Updating hello-jnicallback to support large screen devices

Timer does not restart on configuration changes and utilizes a ViewModel to preserve state. Button added to start and stop.
Small naming convention fix with stopTimer

Author: Matt Monasch

hello-jniCallback/app/src/main/cpp/hello-jnicallback.c

@@ -284,7 +284,7 @@ Java_com_example_hellojnicallback_MainActivity_startTicks(JNIEnv *env, jobject i
  *    for a clean shutdown. The caller is from onPause
  */
 JNIEXPORT void JNICALL
-Java_com_example_hellojnicallback_MainActivity_StopTicks(JNIEnv *env, jobject instance) {
+Java_com_example_hellojnicallback_MainActivity_stopTicks(JNIEnv *env, jobject instance) {
     pthread_mutex_lock(&g_ctx.lock);
     g_ctx.done = 1;
     pthread_mutex_unlock(&g_ctx.lock);

hello-jniCallback/app/src/main/java/com/example/hellojnicallback/MainActivity.java

@@ -17,56 +17,98 @@
 
 import androidx.annotation.Keep;
 import androidx.appcompat.app.AppCompatActivity;
+import androidx.lifecycle.ViewModelProvider;
+import androidx.lifecycle.ViewModelProvider.AndroidViewModelFactory;
+
 import android.os.Bundle;
+import android.view.View;
+import android.widget.Button;
 import android.widget.TextView;
 
 public class MainActivity extends AppCompatActivity {
 
-    int hour = 0;
-    int minute = 0;
-    int second = 0;
+    MainViewModel model;
     TextView tickView;
+    TextView helloJniMsg;
+    Button pauseBtn;
+    
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
-        tickView = (TextView) findViewById(R.id.tickView);
+        tickView = findViewById(R.id.tickView);
+        helloJniMsg = findViewById(R.id.hellojniMsg);
+        pauseBtn = findViewById(R.id.pauseBtn);
+
+        // Fetch the ViewModel, or have one instantiated
+        model = new ViewModelProvider(this,
+                AndroidViewModelFactory.getInstance(this.getApplication())
+        ).get(MainViewModel.class);
+
+        // if the timer has yet to be started, toggle it's state to trigger it's first run
+        // otherwise, and only if it was previously running, start the jni thread for tick callbacks
+        if(!model.started) {
+            model.started = true;
+            toggleTimerState();
+        } else {
+            if(model.running) {
+                startTicks();
+            }
+        }
+
+        setText();
     }
+
+    /*
+     * onDestroy gets called for configuration changes.
+     * We make sure that the jni context cleans up so we don't lose track of threads.
+     */
     @Override
-    public void onResume() {
-        super.onResume();
-        hour = minute = second = 0;
-        ((TextView)findViewById(R.id.hellojniMsg)).setText(stringFromJNI());
-        startTicks();
+    protected void onDestroy(){
+        super.onDestroy();
+        if(model.running) {
+            stopTicks();
+        }
     }
 
-    @Override
-    public void onPause () {
-        super.onPause();
-        StopTicks();
+    private void toggleTimerState() {
+        model.running = !model.running;
+        setText();
+        if(model.running){
+            model.resetTimer();
+            startTicks();
+        } else {
+            stopTicks();
+        }
+    }
+
+    private void setText() {
+        helloJniMsg.setText(stringFromJNI());
+        tickView.setText(model.time());
+
+        if(model.running) {
+            pauseBtn.setText(R.string.pause);
+        } else {
+            pauseBtn.setText(R.string.resume);
+        }
+
+
+    }
+
+    public void onPauseBtn(View v){
+        toggleTimerState();
     }
 
     /*
      * A function calling from JNI to update current timer
      */
     @Keep
     private void updateTimer() {
-        ++second;
-        if(second >= 60) {
-            ++minute;
-            second -= 60;
-            if(minute >= 60) {
-                ++hour;
-                minute -= 60;
-            }
-        }
+        model.updateTimer();
         runOnUiThread(new Runnable() {
             @Override
             public void run() {
-                String ticks = "" + MainActivity.this.hour + ":" +
-                        MainActivity.this.minute + ":" +
-                        MainActivity.this.second;
-                MainActivity.this.tickView.setText(ticks);
+                MainActivity.this.tickView.setText(model.time());
             }
         });
     }
@@ -75,5 +117,5 @@ public void run() {
     }
     public native  String stringFromJNI();
     public native void startTicks();
-    public native void StopTicks();
+    public native void stopTicks();
 }

hello-jniCallback/app/src/main/java/com/example/hellojnicallback/MainViewModel.java

@@ -0,0 +1,31 @@
+package com.example.hellojnicallback;
+
+import androidx.lifecycle.ViewModel;
+
+public class MainViewModel extends ViewModel {
+    private int hour = 0;
+    private int minute = 0;
+    private int second = 0;
+    public boolean started = false;
+    public boolean running = false;
+
+    public void updateTimer() {
+        ++second;
+        if(second >= 60) {
+            ++minute;
+            second -= 60;
+            if(minute >= 60) {
+                ++hour;
+                minute -= 60;
+            }
+        }
+    }
+
+    public void resetTimer() {
+        hour = minute = second = 0;
+    }
+
+    public String time() {
+        return hour + ":" + minute + ":" + second;
+    }
+}

hello-jniCallback/app/src/main/res/layout/activity_main.xml

@@ -43,5 +43,16 @@
         app:layout_constraintBottom_toBottomOf="@+id/activity_hello_jnicallback"
         tools:layout_constraintBottom_creator="0" />
 
+    <Button
+        android:id="@+id/pauseBtn"
+        android:layout_width="wrap_content"
+        android:layout_height="48dp"
+        android:layout_marginTop="32dp"
+        android:onClick="onPauseBtn"
+        android:text="Button"
+        app:layout_constraintEnd_toEndOf="parent"
+        app:layout_constraintStart_toStartOf="parent"
+        app:layout_constraintTop_toBottomOf="@+id/tickView" />
+
 
 </androidx.constraintlayout.widget.ConstraintLayout>

hello-jniCallback/app/src/main/res/values/strings.xml

@@ -1,3 +1,5 @@
 <resources>
     <string name="app_name">Hello-jniCallback</string>
+    <string name="pause">PAUSE</string>
+    <string name="resume">RESUME</string>
 </resources>