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🌉 middlend Layer — Bridge Documentation

This folder contains the Contract that defines how the C++ Backend and the Svelte Frontend communicate. This approach is inspired by tRPC, ensuring that both sides speak the same "language" with strong typing.

📜 The Contract

The definition lives in contract.ts.

📤 Methods (Frontend ➔ Backend)

These are actions triggered by the UI that execute in C++.

Method	Description	Return
`openFileDialog()`	Opens a native file picker.	`Promise<string>` (File Path)
`loadPdf(path)`	Commands the backend to read a PDF file.	`void`
`translate(text, lang)`	Sends text to the C++ translation engine.	`void`

📥 Signals (Backend ➔ Frontend)

These are events emitted by C++ that the UI listens to.

Signal	Payload	Description
`pdfLoaded`	`(data, pages)`	Triggered after `loadPdf` finishes processing.
`translationReady`	`(orig, trans)`	Triggered when a translation result is available.
`errorOccurred`	`(message)`	Triggered on backend-side errors.

🛠 How to implement new features

Update the Contract: Add the new method or signal to middlend/contract.ts.
C++ Side:
- Open backend/bridge.h and add the corresponding Q_INVOKABLE method or Q_SIGNAL.
- Implement logic in backend/bridge.cpp.
Svelte Side:
- The Typescript compiler will immediately show errors if the UI doesn't match the new contract.
- Use the bridgeStore to call the new method.

🔗 Architecture

graph LR
    subgraph "Frontend (Svelte)"
        UI[App.svelte] --> Store[bridgeStore]
    end
    
    subgraph "middlend (Contract)"
        Store -- typed calls --> Contract((contract.ts))
    end
    
    subgraph "Backend (Qt/C++)"
        Contract -- QWebChannel --> QObj[Bridge Object]
    end

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