1. Qt的Quick项目实现多语言加载主要步骤是什么：

1.1CMake怎么指定创建TS文件

# Add other components as you want.
find_package(Qt5 REQUIRED COMPONENTS Quick Qml LinguistTools)
target_link_libraries(${PROJECT_NAME} PRIVATE Qt5::Quick Qt5::Qml)

set(TS_FILES
        ${CMAKE_SOURCE_DIR}/translations/${PROJECT_NAME}_en.ts
        ${CMAKE_SOURCE_DIR}/translations/${PROJECT_NAME}_zh-CN.ts
        ${CMAKE_SOURCE_DIR}/translations/${PROJECT_NAME}_de.ts
        ${CMAKE_SOURCE_DIR}/translations/${PROJECT_NAME}_es.ts
        ${CMAKE_SOURCE_DIR}/translations/${PROJECT_NAME}_fr.ts
        ${CMAKE_SOURCE_DIR}/translations/${PROJECT_NAME}_it.ts
        ${CMAKE_SOURCE_DIR}/translations/${PROJECT_NAME}_ja.ts
        ${CMAKE_SOURCE_DIR}/translations/${PROJECT_NAME}_ko.ts
        ${CMAKE_SOURCE_DIR}/translations/${PROJECT_NAME}_pt-PT.ts)

# Generate TS file for App.
add_custom_target(
    lupdate
    COMMAND ${Qt5_LUPDATE_EXECUTABLE} ${CMAKE_SOURCE_DIR} -ts ${TS_FILES})

# Generate QM files with TS files include plugins.
add_custom_target(
    lrelease
    COMMAND ${Qt5_LRELEASE_EXECUTABLE} ${TS_FILES})

查阅官网资料你可能会发现生成ts文件和qm文件并不是上面创建target的方式，而是以下的方式，只不过Qt 6版本前功能是有缺陷的。

qt5_create_translation(QM_FILES ${CMAKE_SOURCE_DIR} app_zh_CN.ts)
add_executable(app main.cpp qml.qrc ${QM_FILES})

1.2如何代码中指定加载指定的qm文件

#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include <QLocale>
#include <QTranslator>

int main(int argc, char *argv[]) {
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);

    // ======== Make sure installing translation before loading qml. ============= //
    QTranslator translator;
    QString filePath = qApp->applicationDirPath() + "/translations/xxxApp_zh_CN.qm";
    translator.load(filePath);
    app.installTranslator(&translator);
   // ======== Make sure installing translation before loading qml. ============= //

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

如果软件多语言切换无需立即生效，那么以上代码就能满足，需要注意的是app.installTranslator(&translator)必须在加载qml之前，否则会失效。

1.3如何操作生成TS文件和QM文件

• 生成ts文件：在build里选择target lupdate，然后项目右键rebuild，ts文件将产生在项目根目录的translations里。
• 生成qm文件：在build里选择target lrelease，然后项目右键rebuild，qm文件将产生在ts所在的目录里，即：根目录的translations里。

generate_ts.png

2. 语言切换如何立即生效（无需重启）

2.1 为了方便集成多语言切换，我们写了一个多语言切换的实现类

Localization.h 内容如下：

#ifndef LOCALIZATION_H
#define LOCALIZATION_H

#include <QObject>
#include <QMap>
#include <QTranslator>

class Localization : public QObject {
    Q_OBJECT
    Q_PROPERTY(QString currentLanguage READ currentLanguage WRITE setCurrentLanguage NOTIFY currentLanguageChanged)

public:
    const QMap<QString, int> SUPPORT_LANGS = {{"en", 0x0009},{"zh-CN", 0x0004}, {"ja", 0x0011}, {"de", 0x0007},
                                               {"es", 0x000A}, {"fr", 0x000C}, {"it", 0x0010}, {"pt-PT", 0x0016},
                                               {"ko", 0x0012}};

    explicit Localization(const QString &moduleName, QObject *parent = nullptr);

    QString currentLanguage() const;
    void setCurrentLanguage(const QString &language);

   /**
     * @brief Call it once App started that notified from Qml,
     * so that plugins in App can load current language.
     */
    Q_INVOKABLE virtual void loadCurrentLanguge() = 0;

    /**
     * @brief Override to implement saving current language into config file.
     */
    virtual bool saveCurrentLanguage(const QString &language) = 0;

private:
    bool loadLanguage(const QString &language);

signals:
    void currentLanguageChanged();

protected:
    QString mModuleName{};
    QTranslator mTranslator{};
    QString mCurrentLanguage{};
};

#endif // LOCALIZATION_H

Localization.cpp 内容如下：

#include "Localization.h"

#include <QCoreApplication>
#include <QFile>
#include "Logger.h"

Localization::Localization(const QString &moduleName, QObject *parent)
    : QObject{parent},
      mModuleName(moduleName) {
}

QString Localization::currentLanguage() const { return mCurrentLanguage; }

void Localization::setCurrentLanguage(const QString &language) {
    if (mCurrentLanguage != language && loadLanguage(language)) {
        mCurrentLanguage = language;
        saveCurrentLanguage(language);
        emit currentLanguageChanged();
    }
}

bool Localization::loadLanguage(const QString &language) {
    if (language.isEmpty()) {
        LOGERROR("Language to load is empty.");
        return false;
    }

    if (!SUPPORT_LANGS.contains(language)) {
        LOGERROR("Language to load is not supported.");
        return false;
    }

    QString filePath = qApp->applicationDirPath() + QString("/translations/%1_%2.qm").arg(mModuleName, language);
    if (!QFile(filePath).exists()) {
        LOGERROR("Langauge file to load is not exist: %s", filePath.toStdString().c_str());
        return false;
    }

    if (!mTranslator.load(filePath)) {
        LOGERROR("Load language failed: %s", filePath.toStdString().c_str());
        return false;
    }

    qApp->installTranslator(&mTranslator);
    LOGINFO("Load langauge file successfully: %s", filePath.toStdString().c_str());

    emit currentLanguageChanged();
    return true;
}

需要注意的是：

在头文件里有两个虚函数需要自己实现，假如你的项目里有个名字类似AppSettings.h的实现用于集中实现App的所有配置，那么你可以考虑让他继承Localization并实现此虚函数，即：

• saveCurrentLanguage()：将设置的当前语言存入你当前项目的配置文件或者数据库中。
• loadCurrentLange()**: App启动过程中，从你当前项目的配置文件或者数据库种查询当前配置的语言并调用setCurrentLanguage(language)缓存。

class AppSettings : public Localization {
    // other code

public:
    // Save selected language to config file or database in your App.
    bool saveCurrentLanguage(const QString &language) override;

    // Find current language from your App's config file or database.
    void loadCurrentLanguage() overload;
}

读了代码，你会知道最终程序切换语言所加载的qm文件存在可执行文件目录下的translations里，并且切换成功后会发信号告知外面，通知外部为的是告知main.cpp里的QQmlApplicationEngine执行retranslate()，这样App和Plugin将全局刷新翻译，后面再讲。

2.2 如何在main.cpp里集成Localizations使得App切换翻译能立即生效

#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include <QLocale>
#include <QTranslator>

int main(int argc, char *argv[]) {
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl) {
            QCoreApplication::exit(-1);
        }
    }, Qt::QueuedConnection);

    // Hot reload translation overall.
    AppSettings appSettings{};
    QObject::connect(&appSettings, &AppSettings::currentLanguageChanged, &engine, [&engine]() -> void {
        // Refresh entire App's translation even plugins.
        engine.retranslate();
    });

    engine.load(url);
    return app.exec();
}

2.3 App加载好后通知App以及Plugin全局刷新翻译

import QtQuick 2.15
import QtQuick.Window 2.15

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    Component.onCompleted: {
        appSettings.loadCurrentLanguage()
    }
}

如果你的项目无需支持翻译切换即时生效，也可以在main.cpp里QQmlApplicationEngine engine创建之前直接调用appSettings.loadCurrentLanguage()。

2.4 Plugin是如何做到跟随App一起切换语言的呢？

如标题所说，plugin的语言切换是跟随App一起切换的：

ImagePlugin {
    id: imagePlugin
    currentLanguage: appSettings.currentLanguage
}

一般你的plugin可能会有个对外暴露的class，需要让其继承Localization，只不过无需实现Localization内置的两个虚函数，因为plugin是无需记录语言配置的，这个工作是交给App来做的：

class ImagePlugin : public Localization {
public:
    ImagePlugin (QObject *parent = nullptr);
}