Allow monitoring current upstreams

Fix #139.

mobile/src/main/java/be/mygod/vpnhotspot/preference/UpstreamsPreference.kt

@@ -0,0 +1,65 @@
+package be.mygod.vpnhotspot.preference
+
+import android.content.Context
+import android.graphics.Typeface
+import android.net.LinkProperties
+import android.text.SpannableStringBuilder
+import android.text.style.StyleSpan
+import android.util.AttributeSet
+import androidx.lifecycle.DefaultLifecycleObserver
+import androidx.lifecycle.LifecycleOwner
+import androidx.lifecycle.lifecycleScope
+import androidx.preference.Preference
+import be.mygod.vpnhotspot.R
+import be.mygod.vpnhotspot.net.monitor.FallbackUpstreamMonitor
+import be.mygod.vpnhotspot.net.monitor.UpstreamMonitor
+import be.mygod.vpnhotspot.util.SpanFormatter
+import be.mygod.vpnhotspot.util.parseNumericAddress
+
+class UpstreamsPreference(context: Context, attrs: AttributeSet) : Preference(context, attrs),
+        DefaultLifecycleObserver {
+    companion object {
+        private val internetAddress = parseNumericAddress("8.8.8.8")
+    }
+
+    private data class Interface(val ifname: String, val internet: Boolean)
+    private inner class Monitor : UpstreamMonitor.Callback {
+        private var currentInterface: Interface? = null
+        val charSequence get() = currentInterface?.run {
+            if (internet) SpannableStringBuilder(ifname).apply {
+                setSpan(StyleSpan(Typeface.BOLD), 0, length, 0)
+            } else ifname
+        } ?: "∅"
+
+        override fun onAvailable(ifname: String, properties: LinkProperties) {
+            currentInterface = Interface(ifname, properties.routes.any { it.matches(internetAddress) })
+            onUpdate()
+        }
+
+        override fun onLost() {
+            currentInterface = null
+            onUpdate()
+        }
+    }
+
+    private val primary = Monitor()
+    private val fallback = Monitor()
+
+    init {
+        (context as LifecycleOwner).lifecycle.addObserver(this)
+    }
+
+    override fun onStart(owner: LifecycleOwner) {
+        UpstreamMonitor.registerCallback(primary)
+        FallbackUpstreamMonitor.registerCallback(fallback)
+    }
+    override fun onStop(owner: LifecycleOwner) {
+        UpstreamMonitor.unregisterCallback(primary)
+        FallbackUpstreamMonitor.unregisterCallback(fallback)
+    }
+
+    private fun onUpdate() = (context as LifecycleOwner).lifecycleScope.launchWhenStarted {
+        summary = SpanFormatter.format(context.getText(R.string.settings_service_upstream_monitor_summary),
+                primary.charSequence, fallback.charSequence)
+    }
+}