Fix: ViewTreeObserver is unregister when start second Activity because onStop call delay

android/build.gradle

@@ -22,7 +22,7 @@ rootProject.allprojects {
 apply plugin: 'com.android.library'
 
 android {
-    compileSdkVersion 27
+    compileSdkVersion 28
 
     defaultConfig {
         minSdkVersion 16

android/src/main/java/com/github/adee42/keyboardvisibility/KeyboardVisibilityPlugin.java

@@ -1,24 +1,19 @@
 package com.github.adee42.keyboardvisibility;
 
-import io.flutter.plugin.common.EventChannel;
-import io.flutter.plugin.common.EventChannel.EventSink;
-import io.flutter.plugin.common.EventChannel.StreamHandler;
-import io.flutter.plugin.common.MethodChannel;
-import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
-import io.flutter.plugin.common.MethodChannel.Result;
-import io.flutter.plugin.common.MethodCall;
-import io.flutter.plugin.common.PluginRegistry;
-import io.flutter.plugin.common.PluginRegistry.Registrar;
-
 import android.app.Activity;
 import android.app.Application;
-import android.content.Intent;
 import android.graphics.Rect;
 import android.os.Bundle;
 import android.view.View;
 import android.view.ViewGroup;
 import android.view.ViewTreeObserver;
 
+import io.flutter.app.FlutterActivity;
+import io.flutter.plugin.common.EventChannel;
+import io.flutter.plugin.common.EventChannel.EventSink;
+import io.flutter.plugin.common.EventChannel.StreamHandler;
+import io.flutter.plugin.common.PluginRegistry.Registrar;
+
 
 public class KeyboardVisibilityPlugin implements StreamHandler, Application.ActivityLifecycleCallbacks, ViewTreeObserver.OnGlobalLayoutListener {
     private static final String STREAM_CHANNEL_NAME = "github.com/adee42/flutter_keyboard_visibility";
@@ -59,12 +54,13 @@ public void onActivityCreated(Activity activity, Bundle bundle) {
 
     @Override
     public void onActivityStarted(Activity activity) {
-        try {
-            mainView = ((ViewGroup)activity.findViewById(android.R.id.content)).getChildAt(0);
-            mainView.getViewTreeObserver().addOnGlobalLayoutListener(this);
-        }
-        catch (Exception e) {
-            // do nothing
+        if (activity instanceof FlutterActivity) {
+            try {
+                mainView = ((ViewGroup) activity.findViewById(android.R.id.content)).getChildAt(0);
+                mainView.getViewTreeObserver().addOnGlobalLayoutListener(this);
+            } catch (Exception e) {
+                // do nothing
+            }
         }
     }
 
@@ -78,7 +74,9 @@ public void onActivityPaused(Activity activity) {
 
     @Override
     public void onActivityStopped(Activity activity) {
-        unregisterListener();
+        if (activity instanceof FlutterActivity) {
+            unregisterListener();
+        }
     }
 
     @Override
@@ -87,8 +85,9 @@ public void onActivitySaveInstanceState(Activity activity, Bundle bundle) {
 
     @Override
     public void onActivityDestroyed(Activity activity) {
-
-        unregisterListener();
+        if (activity instanceof FlutterActivity) {
+            unregisterListener();
+        }
     }
 
     private void unregisterListener() {

example/android/app/build.gradle

@@ -25,7 +25,7 @@ apply plugin: 'com.android.application'
 apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
 
 android {
-    compileSdkVersion 27
+    compileSdkVersion 28
 
     lintOptions {
         disable 'InvalidPackage'
@@ -35,7 +35,7 @@ android {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
         applicationId "com.github.adee42.keyboardvisibilityexample"
         minSdkVersion 16
-        targetSdkVersion 27
+        targetSdkVersion 28
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"

example/android/build.gradle

@@ -5,7 +5,7 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.2.1'
+        classpath 'com.android.tools.build:gradle:3.5.3'
     }
 }
 

example/android/gradle.properties

@@ -1 +1,2 @@
 org.gradle.jvmargs=-Xmx1536M
+android.enableR8=true

example/android/gradle/wrapper/gradle-wrapper.properties

@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.10.2-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-5.4.1-all.zip

example/ios/Flutter/Debug.xcconfig

@@ -1 +1,2 @@
+#include "Pods/Target Support Files/Pods-Runner/Pods-Runner.debug.xcconfig"
 #include "Generated.xcconfig"

example/ios/Flutter/Release.xcconfig

@@ -1 +1,2 @@
+#include "Pods/Target Support Files/Pods-Runner/Pods-Runner.release.xcconfig"
 #include "Generated.xcconfig"