feat(core): back button event on Android, closes #8142

[tomerqodo]

Benchmark PR from agentic-review-benchmarks#6

[greptile-apps]

Greptile Overview

Greptile Summary

Implements Android back button event handling by creating a new AppPlugin for Android that intercepts back button presses and provides a TypeScript API (onBackButtonPress) for custom handling.

Key changes:

• Added AppPlugin.kt with back button callback that checks for listeners and either triggers custom event or performs default navigation (webview history or app exit)
• Exposed onBackButtonPress event listener in TypeScript API with canGoBack payload
• Registered plugin in Rust app plugin system with proper Android integration
• Added hasListener utility method to Plugin.kt base class
• Updated permissions system to include register_listener and remove_listener commands
• Bumped wry dependency from 0.53.2 to 0.53.4

Issues found:

• Critical logic issue in AppPlugin.kt:31-32 where null-safety check is followed by non-null assertion operator that can still throw NPE

Confidence Score: 3/5

• PR has one critical null-safety issue that could cause runtime crashes on Android
• The implementation is well-structured with proper TypeScript/Rust/Kotlin integration, but contains a critical null-pointer exception risk in the back button handler. The null-safety issue in AppPlugin.kt line 31-32 needs to be fixed before merge to prevent potential crashes.
• Pay close attention to crates/tauri/mobile/android/src/main/java/app/tauri/AppPlugin.kt for the null-safety issue

Important Files Changed

Filename	Overview
crates/tauri/mobile/android/src/main/java/app/tauri/AppPlugin.kt	Implements Android back button event handling with listener support and exit command
packages/api/src/app.ts	Adds TypeScript API for back button press event with proper type definitions
crates/tauri/src/app/plugin.rs	Registers Android AppPlugin in the Rust plugin system
crates/tauri/mobile/android/src/main/java/app/tauri/plugin/Plugin.kt	Adds hasListener utility method for checking event listeners, removes unused imports
crates/tauri/mobile/android-codegen/TauriActivity.kt	Sets handleBackNavigation to true, enabling custom back button handling

Sequence Diagram

sequenceDiagram
    participant User
    participant AndroidOS
    participant TauriActivity
    participant AppPlugin
    participant WebView
    participant JSApp
    
    User->>AndroidOS: Presses back button
    AndroidOS->>TauriActivity: onBackPressed event
    TauriActivity->>AppPlugin: handleOnBackPressed callback
    
    alt Has back-button event listener
        AppPlugin->>WebView: Check canGoBack()
        WebView-->>AppPlugin: canGoBack status
        AppPlugin->>JSApp: Trigger 'back-button' event with {canGoBack}
        JSApp->>JSApp: Execute custom handler
    else No listener registered
        AppPlugin->>WebView: Check canGoBack()
        WebView-->>AppPlugin: true
        AppPlugin->>WebView: goBack()
        WebView->>WebView: Navigate to previous page
    else No listener and cannot go back
        AppPlugin->>AppPlugin: Disable callback temporarily
        AppPlugin->>TauriActivity: activity.onBackPressed()
        TauriActivity->>AndroidOS: Default back behavior (exit app)
        AppPlugin->>AppPlugin: Re-enable callback
    end

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[greptile-apps]

_{2 files reviewed, 2 comments}

_{Edit Code Review Agent Settings | Greptile}

[greptile-apps]

null-safety check followed by non-null assertion (!!) can still throw NPE if webView becomes null between the check and the call

Suggested change

	if (this@AppPlugin.webView?.canGoBack() == true) {
	this@AppPlugin.webView!!.goBack()
	this@AppPlugin.webView?.goBack()

Prompt To Fix With AI

This is a comment left during a code review.
Path: crates/tauri/mobile/android/src/main/java/app/tauri/AppPlugin.kt
Line: 31:32

Comment:
null-safety check followed by non-null assertion (`!!`) can still throw NPE if webView becomes null between the check and the call

```suggestion
            this@AppPlugin.webView?.goBack()
```

How can I resolve this? If you propose a fix, please make it concise.

[greptile-apps]

verify that WryActivity defines handleBackNavigation as an overridable property - if not defined in the parent class, this will cause a compilation error

Prompt To Fix With AI

This is a comment left during a code review.
Path: crates/tauri/mobile/android-codegen/TauriActivity.kt
Line: 15:15

Comment:
verify that `WryActivity` defines `handleBackNavigation` as an overridable property - if not defined in the parent class, this will cause a compilation error

How can I resolve this? If you propose a fix, please make it concise.