要实现一个完整的 Harmattan 应用，涵盖**预览页面**、**登录页面**、**主时间线页面**，并且包含导航逻辑和 API 调用，可以基于 QML 和 C++ 代码进行。以下是一个基于你的需求的完整项目结构及其代码示例。

### 项目结构

- `main.cpp`: 应用入口，初始化 QML 环境和 C++ 类。
- `YourApp.qml`: 主要的 UI 界面，包括预览、登录和主时间线页面。
- `YourCppClass.cpp` / `YourCppClass.h`: C++ 类，用于处理 API 请求和数据通信。

---

### 1. `main.cpp` - 应用入口

```cpp
#include <QtGui/QApplication>
#include <QtDeclarative>
#include "YourCppClass.h"

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

    QDeclarativeView view;
    YourCppClass yourCppClassInstance;

    view.rootContext()->setContextProperty("yourCppClassInstance", &yourCppClassInstance);
    view.setSource(QUrl::fromLocalFile("qml/YourApp.qml"));

    view.show();
    return app.exec();
}
```

---

### 2. `YourCppClass.h` - C++ 类定义

```cpp
#ifndef YOURCPPCLASS_H
#define YOURCPPCLASS_H

#include <QObject>
#include <QtNetwork>

class YourCppClass : public QObject
{
    Q_OBJECT
public:
    explicit YourCppClass(QObject *parent = nullptr);

    Q_INVOKABLE void loadPublicTimeline(const QString &instanceUrl);
    Q_INVOKABLE void login(const QString &instanceUrl, const QString &username, const QString &password);
    Q_INVOKABLE void loadHomeTimeline();

signals:
    void publicTimelineLoaded(const QVariantList &timelineData);
    void loginSuccess();
    void homeTimelineLoaded(const QVariantList &timelineData);

private slots:
    void onPublicTimelineReceived();
    void onLoginFinished();
    void onHomeTimelineReceived();

private:
    QNetworkAccessManager *networkManager;
    QString accessToken;
};

#endif // YOURCPPCLASS_H
```

---

### 3. `YourCppClass.cpp` - C++ 类实现

```cpp
#include "YourCppClass.h"
#include <QJsonDocument>
#include <QJsonArray>
#include <QJsonObject>

YourCppClass::YourCppClass(QObject *parent) : QObject(parent)
{
    networkManager = new QNetworkAccessManager(this);
}

void YourCppClass::loadPublicTimeline(const QString &instanceUrl)
{
    QUrl url(instanceUrl + "/api/v1/timelines/public");
    QNetworkRequest request(url);

    QNetworkReply *reply = networkManager->get(request);
    connect(reply, &QNetworkReply::finished, this, &YourCppClass::onPublicTimelineReceived);
}

void YourCppClass::onPublicTimelineReceived()
{
    QNetworkReply *reply = qobject_cast<QNetworkReply*>(sender());
    if (reply) {
        QByteArray response = reply->readAll();
        QJsonDocument jsonDoc = QJsonDocument::fromJson(response);
        QJsonArray jsonArray = jsonDoc.array();

        QVariantList timelineData;
        for (const QJsonValue &value : jsonArray) {
            QJsonObject obj = value.toObject();
            QVariantMap post;
            post["username"] = obj["account"].toObject()["username"].toString();
            post["content"] = obj["content"].toString();
            timelineData.append(post);
        }

        emit publicTimelineLoaded(timelineData);
    }
}

void YourCppClass::login(const QString &instanceUrl, const QString &username, const QString &password)
{
    QUrl url(instanceUrl + "/oauth/token");
    QNetworkRequest request(url);

    request.setHeader(QNetworkRequest::ContentTypeHeader, "application/x-www-form-urlencoded");

    QByteArray data;
    data.append("username=" + username + "&password=" + password);

    QNetworkReply *reply = networkManager->post(request, data);
    connect(reply, &QNetworkReply::finished, this, &YourCppClass::onLoginFinished);
}

void YourCppClass::onLoginFinished()
{
    QNetworkReply *reply = qobject_cast<QNetworkReply*>(sender());
    if (reply) {
        QByteArray response = reply->readAll();
        QJsonDocument jsonDoc = QJsonDocument::fromJson(response);
        QJsonObject jsonObj = jsonDoc.object();

        accessToken = jsonObj["access_token"].toString();
        emit loginSuccess();
    }
}

void YourCppClass::loadHomeTimeline()
{
    QUrl url("https://mastodon.example.com/api/v1/timelines/home");
    QNetworkRequest request(url);
    request.setRawHeader("Authorization", "Bearer " + accessToken.toUtf8());

    QNetworkReply *reply = networkManager->get(request);
    connect(reply, &QNetworkReply::finished, this, &YourCppClass::onHomeTimelineReceived);
}

void YourCppClass::onHomeTimelineReceived()
{
    QNetworkReply *reply = qobject_cast<QNetworkReply*>(sender());
    if (reply) {
        QByteArray response = reply->readAll();
        QJsonDocument jsonDoc = QJsonDocument::fromJson(response);
        QJsonArray jsonArray = jsonDoc.array();

        QVariantList timelineData;
        for (const QJsonValue &value : jsonArray) {
            QJsonObject obj = value.toObject();
            QVariantMap post;
            post["username"] = obj["account"].toObject()["username"].toString();
            post["content"] = obj["content"].toString();
            timelineData.append(post);
        }

        emit homeTimelineLoaded(timelineData);
    }
}
```

---

### 4. `YourApp.qml` - QML UI 文件

```qml
import QtQuick 1.1
import com.nokia.meego 1.0

PageStackWindow {
    id: appWindow
    initialPage: previewPage

    Component {
        id: previewPage
        Page {
            title: "Public Timeline Preview"
            
            Column {
                spacing: 10
                width: parent.width

                TextField {
                    id: instanceUrl
                    placeholderText: "Enter instance URL"
                    onTextChanged: {
                        yourCppClassInstance.loadPublicTimeline(instanceUrl.text);
                    }
                }

                ListView {
                    id: publicTimeline
                    model: publicTimelineModel

                    delegate: Item {
                        width: parent.width
                        height: 80
                        Column {
                            Text { text: model.username }
                            Text { text: model.content }
                        }
                    }

                    OpacityMask {
                        anchors.fill: parent
                        maskSource: Gradient {
                            GradientStop { position: 0.8; color: "transparent" }
                            GradientStop { position: 1.0; color: "black" }
                        }
                    }
                }

                Button {
                    text: "Login"
                    onClicked: {
                        appWindow.pageStack.push(loginPage);
                    }
                }
            }

            ListModel {
                id: publicTimelineModel
            }

            Connections {
                target: yourCppClassInstance
                onPublicTimelineLoaded: {
                    publicTimelineModel.clear();
                    for (var i = 0; i < timelineData.length; i++) {
                        publicTimelineModel.append(timelineData[i]);
                    }
                }
            }
        }
    }

    Component {
        id: loginPage
        Page {
            title: "Login"

            Column {
                TextField { id: username; placeholderText: "Username" }
                TextField { id: password; placeholderText: "Password"; echoMode: TextInput.Password }

                Button {
                    text: "Login"
                    onClicked: {
                        yourCppClassInstance.login(instanceUrl.text, username.text, password.text);
                    }
                }
            }

            Connections {
                target: yourCppClassInstance
                onLoginSuccess: {
                    appWindow.pageStack.push(mainTimelinePage);
                }
            }
        }
    }

    Component {
        id: mainTimelinePage
        Page {
            title: "Home Timeline"

            Column {
                ListView {
                    id: homeTimeline
                    model: homeTimelineModel
                    delegate: Item {
                        width: parent.width
                        height: 80
                        Column {
                            Text { text: model.username }
                            Text { text: model.content }
                        }
                    }
                }

                Row {
                    anchors.bottom: parent.bottom
                    Button { text: "Home" }
                    Button { text: "Notifications" }
                    Button { text: "Account" }
                }
            }

            Connections {
                target: yourCppClassInstance
                onHomeTimelineLoaded: {
                    homeTimelineModel.clear();
                    for (var i = 0; i < timelineData.length; i++) {
                        homeTimelineModel.append(timelineData[i]);
                    }
                }
            }
        }
    }
}
```

---

### 5. 总结

这个完整的应用包含了预览页面、登录页面、主时间线页面，支持实例公共时间线的查看、用户登录，以及主时间线的显示。你可以根据实际情况调整 API 请求、页面布局以及样式。

主要功能：

1. **预览页面**：输入实例地址并查看公共时间线，带有渐隐效果。
2. **登录页面**：用户输入用户名和密码，执行 Mastodon 实例的登录验证。
3. **主时间线页面**：显示用户的主时间线，并有底部导航栏切换到其他页面（通知、个人账户等）。