feat: camper van energy monitoring widget for Plasma 6main

Pure QML KPackage widget with Rust background service for real-time
Victron energy system monitoring via MQTT and D-Bus.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 63 insertions, 0 deletions

diff --git a/service/src/cache.rs b/service/src/cache.rs
new file mode 100644
index 0000000..acaf9d3
--- /dev/null
+++ b/service/src/cache.rs
@@ -0,0 +1,63 @@
+use std::{
+    path::PathBuf,
+    time::{Duration, SystemTime},
+};
+
+use anyhow::{Result, anyhow};
+
+#[derive(serde::Serialize, serde::Deserialize, Debug, Clone)]
+struct SerialCacheEntry {
+    serial: String,
+    expires_at_unix: u64,
+}
+
+fn cache_file_path() -> Result<PathBuf> {
+    let proj_dir = dirs::cache_dir()
+        .ok_or_else(|| anyhow!("No cache dir"))?
+        .join("camper-widget-refresh");
+    Ok(proj_dir.join("serial.json"))
+}
+
+pub async fn read_serial_cache() -> Result<Option<String>> {
+    let path = cache_file_path()?;
+    if !path.exists() {
+        return Ok(None);
+    }
+    let content = tokio::fs::read_to_string(&path)
+        .await
+        .map_err(|e| anyhow!("Failed to read cache file at {}: {}", path.display(), e))?;
+    let entry: SerialCacheEntry = serde_json::from_str(&content)?;
+    let now = SystemTime::now()
+        .duration_since(SystemTime::UNIX_EPOCH)?
+        .as_secs();
+    if now <= entry.expires_at_unix {
+        Ok(Some(entry.serial))
+    } else {
+        Ok(None)
+    }
+}
+
+pub async fn write_serial_cache(serial: &str, ttl: Duration) -> Result<()> {
+    let path = cache_file_path()?;
+    if let Some(parent) = path.parent() {
+        tokio::fs::create_dir_all(parent).await.map_err(|e| {
+            anyhow!(
+                "Failed to create cache directory {}: {}",
+                parent.display(),
+                e
+            )
+        })?;
+    }
+    let now = SystemTime::now()
+        .duration_since(SystemTime::UNIX_EPOCH)?
+        .as_secs();
+    let entry = SerialCacheEntry {
+        serial: serial.to_string(),
+        expires_at_unix: now + ttl.as_secs(),
+    };
+    let data = serde_json::to_string(&entry)?;
+    tokio::fs::write(&path, data)
+        .await
+        .map_err(|e| anyhow!("Failed to write cache file at {}: {}", path.display(), e))?;
+    Ok(())
+}