dumpsys window windows output

Question

I'm trying to list all UI elements being displayed on an Android device. I'm doing this by running "dumpsys window windows | grep "Window #" in an adb shell. This gives me a list of windows from window n to 0.

I'd like to know how the output order is determined. Is window n on top and window 0 at the bottom of the stack? Also, what do the fields mean? For example, the first line shown as as follows

Window #64 Window{793212d u0 NavigationBar}:

What do the values 793212d and u0 indicate?

Answer 1

Regarding the output order:

• The windows are traversed top-to-bottom in z-order, so the top-most windows come first.

Regarding the fields:

• The 793212d is the (unique) ID of the window, obtained with System#identityHashCode.

• The u0 refers to the user ID to which the window belongs. The user id is not the same as the Unix UID, it is a more high-level concept. Android groups several Unix UIDs to belong to one user ID, i.e. the first 100.000 Unix UIDs belong to user ID 0 and so on (reference). To determine the user ID of the window, Android would look up the Unix UID of the window (with each app having its own Unix UID), and then map the Unix UID to the user ID.

• The NavigationBar is the title of the window.

Technical details: When calling dumpsys window windows, this will trigger a dump request at the WindowManagerService (link). This class has a member mRoot of type RootWindowContainer, to which the dump request will be forwarded (link). The relevant code is:

        forAllWindows((w) -> {
            if (windows == null || windows.contains(w)) {
                pw.println("  Window #" + index[0] + " " + w + ":");
                w.dump(pw, "    ", dumpAll || windows != null);
                index[0] = index[0] + 1;
            }
        }, true /* traverseTopToBottom */);

The w is of type WindowState, which overrides toString to obtain the String representation you see in the dumpsys output (link). The relevant code is:

mStringNameCache = "Window{" + Integer.toHexString(System.identityHashCode(this))
    + " u" + UserHandle.getUserId(mOwnerUid)
    + " " + mLastTitle + (mAnimatingExit ? " EXITING}" : "}");

The RootWindowContainer#forAllWindows method traverses the mChildren list, which is specified to be in z-order (link, link):

    // List of children for this window container. List is in z-order as the children appear on
    // screen with the top-most window container at the tail of the list.
    protected final WindowList<E> mChildren = new WindowList<E>();

    boolean forAllWindows(ToBooleanFunction<WindowState> callback, boolean traverseTopToBottom) {
        if (traverseTopToBottom) {
            for (int i = mChildren.size() - 1; i >= 0; --i) {
                if (mChildren.get(i).forAllWindows(callback, traverseTopToBottom)) {
                    return true;
                }
            }
        ...