diff --git a/src/main.rs b/src/main.rs
index c74861e..80875cc 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -11,8 +11,9 @@ use input::{rebuild_keyboard_layout_map, INPUT_STATE};
 use log::debug;
 use once_cell::sync::OnceCell;
 use platform::{
-    ensure_accessibility_permission, run_event_listener, send_backspace, send_string, Handle,
-    KeyModifier, PressedKey, KEY_DELETE, KEY_ENTER, KEY_ESCAPE, KEY_SPACE, KEY_TAB, RAW_KEY_GLOBE,
+    add_app_change_callback, ensure_accessibility_permission, run_event_listener, send_backspace,
+    send_string, Handle, KeyModifier, PressedKey, KEY_DELETE, KEY_ENTER, KEY_ESCAPE, KEY_SPACE,
+    KEY_TAB, RAW_KEY_GLOBE,
 };
 
 use ui::{UIDataAdapter, UPDATE_UI};
@@ -88,7 +89,6 @@ unsafe fn auto_toggle_vietnamese() {
 
 fn event_handler(handle: Handle, pressed_key: Option<PressedKey>, modifiers: KeyModifier) -> bool {
     unsafe {
-        auto_toggle_vietnamese();
         let pressed_key_code = pressed_key.and_then(|p| match p {
             PressedKey::Char(c) => Some(c),
             _ => None,
@@ -207,6 +207,9 @@ fn main() {
         thread::spawn(|| {
             run_event_listener(&event_handler);
         });
+        add_app_change_callback(|| {
+            unsafe { auto_toggle_vietnamese() };
+        });
         _ = app.launch(UIDataAdapter::new());
     }
 }
diff --git a/src/platform/macos.rs b/src/platform/macos.rs
index 18668fd..dd355b7 100644
--- a/src/platform/macos.rs
+++ b/src/platform/macos.rs
@@ -207,6 +207,13 @@ pub fn send_string(handle: Handle, string: &str) -> Result<(), ()> {
     Ok(())
 }
 
+pub fn add_app_change_callback<F>(cb: F)
+where
+    F: Fn() + Send + 'static,
+{
+    macos_ext::add_app_change_callback(cb);
+}
+
 pub fn run_event_listener(callback: &CallbackFn) {
     let current = CFRunLoop::get_current();
     if let Ok(event_tap) = new_tap::CGEventTap::new(
diff --git a/src/platform/macos_ext.rs b/src/platform/macos_ext.rs
index c54b133..6f709d4 100644
--- a/src/platform/macos_ext.rs
+++ b/src/platform/macos_ext.rs
@@ -1,7 +1,7 @@
 use cocoa::appkit::{
     NSApp, NSApplication, NSButton, NSMenu, NSMenuItem, NSStatusBar, NSStatusItem,
 };
-use cocoa::base::{nil, YES};
+use cocoa::base::{id, nil, YES};
 use cocoa::foundation::{NSAutoreleasePool, NSString};
 use core_foundation::dictionary::CFDictionaryRef;
 use core_foundation::string::CFStringRef;
@@ -12,10 +12,10 @@ use core_graphics::{
 use druid::{Data, Lens};
 use libc::c_void;
 use objc::{
+    class,
     declare::ClassDecl,
     msg_send,
-    runtime::Class,
-    runtime::{Object, Sel},
+    runtime::{Class, Object, Sel},
     sel, sel_impl, Message,
 };
 use objc_foundation::{INSObject, NSObject};
@@ -342,3 +342,26 @@ extern "C" {
     pub fn AXIsProcessTrustedWithOptions(options: CFDictionaryRef) -> bool;
     pub static kAXTrustedCheckOptionPrompt: CFStringRef;
 }
+
+#[link(name = "AppKit", kind = "framework")]
+extern "C" {
+    pub static NSWorkspaceDidActivateApplicationNotification: CFStringRef;
+}
+
+pub fn add_app_change_callback<F>(cb: F)
+where
+    F: Fn() + Send + 'static,
+{
+    unsafe {
+        let shared_workspace: id = msg_send![class!(NSWorkspace), sharedWorkspace];
+        let notification_center: id = msg_send![shared_workspace, notificationCenter];
+        let cb_obj = Callback::from(Box::new(cb));
+
+        let _: id = msg_send![notification_center,
+            addObserver:cb_obj
+            selector:sel!(call)
+            name:NSWorkspaceDidActivateApplicationNotification
+            object:nil
+        ];
+    }
+}
diff --git a/src/platform/mod.rs b/src/platform/mod.rs
index c0f0d74..9a3c590 100644
--- a/src/platform/mod.rs
+++ b/src/platform/mod.rs
@@ -7,9 +7,9 @@ use std::fmt::Display;
 
 use bitflags::bitflags;
 pub use os::{
-    ensure_accessibility_permission, get_active_app_name, get_home_dir, is_in_text_selection,
-    is_launch_on_login, run_event_listener, send_backspace, send_string, update_launch_on_login,
-    Handle, SYMBOL_ALT, SYMBOL_CTRL, SYMBOL_SHIFT, SYMBOL_SUPER,
+    add_app_change_callback, ensure_accessibility_permission, get_active_app_name, get_home_dir,
+    is_in_text_selection, is_launch_on_login, run_event_listener, send_backspace, send_string,
+    update_launch_on_login, Handle, SYMBOL_ALT, SYMBOL_CTRL, SYMBOL_SHIFT, SYMBOL_SUPER,
 };
 
 #[cfg(target_os = "macos")]