Example Use Cases for QPlainTextEdit::blockCountChanged()

What is QPlainTextEdit::blockCountChanged()?

The blockCountChanged() signal is emitted whenever the number of blocks (paragraphs) in the QPlainTextEdit changes. It takes a single integer argument, newBlockCount, which represents the new number of blocks after the change.

When is it Emitted?

This signal is emitted in various scenarios:

• Text formatting changes
  In some cases, text formatting changes can lead to block count changes. For example, inserting a hard line break might create a new block.
• Programmatic changes
  When you insert or remove blocks using the QTextCursor class or other methods.
• User input
  When the user presses Enter to create a new paragraph.

How to Use it?

#include <QPlainTextEdit>

void MyWidget::onBlockCountChanged(int newBlockCount)
{
    // Do something when the block count changes
    qDebug() << "Block count changed to:" << newBlockCount;
}

void MyWidget::initUi()
{
    // ...
    QPlainTextEdit *plainTextEdit = new QPlainTextEdit(this);
    connect(plainTextEdit, &QPlainTextEdit::blockCountChanged, this, &MyWidget::onBlockCountChanged);
    // ...
}

In the onBlockCountChanged slot, you can perform actions based on the new block count. For example:

• Perform custom text processing or analysis.
• Enable or disable certain features based on the number of blocks.
• Update a counter displaying the number of paragraphs.

Example Use Cases

• Custom formatting
  Apply custom formatting to paragraphs based on their content or position.
• UI updates
  Update the UI to reflect changes in the text, such as showing a different toolbar or context menu based on the content.
• Text analysis
  Perform analysis on each paragraph as it's added or removed.
• Word counting
  Track the number of paragraphs to calculate the total word count.

• Document structure
  The QTextDocument class provides a higher-level view of the text document structure, including blocks, frames, and tables.
• Text cursor
  For more fine-grained control over text manipulation, use the QTextCursor class.
• Performance
  If you need to perform heavy computations in response to blockCountChanged, consider optimizing your code or using a debouncing mechanism to avoid performance issues.

By understanding and effectively using QPlainTextEdit::blockCountChanged(), you can create more interactive and responsive text editing applications.

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Basic Example: Counting Paragraphs

#include <QPlainTextEdit>
#include <QLabel>
#include <QVBoxLayout>
#include <QWidget>

class ParagraphCounter : public QWidget {
public:
    ParagraphCounter(QWidget *parent = nullptr) : QWidget(parent) {
        plainTextEdit = new QPlainTextEdit(this);
        paragraphCountLabel = new QLabel(this);

        connect(plainTextEdit, &QPlainTextEdit::blockCountChanged, this, &ParagraphCounter::updateParagraphCount);

        QVBoxLayout *layout = new QVBoxLayout(this);
        layout->addWidget(plainTextEdit);
        layout->addWidget(paragraphCountLabel);
    }

private slots:
    void updateParagraphCount(int newBlockCount) {
        paragraphCountLabel->setText(QString("Paragraphs: %1").arg(newBlockCount));
    }

private:
    QPlainTextEdit *plainTextEdit;
    QLabel *paragraphCountLabel;
};

Example with Line Numbers

#include <QPlainTextEdit>
#include <QWidget>
#include <QPainter>

class LineNumberArea : public QWidget {
public:
    LineNumberArea(QPlainTextEdit *editor) : QWidget(editor), codeEditor(editor) {
        codeEditor->blockCountChanged.connect(this, &LineNumberArea::updateLineNumberAreaWidth);
        codeEditor->updateRequest.connect(this, &LineNumberArea::updateLineNumberArea);
        codeEditor->cursorPositionChanged.connect(this, &LineNumberArea::highlightCurrentLine);
        updateLineNumberAreaWidth(0);
        highlightCurrentLine();
    }

    QSize sizeHint() const override {
        return QSize(lineNumberAreaWidth(), 0);
    }

protected:
    void paintEvent(QPaintEvent *event) override {
        // ... paint line numbers here ...
    }

private:
    void updateLineNumberAreaWidth(int newBlockCount) {
        // ... calculate width based on new block count ...
    }

    void updateLineNumberArea(const QRect &rect, int dy) {
        // ... update line number area ...
    }

    void highlightCurrentLine() {
        // ... highlight current line ...
    }

    QPlainTextEdit *codeEditor;
    int lineNumberAreaWidth;
};

#include <QPlainTextEdit>
#include <QTextBlockFormat>

class CustomParagraphFormatter : public QPlainTextEdit {
public:
    CustomParagraphFormatter(QWidget *parent = nullptr) : QPlainTextEdit(parent) {
        connect(this, &QPlainTextEdit::blockCountChanged, this, &CustomParagraphFormatter::formatNewParagraph);
    }

private slots:
    void formatNewParagraph(int newBlockCount) {
        if (newBlockCount <= 1) {
            return; // No need to format the first paragraph
        }

        QTextBlockFormat format;
        format.setIndent(1); // Indent new paragraphs
        format.setTopMargin(12); // Add top margin

        QTextBlock block = document()->findBlockByLineNumber(newBlockCount - 1);
        block.setBlockFormat(format);
    }
};

These examples demonstrate different ways to utilize the blockCountChanged signal to enhance your QPlainTextEdit functionality. You can combine these techniques or create your own custom logic to achieve the desired behavior.

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QTextDocument Signals:

• undoCommandAdded() and redoCommandAdded()
  These signals indicate changes in the document's undo/redo history, which might indirectly reflect block count changes. However, relying on these signals can be complex and might not always accurately reflect the desired behavior.
• contentsChanged()
  This signal is emitted whenever the contents of the document are modified, including changes in block count. It's more granular than blockCountChanged() but can be less efficient if you're only interested in block count changes.

Custom Text Manipulation:

• Regular expressions or text parsing
  Depending on the specific requirements, you can analyze the text content using regular expressions or custom parsing logic to determine the number of paragraphs. This might be suitable for complex text structures or when you need to extract additional information from the text.
• Manual block counting
  If you have full control over how text is inserted and removed, you can maintain a separate counter and increment/decrement it accordingly. This approach offers more flexibility but requires careful implementation to avoid inconsistencies.

When to Consider Alternatives:

• Complexity
  For complex text processing scenarios or when integrating with other components, custom approaches might offer more flexibility.
• Granularity
  If you need more detailed information about text changes than just block count, using contentsChanged() or custom text manipulation might be more appropriate.
• Performance
  If blockCountChanged() is causing performance issues, especially with frequent updates, exploring alternatives might be necessary.

• Maintainability
  Consider the long-term maintainability of the code when implementing custom logic.
• Accuracy
  Ensure that the chosen alternative accurately reflects the desired block count behavior.
• Efficiency
  Be mindful of the performance implications of alternative approaches, especially when dealing with large text documents or frequent updates.