Difference between revisions of "Documentation/DevGuide/ProUNO/Bridge/Mapping of Sequence"

Revision as of 13:03, 15 February 2008

Arrays in Automation have a particular type. The SAFEARRAY. A SAFEARRAY array is used when a UNO function takes a sequence as an argument. To create a SAFEARRAY in C++, use Windows API functions. The C++ name is also SAFEARRAY, but in other languages it might be named differently. In VB for example, the type does not even exist, because it is mapped to an ordinary VB array:

 Dim myarr(9) as String

JScript is different. It does not have a method to create a SAFEARRAY. Instead, JScript features an Array object that can be used as a common array in terms of indexing and accessing its values. It is represented by a dispatch object internally. JScript offers a VBArray object that converts a SAFEARRAY into an Array object, which can then be processed further.

The Automation bridge accepts both, SAFEARRAY and Array object, for arguments whose UNO type is a sequence.

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UNO does not recognize multi-dimensional sequences. Instead, a sequences can have elements that are also sequences. Those “inner” sequences can have different lengths, whereas the elements of a dimension of a multi-dimensional array are all the same length.

To provide an argument for a sequence of sequences, a SAFEARRAY containing VARIANTs of SAFEARRAYs has to be created. For example:

 //UNO method
 void foo([in] sequence< sequence< long > > value);
 
 
 Dim seq(1) As Variant
 Dim ar1(3) As Long
 Dim ar2(4) As Long 
 'fill ar1, ar2
 ...
 
 seq(0) = ar1
 seq(1) = ar2
 
 objUno.foo seq

The array seq corresponds to the “outer” sequence and contains two VARIANTs, which in turn contain SAFEARRAYs of different lengths.

It is also possible to use a multi-dimensional SAFEARRAY if the elements of the sequence are all the same length:

 Dim seq(9, 1) As Long
 'fill the sequence 
 ...
 objUno.foo seq

Be aware that Visual Basic uses a column-oriented ordering in contrast to C. That is, the C equivalent to the VB array is

 long seq[2][10]

The highest dimension in VB is represented by the right-most number.

This language binding specifies that the “outer” sequence corresponds to the highest dimension. Therefore, the VB array seq(9,1) would map to a sequence of sequences where the outer sequence has two elements and the inner sequences each have ten elements.

Returned sequences are converted into SAFEARRAYs containing VARIANTs. If a sequence of sequences is returned, then the VARIANTs contain again SAFEARRAYs.

To process a returned SAFEARRAY in Jscript, use the VBArray object to convert the SAFEARRAY into a JScript Array.

When a method of an Automation object is called from UNO and a parameter is a SAFEARRAY, then a sequence is used on the UNO side. The element type of the sequence should correspond to the element type of the SAFEARRAY according to the default mapping. If it does not, the bridge tries to convert the elements into the expected element type.

If the parameter is a multi–dimensional SAFEARRAY, then one has to provide a sequence containing sequences has to be provided. The number of nested sequences corresponds to the number of dimensions. Since the elements of a dimension have the same length, the sequences that represent that dimension should also have the same length. For example, assume the expected SAFEARRAY can be expressed in C as

 long ar[2][10]

Then the outer sequence must have two elements and each of those sequences has 10 elements. That a returned sequence maps to a SAFEARRAY of VARIANTs is not ideal because it is ambiguous when the array is passed back to UNO. However, the bridge solves this problem by using UNO type information. For example, a returned sequence of longs will result in a SAFEARRAY of VARIANTs containing long values. When the SAFEARRAY is passed in a method as an argument for a parameter of type sequence<long > then it is converted accordingly. However, if the parameter is an any, then the bridge does not have the necessary type information and converts the SAFEARRAY to sequence<any>. That is, the called method receives an any containing a sequence<any>. If the method now expects the any to contain a sequence<long> then it may fail. This is confusing if there are pairs of methods like getxxx and setxxx, which take any arguments. Then you may get a SAFEARRAY as a return value, which cannot be used in the respective setXXX call. For example:

 //UNO IDL
 any getByIndex();
 void setByIndex([in] any value);
 
 'VB
 Dim arLong() As Variant 
 arLong = objUno.getByIndex() 'object returns sequence<long> in any
 objUno.setByIndex arLong 'object receives sequence<any> in any and may cause an error.

To solve this problem, wrap the argument in a Value Object (Value Objects):

 'VB
 Dim arLong() As Variant 
 arLong = objUno.getByIndex() 'object returns sequence<long> in any
 
 Dim objValueObject As Object
 Set objValueObject = objServiceManager.Bridge_GetValueObject()
 objValueObject.set “[]long”, arLong
 
 objUno.setByIndex objValueObject 'object receives sequence<long>