delphi - 调整实例类内存分配的大小

Question

我使用此示例在运行时更改类:

procedure PatchInstanceClass(Instance: TObject; NewClass: TClass);
type
  PClass = ^TClass;
begin
  if Assigned(Instance) and Assigned(NewClass)
    and NewClass.InheritsFrom(Instance.ClassType)
    and (NewClass.InstanceSize = Instance.InstanceSize) then
  begin
    PClass(Instance)^ := NewClass;
  end;
end;

type
  TMyButton = class(TButton)
  Private
    FLogEvent : TNotifyEvent;
  public
    Property  Log : TNotifyEvent Read FLogEvent Write FLogEvent;
    procedure Click; override;
  end;

procedure TMyButton.Click;
begin
  Inherited;
  if Assigned(FLogEvent) then
     FLogEvent(Self);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  PatchInstanceClass(Button1, TMyButton);
end;

我的问题是 TMyButton 大小与 TButton 不同，因为我将其添加到 TMyButton 的 NotifyEvent 中。我的问题是如何调整 NewClass 同一实例的内存分配大小。 :-)

Answer 1

虽然我同意其他人的观点 - 你应该重新考虑你的方法，因为这听起来有点老套。不过，我喜欢一些古怪的东西 - 所以我将向您展示如何实现您所要求的目标。

关键是不要将任何内容放入新类中(因为 InstanceSize 需要相同)，而应将其放在其他位置 - 如果您使用的是较新的 Delphi 版本(2010 或更高版本)，您可以这样做。否则你需要稍微修改一下代码，但我想你明白了:

uses
  Generics.Collections;

procedure PatchInstanceClass(Instance: TObject; NewClass: TClass);
begin
  if Assigned(Instance) and Assigned(NewClass)
    and NewClass.InheritsFrom(Instance.ClassType)
    and (NewClass.InstanceSize = Instance.InstanceSize) then
  begin
    PPointer(Instance)^ := NewClass;
  end;
end;

type
  TMyButton = class(TButton)
  private
    function GetLogEvent: TNotifyEvent;
    procedure SetLogEvent(const Value: TNotifyEvent);
  public
    destructor Destroy; override;
    procedure Click; override;

    property LogEvent: TNotifyEvent read GetLogEvent write SetLogEvent;
  end;

  TMyButtonHelper = class helper for TMyButton
  private
    class var fLogEvents: TDictionary<TObject, TNotifyEvent>;
  public
    class constructor Create;
    class destructor Destroy;
  end;

{ TMyButtonHelper }

class constructor TMyButtonHelper.Create;
begin
  fLogEvents := TDictionary<TObject, TNotifyEvent>.Create;
end;

class destructor TMyButtonHelper.Destroy;
begin
  fLogEvents.Free;
end;

{ TMyButton }

destructor TMyButton.Destroy;
begin
  fLogEvents.Remove(Self);
  inherited;
end;

procedure TMyButton.Click;
begin
  inherited;
  if Assigned(LogEvent) then
     LogEvent(Self);
end;

function TMyButton.GetLogEvent: TNotifyEvent;
begin
  fLogEvents.TryGetValue(Self, Result);
end;

procedure TMyButton.SetLogEvent(const Value: TNotifyEvent);
begin
  fLogEvents.AddOrSetValue(Self, Value);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  PatchInstanceClass(Button1, TMyButton);
  TMyButton(Button1).LogEvent := Button1Click;
end;