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:
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:
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:
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.
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.
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.
