Accessibility Zoom Widget

An Android application that provides a home screen widget to quickly toggle the screen's display size (zoom level) between the system default and a magnified setting.

Core Functionality

• A simple home screen widget with a single button.
• Tapping the widget will cycle between the phone's default display size and a larger, "zoomed-in" display size.
• The app itself will contain a simple instruction screen explaining how to grant the required permission.

Technology Stack

• Language: Kotlin
• UI Framework:
  • Widget: Standard Android AppWidgetProvider with RemoteViews.
  • Instruction Screen: Jetpack Compose.
• Architecture: A BroadcastReceiver will handle widget taps to execute the zoom change.

CRITICAL: Special Permission Required

To change the screen zoom, the app needs the WRITE_SECURE_SETTINGS permission. This is a highly protected permission.

• You cannot grant this permission from your phone directly.
• You will need to connect your phone to a computer and run a single command using the Android Debug Bridge (adb).

The main screen of the app will detect if the permission is granted and will show you the exact adb command you need to run. This is a one-time setup step.

Alternatively, this repository includes automated setup scripts in the scripts/ directory:

• scripts/grant_permission.py: Automatically finds adb on your system and grants the WRITE_SECURE_SETTINGS permission to the connected device.
• scripts/install_latest.py: Automatically downloads the latest release APK from GitHub, uninstalls any conflicting versions, installs the new APK, and grants the required permissions using grant_permission.py.

Implementation Steps

1. Project Setup: Create a new, empty Android project configured for Kotlin and Jetpack Compose.
2. Instruction Screen: Build a simple UI that checks for the WRITE_SECURE_SETTINGS permission and displays the necessary adb command if it's missing.
3. Widget UI: Design a basic widget layout with a button and an icon.
4. Widget Logic: Implement the AppWidgetProvider to place the widget on the home screen.
5. Zoom Logic: Implement the density change via reflection (see Technical Details below).
6. State Tracking: The widget will change its icon to indicate whether the zoom is "on" or "off".

How the Zoom Change Works (Technical Details)

Changing the system display density from a third-party app process is heavily restricted in modern Android versions. The working solution uses a combination of techniques:

The Problem

1. Settings.Secure.display_density_forced: Writing to this setting requires the WRITE_SECURE_SETTINGS permission. While the app can successfully write to the setting, the change does not take effect immediately. It only acts as persistence for the next reboot.
2. wm density via Shell: Executing Runtime.getRuntime().exec("wm density ...") from an app process fails on many devices (like Samsung) because SELinux policies block third-party app processes from accessing the WindowManager service.
3. Hidden APIs: The Android framework's internal method to change density is IWindowManager.setForcedDisplayDensityForUser. However, this is a "hidden API" restricted by Android (max-target-o).

The Solution

To bypass these restrictions and apply the density change immediately without rebooting or root access:

1. targetSdk Downshift: The app's targetSdkVersion in build.gradle.kts is intentionally set to 26 (Android O). Because the IWindowManager reflection call is restricted with max-target-o, lowering the target SDK to 26 lifts the hidden API restriction, allowing our app to use it.
2. IWindowManager Reflection: We use Java Reflection to get the IWindowManager binder interface and call setForcedDisplayDensityForUser(0, dpi, userId). This is the exact same method that the wm density command calls internally, but we execute it directly from the app process.
3. User ID Resolution: The userId parameter cannot be -2 (USER_CURRENT) because that requires the system-level INTERACT_ACROSS_USERS_FULL permission. Instead, we calculate the actual user ID from the app's UID (Process.myUid() / 100000), which allows the call to succeed for the current user.
4. WRITE_SECURE_SETTINGS: The WindowManager still checks if the caller has the WRITE_SECURE_SETTINGS permission, which the app possesses thanks to the one-time ADB grant.