Creating a Component with a variable number of output parameters

This topic is a follow-up from a discussion started by Benjamin Golder. In it I will show the necessary steps for creating a custom component that has a variable number of output parameters, based on the data structure of the input.

Public Class GH_ExampleComponent_VarOutput

  Inherits GH_Component

  Public Sub New()

    MyBase.New("Extract Paths", "ExPath", "Extract all the paths from a tree", "Sets", "Tree")

  End Sub

End Class

Now, the RegisterXXXXParams methods:

Protected Overrides Sub RegisterInputParams(ByVal pManager As GH_Component.GH_InputParamManager)

  pManager.Register_GenericParam("Tree", "T", "Data tree to examine", GH_ParamAccess.tree)

End Sub

Protected Overrides Sub RegisterOutputParams(ByVal pManager As GH_Component.GH_OutputParamManager)

  'We'll add one output parameter, just to not have a jagged output.

  pManager.Register_PathParam("Path 1", "1", "1st path in tree")

End Sub

Protected Overrides Sub SolveInstance(ByVal DA As IGH_DataAccess)

  'We have only one input parameter and it is set to Tree, 

  'so SolveInstance will only be called once for every solution.

  'We don't actually need the data inside the input, we're only interested in the paths.

  'So we don't actually need to call DA.GetDataTree, we can just go in and extract the 

  'paths directly:

  Dim paths As IList(Of GH_Path) = Params.Input(0).VolatileData.Paths

  'Abort if there is no tree.

  If (paths.Count = 0) Then Return

  'Post a warning if the number of output parameters does not 

  'equal the number of paths in the tree.

  If (paths.Count < Params.Output.Count) Then

AddRuntimeMessage(GH_RuntimeMessageLevel.Warning, "There are more outputs than paths in the tree.")

  ElseIf (paths.Count > Params.Output.Count) Then

AddRuntimeMessage(GH_RuntimeMessageLevel.Warning, "There are fewer outputs than paths in the tree.")

  End If

  'Iterate over all paths and assign to output parameters.

  For i As Int32 = 0 To Math.Min(Params.Output.Count, paths.Count) - 1

DA.SetData(i, paths(i))

  Next

End Sub

Protected Overrides Sub Menu_AppendCustomComponentItems(ByVal iMenu As System.Windows.Forms.ToolStripDropDown)

  'Add a single item to the component menu.

  Menu_AppendGenericMenuItem(iMenu, "Synch outputs", AddressOf Menu_SynchOutputClicked)

End Sub

Private Sub Menu_SynchOutputClicked(ByVal sender As Object, ByVal e As EventArgs)

  'Here we have to synch the number of output parameters with the number 

  'of paths in the volatile data tree in the input parameter.

  'This requires a few steps:

  '1. Determine whether something needs to happen at all.

  '2. Record an undo event.

  '3. Remove excess outputs or add missing outputs.

  Dim paths As IList(Of GH_Path) = Params.Input(0).VolatileData.Paths

  If (paths.Count = Params.Output.Count) Then Return 'yay, nothing needs to be done.

  'Something needs to be done, record an undo state.

  RecordUndoEvent("Synch output")

  'We either have too few or too many outputs, determine which is the case.

  If (paths.Count > Params.Output.Count) Then

'Add the missing outputs

For i As Int32 = Params.Output.Count + 1 To paths.Count

 Dim param As New Grasshopper.Kernel.Parameters.Param_GenericObject()

 param.Name = "Path " & i.ToString()

 param.NickName = i.ToString()

 If (i.ToString.EndsWith("1")) Then

param.Description = i.ToString() & "st path in tree"

 ElseIf (i.ToString.EndsWith("2")) Then

param.Description = i.ToString() & "nd path in tree"

 ElseIf (i.ToString.EndsWith("3")) Then

param.Description = i.ToString() & "rd path in tree"

 Else

param.Description = i.ToString() & "th path in tree"

 End If

 Params.RegisterOutputParam(param)

Next

  Else

'Remove excessive outputs

Do

 If (Params.Output.Count <= paths.Count) Then Exit Do

 Dim param As IGH_Param = Params.Output(Params.Output.Count - 1)

 Params.UnregisterOutputParameter(param)

Loop

  End If

  Params.OnParametersChanged()

  ExpireSolution(True)

End Sub

Public Overrides Function Write(ByVal writer As GH_IO.Serialization.GH_IWriter) As Boolean

  'We must make sure that the number of output parameters is correctly stored.

  'We'll use a special function on the GH_ComponentParamServer to accompish this

  'without too much sweat.

  Params.WriteParameterTypeData(writer)

  Return MyBase.Write(writer)

End Function

Public Overrides Function Read(ByVal reader As GH_IO.Serialization.GH_IReader) As Boolean

  'Very important, we must make sure all parameters exist before we 

  'start with the main deserialization.

  Params.Clear()

  Params.ReadParameterTypeData(reader)

  Return MyBase.Read(reader)

End Function