Walkthrough: Displaying Matching Braces

Article
02/28/2011

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.

Prerequisites

To complete this walkthrough, you must install the Visual Studio 2010 SDK.

Notes

For more information about the Visual Studio SDK, see Visual Studio Integration SDK. To find out how to download the Visual Studio SDK, see Visual Studio Extensibility Developer Center on the MSDN Web site.

Creating a Managed Extensibility Framework (MEF) Project

To create a MEF project

Create an Editor Classifier project. Name the solution BraceMatchingTest.
Open the source.extension.vsixmanifest file in the VSIX Manifest Editor.
Make sure that the Content heading contains a MEF Component content type and that the Path is set to BraceMatchingTest.dll.
Save and close Source.extension.vsixmanifest.
Delete the existing class files.

Implementing a Brace Matching Tagger

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

Add a class file and name it BraceMatching.

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

using System;
using System.Linq;
using System.Collections.Generic;
using System.ComponentModel.Composition;
using Microsoft.VisualStudio.Text;
using Microsoft.VisualStudio.Text.Editor;
using Microsoft.VisualStudio.Text.Tagging;
using Microsoft.VisualStudio.Utilities;

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

Friend Class BraceMatchingTagger
    Implements ITagger(Of TextMarkerTag)

internal class BraceMatchingTagger : ITagger<TextMarkerTag>

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)

ITextView View { get; set; }
ITextBuffer SourceBuffer { get; set; }
SnapshotPoint? CurrentChar { get; set; }
private Dictionary<char, char> m_braceList;

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

internal BraceMatchingTagger(ITextView view, ITextBuffer sourceBuffer)
{
    //here the keys are the open braces, and the values are the close braces
    m_braceList = new Dictionary<char, char>();
    m_braceList.Add('{', '}');
    m_braceList.Add('[', ']');
    m_braceList.Add('(', ')');
    this.View = view;
    this.SourceBuffer = sourceBuffer;
    this.CurrentChar = null;

    this.View.Caret.PositionChanged += CaretPositionChanged;
    this.View.LayoutChanged += ViewLayoutChanged;
}

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

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

public event EventHandler<SnapshotSpanEventArgs> TagsChanged;

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

void ViewLayoutChanged(object sender, TextViewLayoutChangedEventArgs e)
{
    if (e.NewSnapshot != e.OldSnapshot) //make sure that there has really been a change
    {
        UpdateAtCaretPosition(View.Caret.Position);
    }
}

void CaretPositionChanged(object sender, CaretPositionChangedEventArgs e)
{
    UpdateAtCaretPosition(e.NewPosition);
}
void UpdateAtCaretPosition(CaretPosition caretPosition)
{
    CurrentChar = caretPosition.Point.GetPoint(SourceBuffer, caretPosition.Affinity);

    if (!CurrentChar.HasValue)
        return;

    var tempEvent = TagsChanged;
    if (tempEvent != null)
        tempEvent(this, new SnapshotSpanEventArgs(new SnapshotSpan(SourceBuffer.CurrentSnapshot, 0,
            SourceBuffer.CurrentSnapshot.Length)));
}

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

public IEnumerable<ITagSpan<TextMarkerTag>> GetTags(NormalizedSnapshotSpanCollection spans)
{
    if (spans.Count == 0)   //there is no content in the buffer
        yield break;

    //don't do anything if the current SnapshotPoint is not initialized or at the end of the buffer
    if (!CurrentChar.HasValue || CurrentChar.Value.Position >= CurrentChar.Value.Snapshot.Length)
        yield break;

    //hold on to a snapshot of the current character
    SnapshotPoint currentChar = 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 != currentChar.Snapshot)
    {
        currentChar = currentChar.TranslateTo(spans[0].Snapshot, PointTrackingMode.Positive);
    }

    //get the current char and the previous char
    char currentText = currentChar.GetChar();
    SnapshotPoint lastChar = currentChar == 0 ? currentChar : currentChar - 1; //if currentChar is 0 (beginning of buffer), don't move it back
    char lastText = lastChar.GetChar();
    SnapshotSpan pairSpan = new SnapshotSpan();

    if (m_braceList.ContainsKey(currentText))   //the key is the open brace
    {
        char closeChar;
        m_braceList.TryGetValue(currentText, out closeChar);
        if (BraceMatchingTagger.FindMatchingCloseChar(currentChar, currentText, closeChar, View.TextViewLines.Count, out pairSpan) == true)
        {
            yield return new TagSpan<TextMarkerTag>(new SnapshotSpan(currentChar, 1), new TextMarkerTag("blue"));
            yield return new TagSpan<TextMarkerTag>(pairSpan, new TextMarkerTag("blue"));
        }
    }
    else if (m_braceList.ContainsValue(lastText))    //the value is the close brace, which is the *previous* character 
    {
        var open = from n in m_braceList
                   where n.Value.Equals(lastText)
                   select n.Key;
        if (BraceMatchingTagger.FindMatchingOpenChar(lastChar, (char)open.ElementAt<char>(0), lastText, View.TextViewLines.Count, out pairSpan) == true)
        {
            yield return new TagSpan<TextMarkerTag>(new SnapshotSpan(lastChar, 1), new TextMarkerTag("blue"));
            yield return new TagSpan<TextMarkerTag>(pairSpan, new TextMarkerTag("blue"));
        }
    }
}

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

private static bool FindMatchingCloseChar(SnapshotPoint startPoint, char open, char close, int maxLines, out SnapshotSpan pairSpan)
{
    pairSpan = new SnapshotSpan(startPoint.Snapshot, 1, 1);
    ITextSnapshotLine line = startPoint.GetContainingLine();
    string lineText = line.GetText();
    int lineNumber = line.LineNumber;
    int offset = startPoint.Position - line.Start.Position + 1;

    int stopLineNumber = startPoint.Snapshot.LineCount - 1;
    if (maxLines > 0)
        stopLineNumber = Math.Min(stopLineNumber, lineNumber + maxLines);

    int openCount = 0;
    while (true)
    {
        //walk the entire line
        while (offset < line.Length)
        {
            char currentChar = lineText[offset];
            if (currentChar == close) //found the close character
            {
                if (openCount > 0)
                {
                    openCount--;
                }
                else    //found the matching close
                {
                    pairSpan = new SnapshotSpan(startPoint.Snapshot, line.Start + offset, 1);
                    return true;
                }
            }
            else if (currentChar == open) // this is another open
            {
                openCount++;
            }
            offset++;
        }

        //move on to the next line
        if (++lineNumber > stopLineNumber)
            break;

        line = line.Snapshot.GetLineFromLineNumber(lineNumber);
        lineText = line.GetText();
        offset = 0;
    }

    return false;
}

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

private static bool FindMatchingOpenChar(SnapshotPoint startPoint, char open, char close, int maxLines, out SnapshotSpan pairSpan)
{
    pairSpan = new SnapshotSpan(startPoint, startPoint);

    ITextSnapshotLine line = startPoint.GetContainingLine();

    int lineNumber = line.LineNumber;
    int offset = 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)
    {
        line = line.Snapshot.GetLineFromLineNumber(--lineNumber);
        offset = line.Length - 1;
    }

    string lineText = line.GetText();

    int stopLineNumber = 0;
    if (maxLines > 0)
        stopLineNumber = Math.Max(stopLineNumber, lineNumber - maxLines);

    int closeCount = 0;

    while (true)
    {
        // Walk the entire line
        while (offset >= 0)
        {
            char currentChar = lineText[offset];

            if (currentChar == open)
            {
                if (closeCount > 0)
                {
                    closeCount--;
                }
                else // We've found the open character
                {
                    pairSpan = new SnapshotSpan(line.Start + offset, 1); //we just want the character itself
                    return true;
                }
            }
            else if (currentChar == close)
            {
                closeCount++;
            }
            offset--;
        }

        // Move to the previous line
        if (--lineNumber < stopLineNumber)
            break;

        line = line.Snapshot.GetLineFromLineNumber(lineNumber);
        lineText = line.GetText();
        offset = line.Length - 1;
    }
    return false;
}

Implementing a Brace Matching Tagger Provider

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

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

[Export(typeof(IViewTaggerProvider))]
[ContentType("text")]
[TagType(typeof(TextMarkerTag))]
internal class BraceMatchingTaggerProvider : IViewTaggerProvider

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

public ITagger<T> CreateTagger<T>(ITextView textView, ITextBuffer buffer) where T : ITag
{
    if (textView == null)
        return null;

    //provide highlighting only on the top-level buffer
    if (textView.TextBuffer != buffer)
        return null;

    return new BraceMatchingTagger(textView, buffer) as ITagger<T>;
}

Building and Testing the Code

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

To build and test BraceMatchingTest solution

Build the solution.
When you run this project in the debugger, a second instance of Visual Studio is instantiated.
Create a text file and type some text that includes matching braces.
```
hello {
goodbye}

{}

{hello}
```
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.

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