Using WebSocket to Enable Page‑App Communication for Conditional App Launch or Download

In many real‑world scenarios developers need to prompt users to download a mobile app, but if the app is already installed they want to open it directly. Since JavaScript cannot directly check app installation, the article proposes a workaround using a custom URL schema and a WebSocket link between the web page and the native app.

The Android app must expose a WebSocket server (e.g., on port 8899). The web page’s JavaScript establishes a WebSocket connection to this server. When the user clicks the download button, the page calls download(schema, downUrl) which attempts to invoke the app via its schema.

If the app is running, the WebSocket connection allows the page to communicate with it; otherwise the page must decide whether the call succeeded. The author suggests using a short setTimeout and checking whether the browser lost focus (blur) as an indicator that the app was launched.

Two methods for detecting the blur are described: listening to the window.blur event or monitoring the document.visibilityChange event and checking the document.hidden property. Sample code snippets (shown as images in the original article) illustrate both the simple and compatible implementations.

By combining the blur detection with the WebSocket communication, the page can reliably decide: if the app launch succeeds, the browser stays idle; if it fails, the page proceeds to download the app from the provided URL. The article also notes a limitation—if the app is not alive in the background process, the call cannot succeed, requiring additional work to keep the app resident.

The author invites readers to discuss any errors and promotes the “UC International Technology” public account for more high‑quality technical articles.