diff --git a/services/core/java/com/android/server/am/ActivityMetricsLogger.java b/services/core/java/com/android/server/am/ActivityMetricsLogger.java
new file mode 100644
index 0000000..64f423c
--- /dev/null
+++ b/services/core/java/com/android/server/am/ActivityMetricsLogger.java
@@ -0,0 +1,72 @@
+package com.android.server.am;
+
+import static android.app.ActivityManager.StackId.DOCKED_STACK_ID;
+import static android.app.ActivityManager.StackId.FREEFORM_WORKSPACE_STACK_ID;
+import static android.app.ActivityManager.StackId.FULLSCREEN_WORKSPACE_STACK_ID;
+import static android.app.ActivityManager.StackId.HOME_STACK_ID;
+
+import android.app.ActivityManager.StackId;
+import android.content.Context;
+import android.os.SystemClock;
+
+import com.android.internal.logging.MetricsLogger;
+
+/**
+ * Handles logging into Tron.
+ */
+class ActivityMetricsLogger {
+    // Window modes we are interested in logging. If we ever introduce a new type, we need to add
+    // a value here and increase the {@link #TRON_WINDOW_STATE_VARZ_STRINGS} array.
+    private static final int WINDOW_STATE_STANDARD = 0;
+    private static final int WINDOW_STATE_SIDE_BY_SIDE = 1;
+    private static final int WINDOW_STATE_FREEFORM = 2;
+    private static final int WINDOW_STATE_INVALID = -1;
+
+    // Preallocated strings we are sending to tron, so we don't have to allocate a new one every
+    // time we log.
+    private static final String[] TRON_WINDOW_STATE_VARZ_STRINGS = {
+            "tron_varz_window_time_0", "tron_varz_window_time_1", "tron_varz_window_time_2"};
+
+    private int mWindowState = WINDOW_STATE_STANDARD;
+    private long mLastLogTimeSecs;
+    private final ActivityStackSupervisor mSupervisor;
+    private final Context mContext;
+
+    ActivityMetricsLogger(ActivityStackSupervisor supervisor, Context context) {
+        mLastLogTimeSecs = SystemClock.elapsedRealtime() / 1000;
+        mSupervisor = supervisor;
+        mContext = context;
+    }
+
+    void logWindowState() {
+        final long now = SystemClock.elapsedRealtime() / 1000;
+        if (mWindowState != WINDOW_STATE_INVALID) {
+            // We log even if the window state hasn't changed, because the user might remain in
+            // home/fullscreen move forever and we would like to track this kind of behavior
+            // too.
+            MetricsLogger.count(mContext, TRON_WINDOW_STATE_VARZ_STRINGS[mWindowState],
+                    (int) (now - mLastLogTimeSecs));
+        }
+        mLastLogTimeSecs = now;
+
+        mWindowState = WINDOW_STATE_INVALID;
+        ActivityStack stack = mSupervisor.getStack(DOCKED_STACK_ID);
+        if (stack != null && stack.isStackVisibleLocked()) {
+            mWindowState = WINDOW_STATE_SIDE_BY_SIDE;
+        }
+        if (mWindowState == WINDOW_STATE_INVALID) {
+            stack = mSupervisor.getFocusedStack();
+            if (stack.mStackId == HOME_STACK_ID
+                    || stack.mStackId == FULLSCREEN_WORKSPACE_STACK_ID) {
+                mWindowState = WINDOW_STATE_STANDARD;
+            } else if (stack.mStackId == DOCKED_STACK_ID) {
+                throw new IllegalStateException("Docked stack shouldn't be the focused stack, "
+                        + "because it reported not being visible.");
+            } else if (stack.mStackId == FREEFORM_WORKSPACE_STACK_ID) {
+                mWindowState = WINDOW_STATE_FREEFORM;
+            } else if (StackId.isStaticStack(stack.mStackId)) {
+                throw new IllegalStateException("Unknown stack=" + stack);
+            }
+        }
+    }
+}