Foreground detection mixin service for widgets (#1404)

Signed-off-by: Dan Cunningham <dan@digitaldan.com>
Also-by: Yannick Schaus <github@schaus.net>

bundles/org.openhab.ui/web/src/components/widgets/widget-foreground-service.js

@@ -0,0 +1,50 @@
+export default {
+  data () {
+    return {
+      pageEl: null,
+      inForeground: false
+    }
+  },
+  mounted () {
+    const isInModal = this.$el.closest('.framework7-modals')
+    if (isInModal) {
+      this.inForeground = true
+      this.startForegroundActivity()
+      return
+    }
+
+    this.pageEl = this.$el.closest('.page')
+    if (this.pageEl && this.pageEl.classList.contains('page-current')) {
+      this.inForeground = true
+      this.startForegroundActivity()
+    }
+
+    this.$f7.on('pageAfterIn', this.onPageAfterIn)
+    this.$f7.on('pageBeforeOut', this.onPageBeforeOut)
+  },
+  beforeDestroy () {
+    this.$f7.off('pageAfterIn', this.onPageAfterIn)
+    this.$f7.off('pageBeforeOut', this.onPageBeforeOut)
+    this.stopForegroundActivity()
+  },
+  methods: {
+    startForegroundActivity () {
+      // override this in your widget
+    },
+    stopForegroundActivity () {
+      // override this in your widget
+    },
+    onPageAfterIn (page) {
+      if (page.el === this.pageEl) {
+        this.inForeground = true
+        this.startForegroundActivity()
+      }
+    },
+    onPageBeforeOut (page) {
+      if (page.el === this.pageEl) {
+        this.inForeground = false
+        this.stopForegroundActivity()
+      }
+    }
+  }
+}