov substitution patterns
 Practical, not dogmatic LSP
 “Parent” and “Child” are not absolute
 Depending on the context, you may define a given level
as the “highest” LSP abstract class
 e.g. if you work on server side, you may need some
server-only properties and methods
 LSP idea is to be consistent,
once the abstraction level is defined
From Interfaces to SOLID

37. SOLID design principles
 Liskov substitution patterns
 Practical, not dogmatic LSP
 “Parent” and “Child” are not absolute
 Inheritance is often used as code sharing among classes,
not as an abstraction contract
 In this context, LSP may not apply at class level:
be pragmatic, and write efficient code
 But LSP may apply at interface level,
for a set of methods implemented by the class
From Interfaces to SOLID

38. SOLID design principles
 Liskov substitution code smells
if aObject is aClass then …
case aObject.EnumeratedType of …
function … abstract;
without further override;
unit parent;
uses child1,child2,child3;
From Interfaces to SOLID

39. SOLID design principles
 Interface segregation principle
 Once an interface has become too 'fat'
it shall be split into smaller
and more specific interfaces
so that any clients of the interface will only know
about the methods that pertain to them
 In a nutshell, no client should be forced
to depend on methods it does not use
From Interfaces to SOLID

40. SOLID design principles
 Interface segregation principle
 Smaller dedicated classes should be preferred
 Single Responsibility Principle
 Favor composition over inheritance
 Smaller dedicated interfaces
 Gather operations/methods per context
 Meaningful naming and conventions
 So that the contract would be easier to understand
From Interfaces to SOLID

41. SOLID design principles
 Interface segregation principle
 Perfectly fits the SOA uncoupling pattern
 Stateless MicroServices for horizontal scaling
 Allows to release memory and resources ASAP
 Smaller objects have more bounded dependencies
 Ease unit testing
 Less coverage
 Less coupling
From Interfaces to SOLID

42. SOLID design principles
 Interface segregation principle
 Excludes RAD
 Logic implemented in TForm TDataModule
where methods are procedural code in disguise
 Put your logic behind interfaces
and call them from your UI (over VCL and FMX)
 Favors DDD
 Segregation avoid domain leaking
 Dedicated interfaces, e.g. for third party consumption
From Interfaces to SOLID

43. SOLID design principles
 Dependency Inversion
 High-level modules
should not depend on low-level modules
 Both should depend on abstractions
 Following other SOLI principles
 Abstractions should not depend upon details
 Details should depend upon abstractions
 Business Logic should not depend on database
 Application Layer should not depend on client UI techno
From Interfaces to SOLID

44. SOLID design principles
 Dependency Inversion
 In most conventional programming style:
 You write low-level components (e.g. DB tables)
 Then you integrate them with high-level components
 But this limits the re-use of high-level code
 In fact, it breaks the Liskov substitution principle
 It reduces the testing abilities (e.g. need of a real DB)
From Interfaces to SOLID

45. SOLID design principles
 Dependency Inversion may be implemented
 Via a plug-in system
 e.g. external libraries (for embedded applications)
 Using a service locator
 e.g. SOA catalog (SaaS/cloud)
 class resolution from an interface type
From Interfaces to SOLID

46. SOLID design principles
 Dependency Inversion may be implemented
 Via Dependency Injection
 Concretions are injected to the objects using them
 Only abstractions are known at design/coding time
 Uncoupled implementation injected at runtime
 Modular coding
 Perfect for bigger/complex projects, with a set of teams
 Ease testing, via stub/mock dependencies injection
 e.g. a fake database, defined as a Repository service
 Scaling abilities
From Interfaces to SOLID

47. SOLID design principles
 Dependency Inversion may be implemented
 Via Dependency Injection
 A class will define its dependencies as read-only
interface members
 Implementation will be injected at constructor level
 via explicit constructor parameters
 via automated resolution via RTTI
 via service locator/resolver
From Interfaces to SOLID

48. DI, Stubs and Mocks
 Thanks to SOLID design principles
 All your code logic will now be abstracted
to the implementation underneath
 But you need to inject the implementation
 This is Dependency Injection purpose
 You can also create fake instances to implement
a given interface, and enhance testing
 Introducing Stubs and Mocks
From Interfaces to SOLID

49. DI, Stubs and Mocks
 Dependency Injection
 Define external dependencies as interface
 as (private / protected) read-only members
 To set the implementation instance:
 Either inject the interfaces as constructor parameters
 Or use a Factory / Service locator
From Interfaces to SOLID

50. DI, Stubs and Mocks
 Dependency Injection
 Purpose is to test the following class:
From Interfaces to SOLID
TLoginController = class(TInterfacedObject,ILoginController)
protected
fUserRepository: IUserRepository;
fSmsSender: ISmsSender;
public
constructor Create(const aUserRepository: IUserRepository;
const aSmsSender: ISmsSender);
procedure ForgotMyPassword(const UserName: RawUTF8);
end;
constructor TLoginController.Create(const aUserRepository: IUserRepository;
const aSmsSender: ISmsSender);
begin
fUserRepository := aUserRepository;
fSmsSender := aSmsSender;
end;

51. DI, Stubs and Mocks
 Dependency Injection
 Dependencies are defined as
 Two small, uncoupled, SOLID task-specific interfaces
From Interfaces to SOLID
IUserRepository = interface(IInvokable)
['{B21E5B21-28F4-4874-8446-BD0B06DAA07F}']
function GetUserByName(const Name: RawUTF8): TUser;
procedure Save(const User: TUser);
end;
ISmsSender = interface(IInvokable)
['{8F87CB56-5E2F-437E-B2E6-B3020835DC61}']
function Send(const Text, Number: RawUTF8): boolean;
end;

52. DI, Stubs and Mocks
 Dependency Injection
 Using a dedicated Data Transfer Object (DTO)
 No dependency against storage, nor other classes
From Interfaces to SOLID
IUserRepository = interface(IInvokable)
['{B21E5B21-28F4-4874-8446-BD0B06DAA07F}']
function GetUserByName(const Name: RawUTF8): TUser;
procedure Save(const User: TUser);
end;
TUser = record
Name: RawUTF8;
Password: RawUTF8;
MobilePhoneNumber: RawUTF8;
ID: Integer;
end;

53. DI, Stubs and Mocks
 Dependency Injection
 The high-level method to be tested:
 Open/Closed, Liskov and mainly Dependency
Inversion principles are followed
 Will we need a full database and to send a SMS?
From Interfaces to SOLID
procedure TLoginController.ForgotMyPassword(const UserName: RawUTF8);
var U: TUser;
begin
U := fUserRepository.GetUserByName(UserName);
U.Password := Int32ToUtf8(Random(MaxInt));
if fSmsSender.Send('Your new password is '+U.Password,U.MobilePhoneNumber) then
fUserRepository.Save(U);
end;

54. DI, Stubs and Mocks
 "The Art of Unit Testing" (Osherove, Roy - 2009)
 Stubs are fake objects
implementing a given contract
and returning pre-arranged responses
 They just let the test pass
 They “emulate” some behavior (e.g. a database)
From Interfaces to SOLID

55. DI, Stubs and Mocks
 "The Art of Unit Testing" (Osherove, Roy - 2009)
 Mocks are fake objects like stubs
which will verify if an interaction occurred or not
 They help decide if a test failed or passed
 There should be only one mock per test
From Interfaces to SOLID

56. DI, Stubs and Mocks
From Interfaces to SOLID

57. DI, Stubs and Mocks
 Expect – Run – Verify pattern
From Interfaces to SOLID
procedure TMyTest.ForgotMyPassword;
var SmsSender: ISmsSender;
UserRepository: IUserRepository;
begin
TInterfaceStub.Create(ISmsSender,SmsSender).
Returns('Send',[true]);
TInterfaceMock.Create(IUserRepository,UserRepository,self).
ExpectsCount('Save',qoEqualTo,1);
with TLoginController.Create(UserRepository,SmsSender) do
try
ForgotMyPassword('toto');
finally
Free;
end;
end;

58. DI, Stubs and Mocks
 Expect – Run – Verify pattern
 TInterfaceStub / TInterfaceMock constructors
are in fact Factories for any interface
 Clear distinction between stub and mock
 Mock is linked to its test case (self: TMyTest)
From Interfaces to SOLID
procedure TMyTest.ForgotMyPassword;
var SmsSender: ISmsSender;
UserRepository: IUserRepository;
begin
TInterfaceStub.Create(ISmsSender,SmsSender).
Returns('Send',[true]);
TInterfaceMock.Create(IUserRepository,UserRepository,self).
ExpectsCount('Save',qoEqualTo,1);

59. DI, Stubs and Mocks
 Expect – Run – Verify pattern
 Execution code itself sounds like real-life code
 But all dependencies have been injected
 Stubs will emulate real behavior
 Mock will verify that all expectations are fulfilled
From Interfaces to SOLID
with TLoginController.Create(UserRepository,SmsSender) do
try
ForgotMyPassword('toto');
finally
Free;
end;
end;

60. DI, Stubs and Mocks
 Expect – Run – Verify pattern
From Interfaces to SOLID
procedure TMyTest.ForgotMyPassword;
var SmsSender: ISmsSender;
UserRepository: IUserRepository;
begin
TInterfaceStub.Create(ISmsSender,SmsSender).
Returns('Send',[true]);
TInterfaceMock.Create(IUserRepository,UserRepository,self).
ExpectsCount('Save',qoEqualTo,1);
with TLoginController.Create(UserRepository,SmsSender) do
try
ForgotMyPassword('toto');
finally
Free;
end;
end;

61. DI, Stubs and Mocks
 Run – Verify (aka “Test spy”) pattern
From Interfaces to SOLID
procedure TMyTest.ForgotMyPassword;
var SmsSender: ISmsSender;
UserRepository: IUserRepository;
Spy: TInterfaceMockSpy;
begin
TInterfaceStub.Create(ISmsSender,SmsSender).
Returns('Send',[true]);
Spy := TInterfaceMockSpy.Create(IUserRepository,UserRepository,self);
with TLoginController.Create(UserRepository,SmsSender) do
try
ForgotMyPassword('toto');
finally
Free;
end;
Spy.Verify('Save');
end;

62. DI, Stubs and Mocks
 Another features:
 Return complex values (e.g. a DTO)
 Use a delegate to create a stub/mock
 Using named or indexed variant parameters
 Using JSON array of values
 Access the test case when mocking
 Trace and verify the calls
 With a fluent interface
 Log all calls (as JSON)
From Interfaces to SOLID

63. DI, Stubs and Mocks
 Dependency Injection
 Inheriting from TInjectableObject
type
TServiceToBeTested = class(TInjectableObject,IServiceToBeTested)
protected
fService: IInjectedService;
published
property Service: IInjectedService read fService;
end;
 Will auto-inject interface published properties
 At instance creation
 Handled by TSQLRest.Services
From Interfaces to SOLID

64. DI, Stubs and Mocks
 Dependency Injection
 Inheriting from TInjectableObject
var Test: IServiceToBeTested;
begin
Test := TServiceToBeTested.CreateInjected(
[ICalculator],
[TInterfaceMock.Create(IPersistence,self).
ExpectsCount('SaveItem',qoEqualTo,1),
RestInstance.Services],
[AnyInterfacedObject]);
...
From Interfaces to SOLID

65. DI, Stubs and Mocks
 Dependency Injection
 Inheriting from TInjectableObject
procedure TServiceToBeTested.AnyProcessMethod;
var Service: IInjectedService;
begin
Resolve(IInjectedService,Service);
Service.DoSomething;
end;
From Interfaces to SOLID

66. DI, Stubs and Mocks
 Dependency Injection
 Inheriting from TInjectableAutoCreateFields
type
TServiceToBeTested = class(TInjectableObjectAutoCreateFields,
IServiceToBeTested)
protected
fService: IInjectedService;
fNestedObject: TSynPersistentValue;
published
property Service: IInjectedService read fService;
property NestedObject: TSynPersistentValue read fNestedObject;
end;
 Will auto-define published properties
 Resolve interface services
 Create TPersistent TSynPersistent TAutoCreateField
From Interfaces to SOLID

67. Weak references
 Delphi type reference model
 class
 as weak references (plain pointer) and explicit Free
 with TComponent ownership for the VCL/FMX
 integer Int64 currency double record
widestring variant
 with explicit copy
 string or any dynamic array
 via copy-on-write (COW) with reference counting
 interface
 as strong reference with reference counting
From Interfaces to SOLID

68. Weak references
 Strong reference-counted types (OLE/ COM)
 Will increase the count at assignment
 And decrease the count at owner’s release
 When the count reaches 0, release the instance
 Issue comes when there are
 Circular references
 External list(s) of references
From Interfaces to SOLID

69. Weak references
 Managed languages (C# or Java)
 Will let the Garbage Collector handle
interface variable life time
 This is complex and resource consuming
 But easy to work with
 Unmanaged languages (Delphi or ObjectiveC)
 Need explicit weak reference behavior
 mORMot features zeroing weak pointers
 Like Apple’s ARC model
From Interfaces to SOLID

70. Weak references
 Zeroing weak pointers
From Interfaces to SOLID
IParent = interface
procedure SetChild(const Value: IChild);
function GetChild: IChild;
function HasChild: boolean;
property Child: IChild read GetChild write SetChild;
end;
IChild = interface
procedure SetParent(const Value: IParent);
function GetParent: IParent;
property Parent: IParent read GetParent write SetParent;
end;

71. Weak references
 Zeroing weak pointers
This code will leak memory
From Interfaces to SOLID
IParent = interface
procedure SetChild(const Value: IChild);
function GetChild: IChild;
function HasChild: boolean;
property Child: IChild read GetChild write SetChild;
end;
IChild = interface
procedure SetParent(const Value: IParent);
function GetParent: IParent;
property Parent: IParent read GetParent write SetParent;
end;
procedure TParent.SetChild(const Value: IChild);
begin
FChild := Value;
end;
procedure TChild.SetParent(const Value: IParent);
begin
FParent := Value;
end;

72. Weak references
 Zeroing weak pointers
This code won’t leak memory
FChild and FParent will be set to nil
when the stored instance will be freed
From Interfaces to SOLID
IParent = interface
procedure SetChild(const Value: IChild);
function GetChild: IChild;
function HasChild: boolean;
property Child: IChild read GetChild write SetChild;
end;
IChild = interface
procedure SetParent(const Value: IParent);
function GetParent: IParent;
property Parent: IParent read GetParent write SetParent;
end;
procedure TParent.SetChild(const Value: IChild);
begin
SetWeakZero(self,@FChild,Value);
end;
procedure TChild.SetParent(const Value: IParent);
begin
SetWeakZero(self,@FParent,Value);
end;

73. Weak references
 Delphi NextGen memory model
 Uses ARC for every TObject instance
 This is transparent for TComponent / FMX
 No try … finally Free block needed
 But breaks the proven weak reference model
and a lot of existing code
 Only UTF-16 string type
 Direct 8 bit string type disabled 
 UTF8String RawByteString back in 10.1 Berlin 
From Interfaces to SOLID

74. From Interfaces to SOLID