Window, Activity, and View are often used, but the relationship between the three is still not systematically clarified. Today we will start to sort out the relationship between the three:

Window: The abstract base class for top-level window appearance and behavior policies. The only implementation is the PhoneWindow class.

Activity: One of the four major components, it provides an interface for users to click and slide.

View: represents the basic building block of user interface components, UI components.

1.1 Activity

Activity is not responsible for view control, it just controls the life cycle and handles events. What really controls the view is the Window. An Activity contains a Window, and the Window really represents a window. Activity is like a human-computer interaction interface, coordinating the addition and display of views, and interacting with Window and View through other callback methods.

1.2 Window

WindowThe understanding of is more abstract, Windowit is equivalent to a container, which "holds" a lot View, and these Vieware organized in a tree structure.

Window is an abstract class, and what is actually held in Activity is its subclass PhoneWindow. There is an internal class DecorView in PhoneWindow, by creating DecorView to load the R.layout.activity set in the Activity, Window is the carrier of the view, holds a DecorView inside, and this DecorView is the root layout of the view.
Window loads DecorView through WindowManager, and hands DecorView to ViewRoot for view drawing and other interactions.

1.3 View

In the android system, view is managed through a tree structure. The View tree structure is as follows,

Among them, DecorView is a subclass of FrameLayout, which can be considered as the root node view of the Android view tree.

DecorView is used as the top-level View. Generally, it contains a vertical LinearLayout. There are three parts in the LinearLayout. The above is a ViewStub, which is a lazy-loaded view (should be set to ActionBar, according to Theme settings), and the middle one is The title bar (according to the Theme setting, some layouts do not have it), and the content bar below.

Second, the relationship description

2.1 Activity and Window

Every time an Activity instance is created, then Activity.attach() is called to initialize some content. The Window object is created, initialized and assigned in attach. In attach(), the system creates the Window object to which the Activity belongs and sets a callback interface for it. Since Activity implements Window's Callback interface, when Window receives an external state change, it will call back to the Activity's method.

2.2 Window and View

Before understanding the relationship between Window and View, we need to know what ViewRoot (ViewRootImpl) is. It is the link between WindowManager and DecorView.

ViewRoot is a manager of ViewTree, not the root node of ViewTree.
Strictly speaking, the root node of ViewTree is only DecorView.
ViewRoot "combines" DecorView and PhoneWindow (the Window instance created by Activity).

2.3 Window and View

View is the view presentation method in Android, but View cannot exist alone, it must be attached to the abstract concept of Window, so where there is a view, there is a Window.

The most familiar method is to call the setContentView() method in the Activity.onCreate() method and pass in the specified layout file. Activity hands over the specific implementation to Window, and the specific implementation of Window is that PhoneWindow creates DecorView in setContentView, and DecorView is the root View of the entire View tree, and then adds the formulated layout file to mContentParent of DecorView.

2.4 Relationship between Activity, PhoneWindow and DecorView

Activity and PhoneWindow and DecorView relationship diagram,

Detailed explanation of the relationship between Activity, View and Window in Android

Android system startup

Android System Development Preparation

Android System Development Practice

Android System Development Core Knowledge Reserve

Detailed explanation of Android core functions

1. Background introduction