delphi - Rtti 访问复杂数据结构中的字段和属性

Question

正如Rtti data manipulation and consistency in Delphi 2010中已经讨论的那样可以通过使用一对 TRttiField 和实例指针访问成员来达到原始数据和 rtti 值之间的一致性。对于仅具有基本成员类型(例如整数或字符串)的简单类，这将非常容易。但是如果我们有结构化字段类型怎么办？

这是一个例子:

TIntArray = array [0..1] of Integer;

TPointArray = array [0..1] of Point;

TExampleClass = class
  private
    FPoint : TPoint;
    FAnotherClass : TAnotherClass;
    FIntArray : TIntArray;
    FPointArray : TPointArray;
  public  
    property Point : TPoint read FPoint write FPoint; 
    //.... and so on
end;

为了方便地访问成员，我想构建一个成员节点树，它提供了一个用于获取和设置值、获取属性、序列化/反序列化值等的接口(interface)。

TMemberNode = class
  private
    FMember : TRttiMember;
    FParent : TMemberNode;
    FInstance : Pointer;
  public
    property Value : TValue read GetValue write SetValue; //uses FInstance
end;

因此，最重要的是获取/设置值，如前所述，这是通过使用 TRttiField 的 GetValue 和 SetValue 函数来完成的。

那么 FPoint 成员(member)的实例是什么？假设 Parent 是 TExample 类的节点，其中实例已知且成员是字段，则 Instance 将为:

FInstance := Pointer (Integer (Parent.Instance) + TRttiField (FMember).Offset);

但是如果我想知道记录属性的实例怎么办？在这种情况下没有偏移。那么有没有更好的解决方案来获取数据的指针呢？

对于 FAnotherClass 成员，实例将是:

FInstance := Parent.Value.AsObject;  

到目前为止，该解决方案有效，并且可以使用 rtti 或原始类型来完成数据操作，而不会丢失信息。

但是当使用数组时，事情会变得更加困难。特别是第二个点数组。在这种情况下如何获取积分成员的实例？

Answer 1

TRttiField.GetValue 其中字段类型是值类型，可为您获取副本。这是设计使然。 TValue.MakeWithoutCopy 用于管理接口(interface)和字符串等内容的引用计数；这并不是为了避免这种复制行为。 TValue 故意不设计为模仿 Variant 的 ByRef 行为，您最终可能会引用 TValue 内的(例如)堆栈对象，增加了过时指针的风险。这也是违反直觉的。当您说 GetValue 时，您应该期待一个值，而不是引用。

当值类型的值存储在其他结构中时，操作值类型的值的最有效方法可能是后退一步并添加另一个间接级别:通过计算偏移量而不是直接使用 TValue输入的中间值沿着到达项目的路径前进。

这可以相当简单地封装。我花了大约一个小时编写了一些 TLocation 记录，它使用 RTTI 来执行此操作:

type
  TLocation = record
    Addr: Pointer;
    Typ: TRttiType;
    class function FromValue(C: TRttiContext; const AValue: TValue): TLocation; static;
    function GetValue: TValue;
    procedure SetValue(const AValue: TValue);
    function Follow(const APath: string): TLocation;
    procedure Dereference;
    procedure Index(n: Integer);
    procedure FieldRef(const name: string);
  end;

function GetPathLocation(const APath: string; ARoot: TLocation): TLocation; forward;

{ TLocation }

type
  PPByte = ^PByte;

procedure TLocation.Dereference;
begin
  if not (Typ is TRttiPointerType) then
    raise Exception.CreateFmt('^ applied to non-pointer type %s', [Typ.Name]);
  Addr := PPointer(Addr)^;
  Typ := TRttiPointerType(Typ).ReferredType;
end;

procedure TLocation.FieldRef(const name: string);
var
  f: TRttiField;
begin
  if Typ is TRttiRecordType then
  begin
    f := Typ.GetField(name);
    Addr := PByte(Addr) + f.Offset;
    Typ := f.FieldType;
  end
  else if Typ is TRttiInstanceType then
  begin
    f := Typ.GetField(name);
    Addr := PPByte(Addr)^ + f.Offset;
    Typ := f.FieldType;
  end
  else
    raise Exception.CreateFmt('. applied to type %s, which is not a record or class',
      [Typ.Name]);
end;

function TLocation.Follow(const APath: string): TLocation;
begin
  Result := GetPathLocation(APath, Self);
end;

class function TLocation.FromValue(C: TRttiContext; const AValue: TValue): TLocation;
begin
  Result.Typ := C.GetType(AValue.TypeInfo);
  Result.Addr := AValue.GetReferenceToRawData;
end;

function TLocation.GetValue: TValue;
begin
  TValue.Make(Addr, Typ.Handle, Result);
end;

procedure TLocation.Index(n: Integer);
var
  sa: TRttiArrayType;
  da: TRttiDynamicArrayType;
begin
  if Typ is TRttiArrayType then
  begin
    // extending this to work with multi-dimensional arrays and non-zero
    // based arrays is left as an exercise for the reader ... :)
    sa := TRttiArrayType(Typ);
    Addr := PByte(Addr) + sa.ElementType.TypeSize * n;
    Typ := sa.ElementType;
  end
  else if Typ is TRttiDynamicArrayType then
  begin
    da := TRttiDynamicArrayType(Typ);
    Addr := PPByte(Addr)^ + da.ElementType.TypeSize * n;
    Typ := da.ElementType;
  end
  else
    raise Exception.CreateFmt('[] applied to non-array type %s', [Typ.Name]);
end;

procedure TLocation.SetValue(const AValue: TValue);
begin
  AValue.Cast(Typ.Handle).ExtractRawData(Addr);
end;

此类型可用于使用 RTTI 导航值内的位置。为了让它更容易使用，并且让我编写起来更有趣，我还编写了一个解析器 - Follow 方法:

function GetPathLocation(const APath: string; ARoot: TLocation): TLocation;

  { Lexer }

  function SkipWhite(p: PChar): PChar;
  begin
    while IsWhiteSpace(p^) do
      Inc(p);
    Result := p;
  end;

  function ScanName(p: PChar; out s: string): PChar;
  begin
    Result := p;
    while IsLetterOrDigit(Result^) do
      Inc(Result);
    SetString(s, p, Result - p);
  end;

  function ScanNumber(p: PChar; out n: Integer): PChar;
  var
    v: Integer;
  begin
    v := 0;
    while (p >= '0') and (p <= '9') do
    begin
      v := v * 10 + Ord(p^) - Ord('0');
      Inc(p);
    end;
    n := v;
    Result := p;
  end;

const
  tkEof = #0;
  tkNumber = #1;
  tkName = #2;
  tkDot = '.';
  tkLBracket = '[';
  tkRBracket = ']';

var
  cp: PChar;
  currToken: Char;
  nameToken: string;
  numToken: Integer;

  function NextToken: Char;
    function SetToken(p: PChar): PChar;
    begin
      currToken := p^;
      Result := p + 1;
    end;
  var
    p: PChar;
  begin
    p := cp;
    p := SkipWhite(p);
    if p^ = #0 then
    begin
      cp := p;
      currToken := tkEof;
      Exit(currToken);
    end;

    case p^ of
      '0'..'9':
      begin
        cp := ScanNumber(p, numToken);
        currToken := tkNumber;
      end;

      '^', '[', ']', '.': cp := SetToken(p);

    else
      cp := ScanName(p, nameToken);
      if nameToken = '' then
        raise Exception.Create('Invalid path - expected a name');
      currToken := tkName;
    end;

    Result := currToken;
  end;

  function Describe(tok: Char): string;
  begin
    case tok of
      tkEof: Result := 'end of string';
      tkNumber: Result := 'number';
      tkName: Result := 'name';
    else
      Result := '''' + tok + '''';
    end;
  end;

  procedure Expect(tok: Char);
  begin
    if tok <> currToken then
      raise Exception.CreateFmt('Expected %s but got %s', 
        [Describe(tok), Describe(currToken)]);
  end;

  { Semantic actions are methods on TLocation }
var
  loc: TLocation;

  { Driver and parser }

begin
  cp := PChar(APath);
  NextToken;

  loc := ARoot;

  // Syntax:
  // path ::= ( '.' <name> | '[' <num> ']' | '^' )+ ;;

  // Semantics:

  // '<name>' are field names, '[]' is array indexing, '^' is pointer
  // indirection.

  // Parser continuously calculates the address of the value in question, 
  // starting from the root.

  // When we see a name, we look that up as a field on the current type,
  // then add its offset to our current location if the current location is 
  // a value type, or indirect (PPointer(x)^) the current location before 
  // adding the offset if the current location is a reference type. If not
  // a record or class type, then it's an error.

  // When we see an indexing, we expect the current location to be an array
  // and we update the location to the address of the element inside the array.
  // All dimensions are flattened (multiplied out) and zero-based.

  // When we see indirection, we expect the current location to be a pointer,
  // and dereference it.

  while True do
  begin
    case currToken of
      tkEof: Break;

      '.':
      begin
        NextToken;
        Expect(tkName);
        loc.FieldRef(nameToken);
        NextToken;
      end;

      '[':
      begin
        NextToken;
        Expect(tkNumber);
        loc.Index(numToken);
        NextToken;
        Expect(']');
        NextToken;
      end;

      '^':
      begin
        loc.Dereference;
        NextToken;
      end;

    else
      raise Exception.Create('Invalid path syntax: expected ".", "[" or "^"');
    end;
  end;

  Result := loc;
end;

这是一个示例类型，以及操作它的例程 (P):

type
  TPoint = record
    X, Y: Integer;
  end;
  TArr = array[0..9] of TPoint;

  TFoo = class
  private
    FArr: TArr;
    constructor Create;
    function ToString: string; override;
  end;

{ TFoo }

constructor TFoo.Create;
var
  i: Integer;
begin
  for i := Low(FArr) to High(FArr) do
  begin
    FArr[i].X := i;
    FArr[i].Y := -i;
  end;
end;

function TFoo.ToString: string;
var
  i: Integer;
begin
  Result := '';
  for i := Low(FArr) to High(FArr) do
    Result := Result + Format('(%d, %d) ', [FArr[i].X, FArr[i].Y]);
end;

procedure P;
var
  obj: TFoo;
  loc: TLocation;
  ctx: TRttiContext;
begin
  obj := TFoo.Create;
  Writeln(obj.ToString);

  ctx := TRttiContext.Create;

  loc := TLocation.FromValue(ctx, obj);
  Writeln(loc.Follow('.FArr[2].X').GetValue.ToString);
  Writeln(obj.FArr[2].X);

  loc.Follow('.FArr[2].X').SetValue(42);
  Writeln(obj.FArr[2].X); // observe value changed

  // alternate syntax, not using path parser, but location destructive updates
  loc.FieldRef('FArr');
  loc.Index(2);
  loc.FieldRef('X');
  loc.SetValue(24);
  Writeln(obj.FArr[2].X); // observe value changed again

  Writeln(obj.ToString);
end;

该原理可以扩展到其他类型和 Delphi 表达式语法，或者可以更改 TLocation 以返回新的 TLocation 实例，而不是破坏性的 self 更新，或者非破坏性的 self 更新。可能支持平面数组索引等。