1. Using view model to store the status of fragment.

   Stopwatch will keep working when users switch to other App/Interface

2. Add permission request callback. The map view will be refreshed
   automatically after getting the Location permission

Author: butterflyforever

android/canonical/app/build.gradle

@@ -35,6 +35,8 @@ dependencies {
     implementation 'androidx.legacy:legacy-support-v4:1.0.0'
     implementation 'com.google.android.gms:play-services-maps:17.0.0'
     implementation 'com.google.android.gms:play-services-location:17.0.0'
+    implementation "android.arch.lifecycle:extensions:1.1.1"
+    implementation 'androidx.lifecycle:lifecycle-livedata-ktx:2.2.0'
     testImplementation 'junit:junit:4.12'
     androidTestImplementation 'androidx.test.ext:junit:1.1.1'
     androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'

android/canonical/app/src/main/java/com/google/samples/quickstart/canonical/MainActivity.kt

@@ -1,20 +1,39 @@
 package com.google.samples.quickstart.canonical
 
+import android.content.pm.PackageManager
 import androidx.appcompat.app.AppCompatActivity
 import android.os.Bundle
-import android.view.View
-import android.widget.Button
-import android.widget.Chronometer
-import android.widget.Toast
+import android.util.Log
 import androidx.fragment.app.FragmentTransaction
 import com.google.android.material.bottomnavigation.BottomNavigationView
-import org.jetbrains.annotations.Nullable
+import androidx.lifecycle.MutableLiveData
+import androidx.lifecycle.ViewModel
+
+class StopwatchView : ViewModel(){
+
+  var pauseOffset = MutableLiveData<Long>(0L)
+  var isWorking = MutableLiveData<Boolean>(false)
+  var fragmentPauseStartTime = MutableLiveData<Long>(0L)
+
+  fun setPauseOffset(pause_offset_value:Long){
+    pauseOffset.value = pause_offset_value
+  }
+
+  fun setWorkingStatus(working_status:Boolean){
+    isWorking.value = working_status
+  }
+
+  fun setFragmentPauseStartTime(fragment_pause_start_time:Long){
+    fragmentPauseStartTime.value = fragment_pause_start_time
+  }
+}
 
 class MainActivity : AppCompatActivity() {
 
-  lateinit var runFragment: RunFragment
-  lateinit var mapsFragment: MapsFragment
-  lateinit var meFragment: MeFragment
+  private lateinit var runFragment: RunFragment
+  private lateinit var mapsFragment: MapsFragment
+  private lateinit var meFragment: MeFragment
+
 
   override fun onCreate(savedInstanceState: Bundle?) {
     super.onCreate(savedInstanceState)

android/canonical/app/src/main/java/com/google/samples/quickstart/canonical/MapsFragment.kt

@@ -8,10 +8,12 @@ import android.location.Location
 import androidx.fragment.app.Fragment
 
 import android.os.Bundle
+import android.util.Log
 import android.view.LayoutInflater
 import android.view.View
 import android.view.ViewGroup
 import androidx.core.app.ActivityCompat
+import androidx.core.content.ContextCompat.checkSelfPermission
 import androidx.fragment.app.FragmentTransaction
 import com.google.android.gms.location.FusedLocationProviderClient
 import com.google.android.gms.location.LocationServices
@@ -34,13 +36,30 @@ class MapsFragment : Fragment() {
         private const val LOCATION_PERMISSION_REQUEST_CODE = 1
     }
 
+    override fun onRequestPermissionsResult(requestCode: Int,
+                                            permissions: Array<String>, grantResults: IntArray) {
+        when (requestCode) {
+            LOCATION_PERMISSION_REQUEST_CODE -> {
+                // If request is cancelled, the result arrays are empty.
+                if ((grantResults.isNotEmpty() &&
+                            grantResults[0] == PackageManager.PERMISSION_GRANTED)) {
+                    setUpMap()
+                } else {
+                    // Explain to the user that the feature is unavailable because
+                    // the features requires a permission that the user has denied.
+                }
+                return
+            }
+            else -> {
+                // Ignore all other requests.
+            }
+        }
+    }
+
     private fun setUpMap() {
-        if (ActivityCompat.checkSelfPermission(context as Activity,
+        if (checkSelfPermission(context as Activity,
                 Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
-            ActivityCompat.requestPermissions(context as Activity,
-                arrayOf(Manifest.permission.ACCESS_FINE_LOCATION), LOCATION_PERMISSION_REQUEST_CODE)
-            val mapFragment = childFragmentManager.findFragmentById(R.id.map) as SupportMapFragment?
-            mapFragment?.getMapAsync(callback)
+            requestPermissions(arrayOf(Manifest.permission.ACCESS_FINE_LOCATION), LOCATION_PERMISSION_REQUEST_CODE)
             return
         }
 
@@ -54,7 +73,6 @@ class MapsFragment : Fragment() {
                 map.animateCamera(CameraUpdateFactory.newLatLngZoom(currentLatLng, 12f))
                 map.addMarker(MarkerOptions().position(currentLatLng).title("My location"))
                 map.moveCamera(CameraUpdateFactory.newLatLng(currentLatLng))
-                println("****************** %f %f **************\n".format(location.latitude, location.longitude))
             }
         }
     }

android/canonical/app/src/main/java/com/google/samples/quickstart/canonical/RunFragment.kt

@@ -2,14 +2,15 @@ package com.google.samples.quickstart.canonical
 
 import android.os.Bundle
 import android.os.SystemClock
+import android.util.Log
 import androidx.fragment.app.Fragment
 import android.view.LayoutInflater
 import android.view.View
 import android.view.ViewGroup
 import android.widget.Button
 import android.widget.Chronometer
+import androidx.lifecycle.ViewModelProviders
 import kotlinx.android.synthetic.main.fragment_run.*
-import org.jetbrains.annotations.Nullable
 
 // TODO: Rename parameter arguments, choose names that match
 // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
@@ -25,17 +26,24 @@ class RunFragment : Fragment() {
     // TODO: Rename and change types of parameters
     private var param1: String? = null
     private var param2: String? = null
+
+    private lateinit var stopwatchViewModel: StopwatchView
     private var pauseOffset = 0L
     private var isWorking = false
+    private var fragmentPauseStartTime = 0L
 
     private fun startStopTimer(runBtn: Button, chronometer : Chronometer) {
         isWorking = if (!isWorking) {
             chronometer.base = SystemClock.elapsedRealtime() - pauseOffset
             chronometer.start()
+            chronometer.showContextMenu()
+            stopwatchViewModel.setWorkingStatus(true)
             true
         } else {
             pauseOffset = SystemClock.elapsedRealtime() - chronometer.base
             chronometer.stop()
+            stopwatchViewModel.setPauseOffset(SystemClock.elapsedRealtime() - chronometer.base)
+            stopwatchViewModel.setWorkingStatus(false)
             false
         }
         runBtn.setText(if (!isWorking) R.string.start else R.string.stop)
@@ -47,6 +55,14 @@ class RunFragment : Fragment() {
             param1 = it.getString(ARG_PARAM1)
             param2 = it.getString(ARG_PARAM2)
         }
+
+        stopwatchViewModel = activity?.run {
+            ViewModelProviders.of(this)[StopwatchView::class.java]
+        } ?: throw Exception("Null Activity")
+
+        pauseOffset = stopwatchViewModel.pauseOffset.value!!
+        isWorking = stopwatchViewModel.isWorking.value!!
+        fragmentPauseStartTime = stopwatchViewModel.fragmentPauseStartTime.value!!
     }
 
     override fun onCreateView(
@@ -88,5 +104,24 @@ class RunFragment : Fragment() {
 
     }
 
+    override fun onResume() {
+        super.onResume()
+        if (isWorking) {
+            chronometer?.base = fragmentPauseStartTime - pauseOffset
+            chronometer?.start()
+        } else {
+            chronometer?.base = SystemClock.elapsedRealtime() - pauseOffset
+            chronometer?.stop()
+        }
+    }
 
+    override fun onPause() {
+        super.onPause()
+        if (isWorking) {
+            pauseOffset = SystemClock.elapsedRealtime() - chronometer.base
+            fragmentPauseStartTime = SystemClock.elapsedRealtime()
+            stopwatchViewModel.setPauseOffset(pauseOffset)
+            stopwatchViewModel.setFragmentPauseStartTime(fragmentPauseStartTime)
+        }
+    }
 }