Android GUI architecture - relation between Surface/view/window/canvas

Question

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UPDATE: After several days googling and experiments, I have found the answers for most of those dumb questions. See the answers I submitted.

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What is the responsibility of Android Window?

Here are some questions:

• Is it responsible for collecting and dispatching the input?
• What is the relationship between the view and window? Same as the relationship between surface and window in DFB?
• What is the relationship between an activity and window? Will each Activity has a window?
• Is it possible to create a window from application ? And when it is necessary?
• Does Android support multi-window?

EDIT: Add more questions:

1. What is responsibilities of various class , such as Window, View, Canvas, Surface and how they collaborate with each other?

2. How many windows usually an Activity have?

3.Will all the views in one Activity will be attached to Window? What does attach mean?

1. Every window have surface? Every Canvas has surface?

2. View is responsible for focus/keyEvent/ manager, while Cavus is only responsible for "drawing" operation.

3. WindowManager is responsible for Window stacking? How that is related with SurfaceFlinger?

4. View doesn't own a Surface , the Window the view contained owns?

5. The View draw itself using canvas got by calling surface.lockCanvas().

6. When onDraw(Canvas) will be called? How & who pass the canvas parameters?

7. Does Canvas has size? Will Window's surface always be full screen?

EDIT again:

After watching this wonderful presentatin provided by Romain Guy http://www.youtube.com/watch?v=duefsFTJXzc&feature=feedwll&list=WL , several questions are resolved and add several more :)

1. Will every Activity has one ViewRoot and thus one Window?
2. Is there any need to create a window explictly? and Will the surface for the window always be full screen?
3. Will status bar be in another Window?
4. What is the size of the surface? Will that always be full screen?

Frigg

Answer 1

Is Window responsible for collecting and dispatching the input?

No. ViewRoot is responsible for this.

What is the relationship between the view and window? Same as the relationship between surface and window in DFB?

?

What is the relationship between an activity and window? Will each activity have a window?

Yes, most of the time. However, a SurfaceView has its own window. So, if an Activity has a SurfaceView it will have more than one Window.

Is it possible to create a window from application? And when it is necessary?

Not necessary.

Does Android support multi-window?

Sure. Using HierachyView you can clearly see that there is more than one Window exists in the system.

1.What are the responsibilities of various classes, such as Window, View, Canvas, Surface, and how do they collaborate with each other? 2.How many windows usually an Activity have?

Usually one.

3.Will all the views in one Activity will be attached to a window? What does attach mean? 4.Does every window have a surface? Does every canvas have a surface?

Every Window has a surface and Surface uses Canvas to draw on the surface.

5.View is responsible for managing focus/key events, while Canvas is only responsible for "drawing" operation?

YES.

6.WindowManager is responsible for Window stacking? How does that relate to SurfaceFlinger?

Not Sure of WindowManager's responsibility. (TODO)

SurfaceFlinger is used to compose the Surface that is associated with different Window/Activity.

7.View doesn't own a Surface, the Window the view contained owns?

View will draw on surface using Canvas. The window the view is attached to owns the surface.

This could be understood by implement a customize view, when you should override the onDraw(Canvas) method in your derived class.

8.The View draws itself using canvas got by calling surface.lockCanvas()?

YES.

9.When and how is onDraw(Canvas) called, and who passes the canvas parameters?

onDraw() will be called by the RootView and when invalidate is called. The canvas parameter is passed from the RootView.

10.Does Canvas have a size? Will a Window's surface always be full screen?

I cannot say for sure. But when I create a customize view, the size of the canvas got from onDraw(Canvas) is full screen.

However, in my understanding, for performance sake, the Surface for the window should not always be full screen. But this assumption has not been verified. For example, the statusBar window should not be full screen.

1.Will every Activity have one ViewRoot and thus one Window?

YES.

2.Is there any need to create a window explicitly? Will the surface for the window always be full screen?

No need to create the Window explicitly per se.

3.Will status bar be in another Window?

YES.

4.What is the size of the surface? Will that always be full screen?

Frigg

Answer 2

Appreciating that you asked all those questions. 1) AFAIK every Activity has alteast one ViewRoot and every ViewRoot has atleast one window 2) There is no need to create window explicitly and I think it should always be full window .. although not sure about this 3) Yes it can be, we can place status bar in another windows 4) Not is can be part of window, not always covers full screen.

Please correct my understanding if I stated anything wrong here.

Philcox

Answer 3

Here is a very basic and simple conceptual overview of how interaction happens among the Window, Surface, Canvas, and Bitmap.
Look at this answer by hackbod, great explanation.

Christoforo

Answer 4

for the question

What is the relationship between an activity and window? Will each Activity has a window?

disagree with @pierrotlefou ,cause in google doc SurfaceView provides a dedicated surface, rather than Window.

The surface is Z ordered so that it is behind the window holding its SurfaceView; the SurfaceView punches a hole in its window to allow its surface to be displayed.

Driveway