Understanding QTextBrowser::openExternalLinks in Qt Widgets

Purpose

• By default, openExternalLinks is set to true, meaning the browser will attempt to launch the default application associated with the clicked URL (e.g., web browser for HTTP links, email client for mailto links).
• Controls whether QTextBrowser automatically opens links (URLs) that the user clicks within the displayed HTML content.

How it Works

1. When the user clicks on an anchor element (a clickable link) in the QTextBrowser, the anchorClicked(const QUrl &link) signal is emitted.
2. If openExternalLinks is true, Qt examines the URL and determines the appropriate default application to handle it.
3. Qt then attempts to launch the external application with the clicked URL as an argument.

Customization

1. Disabling Automatic Opening
   Set openExternalLinks to false to prevent automatic opening of links. You can then implement your own logic to handle link clicks, such as displaying a confirmation dialog or opening the link in a separate window within your application.
2. Handling Specific Links
   You can intercept the anchorClicked(const QUrl &link) signal and take custom actions based on the clicked URL's scheme (e.g., "http", "mailto"). This allows you to handle certain link types differently within your application.

Example Code

#include <QApplication>
#include <QTextBrowser>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QTextBrowser browser;
    browser.setOpenExternalLinks(false); // Disable automatic opening

    // Connect to the anchorClicked signal
    QObject::connect(&browser, &QTextBrowser::anchorClicked,
                      [&](const QUrl &link) {
                          // Handle link clicks here, e.g., display a confirmation dialog
                          if (QMessageBox::question(nullptr, "Open Link?",
                                                    "Do you want to open " + link.toString() + "?",
                                                    QMessageBox::Yes | QMessageBox::No) == QMessageBox::Yes) {
                              // Open the link in your desired way (e.g., using QDesktopServices)
                          }
                      });

    browser.setSource(QUrl("path/to/your/html_file.html")); // Load HTML content

    browser.show();

    return app.exec();
}

In this example, clicking a link within the QTextBrowser will prompt the user for confirmation before opening the URL in an external application.

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Opening Links in a Separate Window (within your application)

#include <QApplication>
#include <QTextBrowser>
#include <QUrl>
#include <QDesktopWidget>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QTextBrowser browser;

    // Connect to the anchorClicked signal
    QObject::connect(&browser, &QTextBrowser::anchorClicked,
                      [&](const QUrl &link) {
                          // Create a new QTextBrowser window for the clicked link
                          QTextBrowser *newWindow = new QTextBrowser;
                          newWindow->setSource(link);
                          newWindow->show();
                          newWindow->resize(QApplication::desktop()->availableGeometry().size() * 0.5); // Set window size to half of screen
                      });

    browser.setSource(QUrl("path/to/your/html_file.html"));

    browser.show();

    return app.exec();
}

This example creates a new QTextBrowser window whenever a link is clicked, displaying the linked content within your application.

Handling Specific Link Types

#include <QApplication>
#include <QTextBrowser>
#include <QUrl>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QTextBrowser browser;

    // Connect to the anchorClicked signal
    QObject::connect(&browser, &QTextBrowser::anchorClicked,
                      [&](const QUrl &link) {
                          if (link.scheme() == "http") {
                              // Open HTTP links in a web browser
                              QDesktopServices::openUrl(link);
                          } else if (link.scheme() == "mailto") {
                              // Handle mailto links (e.g., open email client)
                              // ... (implement your custom logic)
                          } else {
                              // Handle other link types (optional)
                              // ...
                          }
                      });

    browser.setSource(QUrl("path/to/your/html_file.html"));

    browser.show();

    return app.exec();
}

This example differentiates between HTTP links and mailto links. HTTP links are opened using QDesktopServices::openUrl, while mailto links can be handled according to your specific requirements (e.g., launching an email client).

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Custom Link Handling with anchorClicked Signal

• Drawbacks
  
  • Requires more development effort compared to openExternalLinks.
• Benefits
  
  • Provides complete control over link behavior.
  • Allows for custom UI elements like confirmation dialogs.
• Implementation
  
  1. Connect to the anchorClicked(const QUrl &link) signal of the QTextBrowser.
  2. Inside the signal handler, examine the clicked URL using methods like scheme(), host(), and path().
  3. Based on the URL properties or other criteria, take appropriate actions:
     • Open the link in an external application using QDesktopServices::openUrl(link).
     • Display a confirmation dialog before opening.
     • Open the link in a separate QTextBrowser window within your application (similar to the example code provided earlier).
     • Handle specific link types (e.g., mailto) differently.
• Description
  This approach involves overriding the default behavior of QTextBrowser and implementing your own logic for handling link clicks.

QTextDocument with Custom Link Highlighting and Click Handling

• Drawbacks
  
  • More complex implementation compared to the anchorClicked signal handler.
• Benefits
  
  • Offers greater control over link appearance and behavior.
  • Can be combined with other text formatting features.
• Implementation
  
  1. Subclass QTextDocument and override the createDocumentFragment(const QString &text) method.
  2. Inside createDocumentFragment, identify and format links within the provided text using regular expressions or other techniques.
  3. Set custom properties on the formatted text fragments to differentiate them from regular text.
  4. Connect to the mousePressEvent of the QTextBrowser to handle clicks.
  5. In the mousePressEvent handler, check if the clicked position corresponds to a formatted link fragment (using the custom property).
  6. If a link is clicked, extract the URL and perform the desired action (similar to the anchorClicked signal approach).
• Description
  This approach involves creating a custom subclass of QTextDocument that overrides link highlighting and click behavior.

• Drawbacks
  
  • Introduces additional dependencies for your project.
  • Might have a steeper learning curve compared to using Qt's built-in widgets.
• Benefits
  
  • May provide more advanced features for handling links and other rich text elements.
• Description
  If your requirements are extensive, consider using third-party rich text editor libraries like QtWebKit or KDE's KTextEditor.