xref: /docs/en/application-dev/windowmanager/window-overview.md

# Window Overview


## Introduction

The **Window** module provides a mechanism for displaying multiple application UIs and allowing the end user to interact with them on the same physical screen.

- For application developers, the **Window** module provides APIs for UI display and user interaction.

- For end users, the **Window** module provides a way to control application UIs.

- For the entire operating system, the **Window** module provides logic for application UI management.


## Features

The **Window** module has the following features:

-  **Provides a `Window` object to hold application and system UIs.** You can load your application UIs through the window to display them to the end user.

-  **Maintains the window relationship (overlay layers and positions).** Different types of application and system windows have different default positions and overlay layers (z-index). End users can adjust the position and overlay layer of a window within a certain range.

-  **Provides window decoration.** Window decoration refers to the title bar and border of a window. The title bar usually provides the Maximize, Minimize, and Close buttons and has the default click behavior. The border can be dragged to relocate or resize the window. Window decoration is a system-level default behavior. You can enable or disable window decoration without paying attention to the implementation at the UI code layer.

-  **Provides window animations.** When a window is displayed, hidden, or switched, an animation is usually used to smooth the interaction process. This is the default behavior for application windows. You do not need to set or modify the code.

-  **Provides guidance for input event distribution.** Events are distributed based on the window status and focus. Touch and mouse events are distributed based on the window position and size, and keyboard events are distributed to the focused window. You can call APIs provided by the **Window** module to set whether a window is touchable and can gain focus.


## Basic Concepts


### Window Type

The **Window** module provides system windows and application windows.
- A **system window** implements specific functionalities of the system. Examples include the volume bar, wallpaper, notification panel, status bar, and navigation bar.
- An **application window** is related to the application display. Based on the displayed content, application windows are further classified into main windows and subwindows.
  - A main window shows the application UI and appears on the **Recents** page.
  - A subwindow is an auxiliary window of an application, such as a dialog box and floating window. It is not displayed on the **Recents** page. Its lifecycle follows that of the main window.




### Application Window Mode

The application window mode refers to the display mode of the main window when it is started. Currently, there are three application window modes: full-screen, split-screen, and freeform window. This support for multiple window modes is known as the multi-window capability.


-  In **full-screen** mode, the main window is displayed on the entire screen when it is started.
-  In **split-screen** mode, the main window occupies part of the screen when it is started, and the other part of the screen is occupied by another window. You can resize the two windows by dragging the split line between them.
-  In **freeform window** mode, the main window may come in any size or position as needed. Multiple freeform windows can be simultaneously displayed on the screen. These freeform windows are arranged on the z-axis in the sequence that they are opened or gain the focus. When a freeform window is clicked or touched, its z-index is incremented and it gains the focus.


![windowMode](figures/windowMode.png)


## Working Principles

The window implementation and development vary according to the application development model, which can be FA model or stage model.

For details about the overall architecture and design ideas of the two models, see [Interpretation of the Application Model](../application-models/application-models.md).

You are advised to use the stage model for window development.


## Constraints

-  You cannot develop system windows in the FA model.

-  The application main window and subwindow have the following size limits: [320, 2560] in width and [240, 2560] in height, both in units of vp.

-  The system window has the following size limits: [0, 2560] in width and [0, 2560] in height, both in units of vp.