Add predictive back support to HelpActivity

app/src/main/java/com/limelight/HelpActivity.java

@@ -2,9 +2,12 @@ package com.limelight;
 
 import android.app.Activity;
 import android.graphics.Bitmap;
+import android.os.Build;
 import android.os.Bundle;
 import android.webkit.WebView;
 import android.webkit.WebViewClient;
+import android.window.OnBackInvokedCallback;
+import android.window.OnBackInvokedDispatcher;
 
 import com.limelight.utils.SpinnerDialog;
 
@@ -13,10 +16,26 @@ public class HelpActivity extends Activity {
     private SpinnerDialog loadingDialog;
     private WebView webView;
 
+    private boolean backCallbackRegistered;
+    private OnBackInvokedCallback onBackInvokedCallback;
+
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
 
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
+            onBackInvokedCallback = new OnBackInvokedCallback() {
+                @Override
+                public void onBackInvoked() {
+                    // We should always be able to go back because we unregister our callback
+                    // when we can't go back. Nonetheless, we will still check anyway.
+                    if (webView.canGoBack()) {
+                        webView.goBack();
+                    }
+                }
+            };
+        }
+
         webView = new WebView(this);
         setContentView(webView);
 
@@ -39,6 +58,8 @@ public class HelpActivity extends Activity {
                             getResources().getString(R.string.help_loading_title),
                             getResources().getString(R.string.help_loading_msg), false);
                 }
+
+                refreshBackDispatchState();
             }
 
             @Override
@@ -47,6 +68,8 @@ public class HelpActivity extends Activity {
                     loadingDialog.dismiss();
                     loadingDialog = null;
                 }
+
+                refreshBackDispatchState();
             }
 
             @Override
@@ -59,6 +82,31 @@ public class HelpActivity extends Activity {
         webView.loadUrl(getIntent().getData().toString());
     }
 
+    private void refreshBackDispatchState() {
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
+            if (webView.canGoBack() && !backCallbackRegistered) {
+                getOnBackInvokedDispatcher().registerOnBackInvokedCallback(
+                        OnBackInvokedDispatcher.PRIORITY_DEFAULT, onBackInvokedCallback);
+                backCallbackRegistered = true;
+            }
+            else if (!webView.canGoBack() && backCallbackRegistered) {
+                getOnBackInvokedDispatcher().unregisterOnBackInvokedCallback(onBackInvokedCallback);
+                backCallbackRegistered = false;
+            }
+        }
+    }
+
+    @Override
+    protected void onDestroy() {
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
+            if (backCallbackRegistered) {
+                getOnBackInvokedDispatcher().unregisterOnBackInvokedCallback(onBackInvokedCallback);
+            }
+        }
+
+        super.onDestroy();
+    }
+
     @Override
     public void onBackPressed() {
         // Back goes back through the WebView history