Developer Notes for Qt Code - bitcoin-core/bitcoin-devwiki GitHub Wiki

1. Git Tips
2. Backward Compatibility
3. QObject Subclassing Style
4. Qt Signal-Slot Connection Tips
5. Writing Code for Translation
6. Debugging Tips

The GUI repo and the main Bitcoin Core repo share the same commit history. If you are going to contribute, including reviewing, to both of the repo, it is recommended to have them locally.

1. Log into GitHub, navigate to the GUI repo, and fork it.

2. Clone the GUI repo locally:

$ git clone https://github.com/bitcoin-core/gui.git && cd gui

3. Set up your forked repo as a local git remote ("satoshi" is used as an example GitHub username)::

$ git remote add satoshi https://github.com/satoshi/gui.git
$ git config remote.pushDefault satoshi

4. Verify steps were performed correctly:

$ git remote -v
origin	https://github.com/bitcoin-core/gui.git (fetch)
origin	https://github.com/bitcoin-core/gui.git (push)
satoshi	https://github.com/satoshi/gui.git (fetch)
satoshi	https://github.com/satoshi/gui.git (push)

5. To synchronize your local and forked repos:

$ git switch master
$ git pull --ff-only
$ git push

The following highlights important steps of the review process using PR 42 as an example.

It is recommended to review pull requests (PR) locally as this allows you to build binaries and test functionality.

To fetch the PR branch to your local repo, run the following command:

$ git fetch origin pull/42/head:pr42.01 && git switch pr42.01

This command clones PR 42 into a new branch named pr42.01 then switches to it. This example branch name includes a sequence number .01. Sequence numbers make it convenient to compare how the PR changes as development occurs.

As a PR goes through the review process, changes are made. When changes occur, you must re-fetch the PR to test it locally. It is recommended to use separate local branches for every fetch. Each fetch should increment the sequence number used.

To examine differences between updates:

$ git diff pr42.03 pr42.04

To examine differences when a PR is rebased:

$ git range-diff master pr42.03 pr42.04

You can delete all local branches of the PR after it has been successfully merged:

$ git switch master
$ git branch -D $(git branch | grep pr42)

The source code must always maintain compatibility with the minimum required Qt version, which is set in configure.ac:

BITCOIN_QT_CONFIGURE([5.9.5])

If an optional feature requires a higher version of Qt, or if a feature was replaced by another one, use the QT_VERSION and QT_VERSION_CHECK macros:

#include <QDate>
#include <QDateTime>
#include <QtGlobal>  // For QT_VERSION and QT_VERSION_CHECK macros.

QDateTime StartOfDay(const QDate& date)
{
#if (QT_VERSION >= QT_VERSION_CHECK(5, 14, 0))
    return date.startOfDay();  // QDate::startOfDay was introduced in Qt 5.14.0.
#else
    return {date};
#endif
}

Do not compare versions of QT_VERSION directly using 0xMMNNPP (MM = major, NN = minor, PP = patch), as this approach is less readable and more error-prone.

Every time the minimum required Qt version is bumped, grep or git grep for all of the QT_VERSION instances and adjust/remove them accordingly.

When subclassing the QObject class follow the pattern below:

#include <QObject>

class MyWidget : public QObject
{
    Q_OBJECT

public:
    // Public members.

public Q_SLOTS:
    // Public slots.

Q_SIGNALS:
    // Signals (inherently public).

private:
    // Private members.

private Q_SLOTS:
    // Private slots.
};

Use the Q_SIGNALS and Q_SLOTS macros instead of the signals and slots keywords of the Qt moc (Meta-Object Compiler). This prevents potential conflicts with 3rd party signal/slot mechanisms.

Use Qt's standard camelCase naming convention for new signals and slots:

void TransactionWidget::transactionChanged()
...

This makes reading Qt code easier.

Note. This rule is an exception from Developer Notes.

Avoid the auto-connection feature in new code:

• https://github.com/bitcoin/bitcoin/pull/18246
• https://bugreports.qt.io/browse/QTBUG-76375

A slot can be connected to a given signal if the signal has at least as many arguments as the slot, and there is implicit conversion among the corresponding argument types between signal and slot.

The best way to drop arguments or convert them is to make it explicit by using a lambda as a slot:

// The signal signature is QAbstractButton::clicked(bool checked).
connect(ui->clearButton, &QAbstractButton::clicked, this, [this] { clear(); });

Use base class functions as this makes the code more general, e.g., use QAbstractButton::clicked instead of QPushButton::clicked.

Throwing an exception from a slot invoked by Qt's signal-slot connection mechanism is considered undefined behaviour. To handle possible exceptions, use GUIUtil::ExceptionSafeConnect function as a drop-in replacement of QObject::connect:

// SendCoinsDialog::sendButtonClicked can throw.
GUIUtil::ExceptionSafeConnect(ui->sendButton, &QAbstractButton::clicked,
                              this, &SendCoinsDialog::sendButtonClicked);

It is assumed that developers follow these guides:

• Translation Process
• Translation Strings Policy

To translate a string literal (quoted text), use the QObject::tr function which returns a translated QString object:

• within classes that are derived from the QObject:

QString need_sig_text = tr("Transaction still needs signature(s).");

• in other cases:

QString NetworkToQString(Network net)
{
    switch (net) {
    case NET_UNROUTABLE: return QObject::tr("Unroutable");
    case NET_IPV4: return "IPv4";
    case NET_IPV6: return "IPv6";
    case NET_ONION: return "Onion";
    case NET_I2P: return "I2P";
    case NET_CJDNS: return "CJDNS";
    case NET_INTERNAL: return QObject::tr("Internal");
    case NET_MAX: assert(false);
    } // no default case, so the compiler can warn about missing cases
    assert(false);
}

It is highly recommended to provide a translation context via translator comments to every translatable string:

void BitcoinGUI::setNetworkActive(bool network_active)
{
    updateNetworkState();
    m_network_context_menu->clear();
    m_network_context_menu->addAction(
        //: A context menu item. The "Peers tab" is an element of the "Node window".
        tr("Show Peers tab"),
        [this] {
            rpcConsole->setTabFocus(RPCConsole::TabTypes::PEERS);
            showDebugWindow();
        });
    m_network_context_menu->addAction(
        network_active ?
            //: A context menu item.
            tr("Disable network activity") :
            //: A context menu item. The network activity was disabled previously.
            tr("Enable network activity"),
        [this, new_state = !network_active] { m_node.setNetworkActive(new_state); });
}

Some substrings are unknown at compile-time or are not intended to be translated, e.g., error messages, command line options, default file names, etc. In such cases, it is recommended to:

• avoid unwanted translation
• not break up the whole string into fragments that are translated separately

To achieve both goals, use numbered place markers with the QString::arg function:

QString EditAddressDialog::getDuplicateAddressWarning() const
{
    QString dup_address = ui->addressEdit->text();
    QString existing_label = model->labelForAddress(dup_address);
    QString existing_purpose = model->purposeForAddress(dup_address);

    if (existing_purpose == "receive" &&
            (mode == NewSendingAddress || mode == EditSendingAddress)) {
        return tr(
            "Address \"%1\" already exists as a receiving address with label "
            "\"%2\" and so cannot be added as a sending address."
            ).arg(dup_address).arg(existing_label);
    }
    return tr(
        "The entered address \"%1\" is already in the address book with "
        "label \"%2\"."
        ).arg(dup_address).arg(existing_label);
}

The order of the numbered place markers can be changed when strings are translated into other languages, but each arg will still replace the lowest-numbered unreplaced place marker, no matter where it appears. Also, if place marker %i appears more than once in the string, the arg replaces all instances.

In Designer UI Files (src/qt/forms/*ui) all <string> elements are subject to translation by default. To avoid such behavior uncheck the "translatable" property checkbox of the GUI element which is not intended to be translated. Alternatively, use the notr attribute in XML code:

<property name="placeholderText">
  <string notr="true">https://example.com/tx/%s</string>
</property>

For debugging, including signal-to-slot connection issues, you can use the QT_FATAL_WARNINGS environment variable:

$ QT_FATAL_WARNINGS=1 src/qt/bitcoin-qt -printtoconsole -debug=qt

This tip can be combined with a debugger.

More notes come soon...