Introduce event dispatcher

Author: simoarpe

android/src/main/java/com/pspdfkit/flutter/pspdfkit/EventDispatcher.java

@@ -0,0 +1,45 @@
+package com.pspdfkit.flutter.pspdfkit;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
+import io.flutter.plugin.common.MethodChannel;
+
+/**
+ * Internal singleton class used to communicate between activities and the PSPDFKit Flutter plugin.
+ */
+public class EventDispatcher {
+    @Nullable
+    private static EventDispatcher instance;
+    /**
+     * A channel for sending events from Java to Flutter. This is set as soon as
+     * the plugin is registered.
+     */
+    @Nullable
+    private MethodChannel channel = null;
+
+    private EventDispatcher() {
+    }
+
+    @NonNull
+    public static synchronized EventDispatcher getInstance() {
+        if (instance == null) {
+            instance = new EventDispatcher();
+        }
+        return instance;
+    }
+
+    public void setChannel(@Nullable MethodChannel channel) {
+        this.channel = channel;
+    }
+
+    public void notifyActivityOnPause() {
+        sendEvent("flutterPdfActivityOnPause");
+    }
+
+    private void sendEvent(@NonNull final String method) {
+        if (channel != null) {
+            channel.invokeMethod(method, null, null);
+        }
+    }
+}

android/src/main/java/com/pspdfkit/flutter/pspdfkit/FlutterPdfActivity.java

@@ -3,14 +3,13 @@
 import android.os.Bundle;
 
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 
 import com.pspdfkit.document.PdfDocument;
 import com.pspdfkit.ui.PdfActivity;
 
 import java.util.concurrent.atomic.AtomicReference;
 
-import io.flutter.plugin.common.MethodChannel;
-import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
 
 /**
@@ -19,18 +18,13 @@
  */
 public class FlutterPdfActivity extends PdfActivity {
 
-    private static FlutterPdfActivity currentActivity;
-    private static MethodChannel channel;
-    private static AtomicReference<Result> loadedDocumentResult = new AtomicReference<>();
+    @Nullable private static FlutterPdfActivity currentActivity;
+    @NonNull private static final AtomicReference<Result> loadedDocumentResult = new AtomicReference<>();
 
     public static void setLoadedDocumentResult(Result result) {
         loadedDocumentResult.set(result);
     }
 
-    public static void setMethodChannel(MethodChannel methodChannel) {
-        channel = methodChannel;
-    }
-
     @Override
     protected void onCreate(Bundle bundle) {
         super.onCreate(bundle);
@@ -39,8 +33,9 @@ protected void onCreate(Bundle bundle) {
 
     @Override
     protected void onPause() {
+        // Notify the Flutter PSPDFKit plugin that the activity is going to enter the onPause state.
+        EventDispatcher.getInstance().notifyActivityOnPause();
         super.onPause();
-        channel.invokeMethod("flutterPdfActivityOnPause", null, null);
     }
 
     @Override
@@ -79,6 +74,7 @@ private void releaseActivity() {
         currentActivity = null;
     }
 
+    @Nullable
     public static FlutterPdfActivity getCurrentActivity() {
         return currentActivity;
     }

android/src/main/java/com/pspdfkit/flutter/pspdfkit/PspdfkitPlugin.java

@@ -59,13 +59,13 @@
  * PSPDFKit plugin to load PDF and image documents.
  */
 public class PspdfkitPlugin implements MethodCallHandler, PluginRegistry.RequestPermissionsResultListener {
+    @NonNull private static final EventDispatcher eventDispatcher = EventDispatcher.getInstance();
     private static final String LOG_TAG = "PSPDFKitPlugin";
     private static final String FILE_SCHEME = "file:///";
-    private final Context context;
-    private final Registrar registrar;
-    private static MethodChannel channel;
+    @NonNull private final Context context;
+    @NonNull private final Registrar registrar;
     /** Atomic reference that prevents sending twice the permission result and throwing exception. */
-    private AtomicReference<Result> permissionRequestResult;
+    @NonNull private final AtomicReference<Result> permissionRequestResult;
 
     private PspdfkitPlugin(Registrar registrar) {
         this.context = registrar.activeContext();
@@ -77,20 +77,18 @@ private PspdfkitPlugin(Registrar registrar) {
      * Plugin registration.
      */
     public static void registerWith(Registrar registrar) {
-        channel = new MethodChannel(registrar.messenger(), "pspdfkit");
+        MethodChannel channel = new MethodChannel(registrar.messenger(), "pspdfkit");
         PspdfkitPlugin pspdfkitPlugin = new PspdfkitPlugin(registrar);
         channel.setMethodCallHandler(pspdfkitPlugin);
+        eventDispatcher.setChannel(channel);
         registrar.addRequestPermissionsResultListener(pspdfkitPlugin);
     }
 
     @Override
     public void onMethodCall(@NonNull MethodCall call, @NonNull Result result) {
         String fullyQualifiedName;
-        FlutterPdfActivity flutterPdfActivity;
         PdfDocument document;
 
-        FlutterPdfActivity.setMethodChannel(channel);
-
         switch (call.method) {
             case "frameworkVersion":
                 result.success("Android " + PSPDFKit.VERSION);