Walkthrough: Displaying Matching Braces

 

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You can implement language-based features such as brace matching by defining the braces you want to match, and then adding a text marker tag to the matching braces when the caret is on one of the braces. You can define braces in the context of a language, or you can define your own file name extension and content type and apply the tags to just that type, or you can apply the tags to an existing content type (such as "text"). The following walkthrough shows how to apply brace matching tags to the "text" content type.

Starting in Visual Studio 2015, you do not install the Visual Studio SDK from the download center. It is included as an optional feature in Visual Studio setup. You can also install the VS SDK later on. For more information, see Installing the Visual Studio SDK.

To create a MEF project

  1. Create an Editor Classifier project. Name the solution BraceMatchingTest.

  2. Add an Editor Classifier item template to the project. For more information, see Creating an Extension with an Editor Item Template.

  3. Delete the existing class files.

To get a brace highlighting effect that resembles the one that is used in Visual Studio, you can implement a tagger of type TextMarkerTag. The following code shows how to define the tagger for brace pairs at any level of nesting. In this example, the brace pairs []. [], and {} are defined in the tagger constructor, but in a full language implementation the relevant brace pairs would be defined in the language specification.

To implement a brace matching tagger

  1. Add a class file and name it BraceMatching.

  2. Import the following namespaces.

    Imports System.ComponentModel.Composition
    Imports Microsoft.VisualStudio.Text
    Imports Microsoft.VisualStudio.Text.Editor
    Imports Microsoft.VisualStudio.Text.Tagging
    Imports Microsoft.VisualStudio.Utilities
    

  3. Define a class BraceMatchingTagger that inherits from ITagger<T> of type TextMarkerTag.

    Friend Class BraceMatchingTagger
        Implements ITagger(Of TextMarkerTag)
    

  4. Add properties for the text view, the source buffer, and the current snapshot point, and also a set of brace pairs.

        Private _View As ITextView
        Private Property View() As ITextView
            Get
                Return _View
            End Get
            Set(ByVal value As ITextView)
                _View = value
            End Set
        End Property
        Private _SourceBuffer As ITextBuffer
        Private Property SourceBuffer() As ITextBuffer
            Get
                Return _SourceBuffer
            End Get
            Set(ByVal value As ITextBuffer)
                _SourceBuffer = value
            End Set
        End Property
        Private _CurrentChar As System.Nullable(Of SnapshotPoint)
        Private Property CurrentChar() As System.Nullable(Of SnapshotPoint)
            Get
                Return _CurrentChar
            End Get
            Set(ByVal value As System.Nullable(Of SnapshotPoint))
                _CurrentChar = value
            End Set
        End Property
        Private m_braceList As Dictionary(Of Char, Char)
    

  5. In the tagger constructor, set the properties and subscribe to the view change events PositionChanged and LayoutChanged. In this example, for illustrative purposes, the matching pairs are also defined in the constructor.

        Friend Sub New(ByVal view As ITextView, ByVal sourceBuffer As ITextBuffer)
            'here the keys are the open braces, and the values are the close braces
            m_braceList = New Dictionary(Of Char, Char)()
            m_braceList.Add("{"c, "}"c)
            m_braceList.Add("["c, "]"c)
            m_braceList.Add("("c, ")"c)
            Me.View = view
            Me.SourceBuffer = sourceBuffer
            Me.CurrentChar = Nothing
    
            AddHandler Me.View.Caret.PositionChanged, AddressOf Me.CaretPositionChanged
            AddHandler Me.View.LayoutChanged, AddressOf Me.ViewLayoutChanged
        End Sub
    

  6. As part of the ITagger<T> implementation, declare a TagsChanged event.

        Public Event TagsChanged As EventHandler(Of SnapshotSpanEventArgs) _
            Implements ITagger(Of TextMarkerTag).TagsChanged
    

  7. The event handlers update the current caret position of the CurrentChar property and raise the TagsChanged event.

        Private Sub ViewLayoutChanged(ByVal sender As Object, ByVal e As TextViewLayoutChangedEventArgs)
            If e.NewSnapshot IsNot e.OldSnapshot Then
                'make sure that there has really been a change
                UpdateAtCaretPosition(View.Caret.Position)
            End If
        End Sub
    
        Private Sub CaretPositionChanged(ByVal sender As Object, ByVal e As CaretPositionChangedEventArgs)
            UpdateAtCaretPosition(e.NewPosition)
        End Sub
    
        Private Sub UpdateAtCaretPosition(ByVal caretPosition As CaretPosition)
            CurrentChar = caretPosition.Point.GetPoint(SourceBuffer, caretPosition.Affinity)
    
            If Not CurrentChar.HasValue Then
                Exit Sub
            End If
    
            RaiseEvent TagsChanged(Me, New SnapshotSpanEventArgs(New SnapshotSpan(SourceBuffer.CurrentSnapshot, 0, SourceBuffer.CurrentSnapshot.Length)))
        End Sub
    

  8. Implement the GetTags method to match braces either when the current character is an open brace or when the previous character is a close brace, as in Visual Studio. When the match is found, this method instantiates two tags, one for the open brace and one for the close brace.

        Public Function GetTags(ByVal spans As NormalizedSnapshotSpanCollection) As IEnumerable(Of ITagSpan(Of TextMarkerTag)) Implements ITagger(Of Microsoft.VisualStudio.Text.Tagging.TextMarkerTag).GetTags
            If spans.Count = 0 Then
                'there is no content in the buffer
                Exit Function
            End If
    
            'don't do anything if the current SnapshotPoint is not initialized or at the end of the buffer
            If Not CurrentChar.HasValue OrElse CurrentChar.Value.Position >= CurrentChar.Value.Snapshot.Length Then
                Exit Function
            End If
    
            'hold on to a snapshot of the current character
            Dim currentChar__1 As SnapshotPoint = CurrentChar.Value
    
            'if the requested snapshot isn't the same as the one the brace is on, translate our spans to the expected snapshot
            If spans(0).Snapshot IsNot currentChar__1.Snapshot Then
                currentChar__1 = currentChar__1.TranslateTo(spans(0).Snapshot, PointTrackingMode.Positive)
            End If
    
            'get the current char and the previous char
            Dim currentText As Char = currentChar__1.GetChar()
            Dim lastChar As SnapshotPoint = If(CInt(currentChar__1) = 0, currentChar__1, currentChar__1 - 1)
            'if currentChar is 0 (beginning of buffer), don't move it back
            Dim lastText As Char = lastChar.GetChar()
            Dim pairSpan As New SnapshotSpan()
    
            If m_braceList.ContainsKey(currentText) Then
                'the key is the open brace
                Dim closeChar As Char
                m_braceList.TryGetValue(currentText, closeChar)
                If BraceMatchingTagger.FindMatchingCloseChar(currentChar__1, currentText, closeChar, View.TextViewLines.Count, pairSpan) = True Then
                    Exit Function
                End If
            ElseIf m_braceList.ContainsValue(lastText) Then
                'the value is the close brace, which is the *previous* character 
                Dim open = From n In m_braceList _
                    Where n.Value.Equals(lastText) _
                    Select n.Key
                If BraceMatchingTagger.FindMatchingOpenChar(lastChar, CChar(open.ElementAt(0)), lastText, View.TextViewLines.Count, pairSpan) = True Then
                    Exit Function
                End If
            End If
        End Function
    

  9. The following private methods find the matching brace at any level of nesting. The first method finds the close character that matches the open character:

        Private Shared Function FindMatchingCloseChar(ByVal startPoint As SnapshotPoint, ByVal open As Char, ByVal close As Char, ByVal maxLines As Integer, ByRef pairSpan As SnapshotSpan) As Boolean
            pairSpan = New SnapshotSpan(startPoint.Snapshot, 1, 1)
            Dim line As ITextSnapshotLine = startPoint.GetContainingLine()
            Dim lineText As String = line.GetText()
            Dim lineNumber As Integer = line.LineNumber
            Dim offset As Integer = startPoint.Position - line.Start.Position + 1
    
            Dim stopLineNumber As Integer = startPoint.Snapshot.LineCount - 1
            If maxLines > 0 Then
                stopLineNumber = Math.Min(stopLineNumber, lineNumber + maxLines)
            End If
    
            Dim openCount As Integer = 0
            While True
                'walk the entire line
                While offset < line.Length
                    Dim currentChar As Char = lineText(offset)
                    If currentChar = close Then
                        'found the close character
                        If openCount > 0 Then
                            openCount -= 1
                        Else
                            'found the matching close
                            pairSpan = New SnapshotSpan(startPoint.Snapshot, line.Start + offset, 1)
                            Return True
                        End If
                    ElseIf currentChar = open Then
                        ' this is another open
                        openCount += 1
                    End If
                    offset += 1
                End While
    
                'move on to the next line
                If System.Threading.Interlocked.Increment(lineNumber) > stopLineNumber Then
                    Exit While
                End If
    
                line = line.Snapshot.GetLineFromLineNumber(lineNumber)
                lineText = line.GetText()
                offset = 0
            End While
    
            Return False
        End Function
    

  10. The following helper method finds the open character that matches a close character:

        Private Shared Function FindMatchingOpenChar(ByVal startPoint As SnapshotPoint, ByVal open As Char, ByVal close As Char, ByVal maxLines As Integer, ByRef pairSpan As SnapshotSpan) As Boolean
            pairSpan = New SnapshotSpan(startPoint, startPoint)
    
            Dim line As ITextSnapshotLine = startPoint.GetContainingLine()
    
            Dim lineNumber As Integer = line.LineNumber
            Dim offset As Integer = startPoint - line.Start - 1
            'move the offset to the character before this one
            'if the offset is negative, move to the previous line
            If offset < 0 Then
                line = line.Snapshot.GetLineFromLineNumber(System.Threading.Interlocked.Decrement(lineNumber))
                offset = line.Length - 1
            End If
    
            Dim lineText As String = line.GetText()
    
            Dim stopLineNumber As Integer = 0
            If maxLines > 0 Then
                stopLineNumber = Math.Max(stopLineNumber, lineNumber - maxLines)
            End If
    
            Dim closeCount As Integer = 0
    
            While True
                ' Walk the entire line
                While offset >= 0
                    Dim currentChar As Char = lineText(offset)
    
                    If currentChar = open Then
                        If closeCount > 0 Then
                            closeCount -= 1
                        Else
                            ' We've found the open character
                            pairSpan = New SnapshotSpan(line.Start + offset, 1)
                            'we just want the character itself
                            Return True
                        End If
                    ElseIf currentChar = close Then
                        closeCount += 1
                    End If
                    offset -= 1
                End While
    
                ' Move to the previous line
                If System.Threading.Interlocked.Decrement(lineNumber) < stopLineNumber Then
                    Exit While
                End If
    
                line = line.Snapshot.GetLineFromLineNumber(lineNumber)
                lineText = line.GetText()
                offset = line.Length - 1
            End While
            Return False
        End Function
    

In addition to implementing a tagger, you must also implement and export a tagger provider. In this case, the content type of the provider is "text". This means that brace matching will appear in all types of text files, but a fuller implementation would apply brace matching only to a specific content type.

To implement a brace matching tagger provider

  1. Declare a tagger provider that inherits from IViewTaggerProvider, name it BraceMatchingTaggerProvider, and export it with a ContentTypeAttribute of "text" and a TagTypeAttribute of TextMarkerTag.

    <Export(GetType(IViewTaggerProvider))> _
    <ContentType("text")> _
    <TagType(GetType(TextMarkerTag))> _
    Friend Class BraceMatchingTaggerProvider
        Implements IViewTaggerProvider
    

  2. Implement the CreateTagger<T> method to instantiate a BraceMatchingTagger.

        Public Function CreateTagger(Of T As ITag)(ByVal textView As ITextView, ByVal buffer As ITextBuffer) As ITagger(Of T) Implements IViewTaggerProvider.CreateTagger
            If textView Is Nothing Then
                Return Nothing
            End If
    
            'provide highlighting only on the top-level buffer
            If textView.TextBuffer IsNot buffer Then
                Return Nothing
            End If
    
            Return TryCast(New BraceMatchingTagger(textView, buffer), ITagger(Of T))
        End Function
    

To test this code, build the BraceMatchingTest solution and run it in the experimental instance.

To build and test BraceMatchingTest solution

  1. Build the solution.

  2. When you run this project in the debugger, a second instance of Visual Studio is instantiated.

  3. Create a text file and type some text that includes matching braces.

    hello {  
    goodbye}  
    
    {}  
    
    {hello}  
    
    
  4. When you position the caret before an open brace, both that brace and the matching close brace should be highlighted. When you position the cursor just after the close brace, both that brace and the matching open brace should be highlighted.

Walkthrough: Linking a Content Type to a File Name Extension

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