关于 datasnap Stream的英文博客能容

转载：http://blogs.embarcadero.com/jimtierney/2009/04/06/31461/

DataSnap Server Method Stream Parameters

This is a continuation of my posts on DataSnap server method parameters and return types.  This post is about  TStream and TDBXStreamValue.    The sample client and server projects that go with this post can be downloaded here: http://cc.embarcadero.com/item/26854

See my earlier posts on “Basic” and “Basic DBXValue” types.   I’ve yet to post about TDBXConnection, TDBXConnectionValue, TDBXReader, TParams, and TDataSet.  However, TDataSet is demonstrated in the sample projects.

Basic	Basic DBXValue	Collection	Connection
AnsiString Boolean Currency TDateTime TDBXDate TDBXTime Double Int64 Integer LongInt OleVariant Single SmallInt WideString	TDBXAnsiStringValue TDBXAnsiCharsValue TDBXBcdValue TDBXBooleanValue TDBXDateValue TDBXDoubleValue TDBXInt16Value TDBXInt32Value TDBXInt64Value TDBXSingleValue TDBXStringValue TDBXTimeStampValue TDBXTimeValue TDBXWideCharsValue TDBXWideStringValue	TDBXReader TDataSet TParams TStream Collection DBXValue TDBXReaderValue TDBXStreamValue	TDBXConnection Connection DBXValue TDBXConnectionValue

The following table summarizes differences between TStream and TDBXStreamValue types:

	Supports null values	Declaration			Accessing Values		Proxy generator
		Default parameter direction	Other parameter directions	function result type	Get	Set
TStream	No	in	in/out: usevar keyword out: use outkeyword	Yes	lhs := AParameter	AParameter := rhs	Yes
TDBXStreamValue	Yes	in/out	None	No	AParameter.IsNull lhs:=AParameter.GetStream(InstanceOwner)	AParameter.SetNull AParameter.SetStream(rhs, InstanceOwner)	No

Supports null values

The TDBXStreamValue type has an IsNull property and a SetNull method.   Use this type instead of TStream a parameter value can be null/nil, in some cases.

Declaration

The var and out keywords can’t be used to specify the parameter direction of a TDBXStreamValue parameter.  The direction is always in/out. A TDBXStreamValue type can’t be used as a function result.

Proxy Generator

The RAD Studio 2007 client proxy generator does not work properly for server methods with TDBXStreamValue parameters. So you will need to hand code the client code or correct the generated proxy. The sample client has hand corrected proxy methods that look like this.

function TTestStreamValueClient.Echo(var I: TStream): TStream;
begin
  if FEchoCommand = nil then
  begin
    FEchoCommand := FDBXConnection.CreateCommand;
    FEchoCommand.CommandType := TDBXCommandTypes.DSServerMethod;
    FEchoCommand.Text := 'TTestStreamValue.Echo';
    FEchoCommand.Prepare;
  end;
  if I = nil then
    FEchoCommand.Parameters[0].Value.SetNull
  else
    FEchoCommand.Parameters[0].Value.SetStream(I, FInstanceOwner);
  FEchoCommand.ExecuteUpdate;
  if FEchoCommand.Parameters[0].Value.IsNull then
    I := nil
  else
    I := FEchoCommand.Parameters[0].Value.GetStream(FInstanceOwner);
  Result := FEchoCommand.Parameters[1].Value.GetStream(FInstanceOwner);
end;

This client method works with a server method declared as follows:

function Echo(I: TDBXStreamValue): TStream;

Calling a server method with a TDBXStreamValue parameter is the same as calling a server method with an TStream parameter, except that you can use SetNull and IsNull methods to work with null values.

For example, compare the following server method declaration and client method implementation to the previous example:

function Echo(I: TStream): TStream; 

function TTestStreamClient.Echo(I: TStream): TStream;
begin
  if FEchoCommand = nil then
  begin
    FEchoCommand := FDBXConnection.CreateCommand;
    FEchoCommand.CommandType := TDBXCommandTypes.DSServerMethod;
    FEchoCommand.Text := 'TTestStream.Echo';
    FEchoCommand.Prepare;
  end;
  FEchoCommand.Parameters[0].Value.SetStream(I, FInstanceOwner);
  FEchoCommand.ExecuteUpdate;
  Result := FEchoCommand.Parameters[1].Value.GetStream(FInstanceOwner);
end;

Accessing Values

The GetStream and SetStream methods have an InstanceOwner parameter.  Passing True indicates that DBX owns the stream and will free it.  Passing False indicates that the caller owns the stream.  To control how the generated proxy classes call SetStream and GetStream, there is an AInstanceOwner parameter on the proxy class constructor:

constructor TTestStreamClient.Create(ADBXConnection: TDBXConnection; AInstanceOwner: Boolean);

The other constructor is equivalent to passing AInstanceOwner as True.

constructor TTestStreamClient.Create(ADBXConnection: TDBXConnection);

Sample Client And Server Applications

The sample server has a few simple server methods for passing values (including null).

  {$METHODINFO ON}
  TTestCollection = class(TComponent)
  strict protected
    procedure LogMessage(const AMessage: string);
  public
    // Server method to get the parameter type names
    procedure GetTypeNames(I: T; ADeclaredName, AActualName: TDBXStringValue);
  end;
  {$METHODINFO OFF}

  TTestStream = class(TTestCollection)
  public
    function Echo(I: TStream): TStream;
    procedure Copy(I: TStream; out J: TStream);
    procedure Swap(var I: TStream; var J: TStream);
  end;

  TTestStreamValue = class(TTestCollection)
  public
    function Echo(I: TDBXStreamValue): TStream;
    procedure Copy(I: TDBXStreamValue; J: TDBXStreamValue);
    procedure Swap(I: TDBXStreamValue; J: TDBXStreamValue);
    function IsNull(I: TDBXStreamValue): Boolean;
    procedure SetNull(I: TDBXStreamValue);
  end;

The sample client tests the server methods by calling them with sample values and verifying the results (e.g; comparing streams). TMemoryStream is used to pass short streams and TFileStream to pass long streams.  Here is a screen shot of the running server and client:

The following table shows the parameter types and return types demonstrated in the sample client and server:

Type	Direction
	(default)	var	out	Result
TDBXStreamValue	X (in/out)
TStream	X (in)	X	X	X

Stream Usage

Check  “Log Stream Usage” to show TStream sizes, type names, creates, destroys, and reads.  The results from 4 different test cases are displayed after “Call Server Methods” is clicked. 

The screen shot below shows the result from a test case with longer streams.  I’ve added highlighting to illustrate these points:

1. When the server reads a longer stream passed from the client, round trips are made to the client.  This makes it possible to send very large streams to the server without consuming lots of memory.
2. When the client reads a longer stream passed from the server, round trips are made to the server.  This makes it possible to return very large streams to the client without consuming lots of memory.
3. The TStream objects that represent longer client and server streams have a Size of –1, meaning the size is unknown.  Rely on the return value of TStream.Read to detect the end of the stream.  Also note that some DBX streams such as TDBXStreamReaderStream do not fully support Seek.

That’s all for now.

附件：（ http://cc.embarcadero.com/item/26854 链接的文件）