Briefly discussion about the View mechanism of Android on Android Taipei 2016/Aug.

1. View的大小事
Charlie Tsai

2. Charlie Tsai@Android Study Group
cha122977@gmail.com
Individual App Developer &
System Software Engineer@NVIDIA (will quit in Sep.)
Today’s source code
https://github.com/chatea/ViewDemo

3. Agenda
• View & ViewGroup
• Mechanism of View
• Live debugging
• Free topics :D

4. View
• Attached to window / Detached from window
• Measure
• Layout
• Draw

5. ViewGroup
• ViewGroup extend View (composition pattern)
• The draw functions are different:
• View.onDraw(Canvas);
• ViewGroup.dispatchDraw(Canvas);

6. Mechanism of View
• What does View do?
• What does View NOT do?
• Need to attach to Window
• measure: analysis the size of view
• layout: determine the position and size of view
• draw: real function to render the View

7. Mechanism of View
• Draw from parent, then to children
• Canvas is given by Android framework
• Cost of overlay?

8. Attach / Detach
• onAttachedToWindow() / onDetachedFromWindow()
• What is Window?
• WindowManager
• ViewRootImpl (composition pattern)
• interactive with WindowManager
• performTraversals() & scheduleTraversals()
• own the Surface and send Canvas to View

9. Measure
• How big this View should be?
• void onMeasure(int, int)
• setMeasuredDimension(int, int)

10. Ex: Square View

11. Layout
• Assign the size and position to self and all
descendants
• onLayout(boolean changed, l, t, r b)
• addOnLayoutChangeListener(…)

12. Draw
• Should I use super.onDraw(Canvas)?
• View.onDraw(): hook for customized drawing
• Why ViewGroup.dispatchDraw()?

13. Live Debugging

14. Let’s Go Deeper…

15. Hardware Acceleration
• View.setLayerType(…)
• LAYER_TYPE_NONE
• LAYER_TYPE_HARDWARE
• LAYER_TYPE_SOFTWARE

16. Hardware Acceleration
• save() -> setXXX -> draw -> restore()
• setXXX - a function which doesn’t effect the
content
• e.g. setX(), setY(), setRotation()…

17. SurfaceView
• SurfaceView doesn’t render from parent
• System directly maintain its Canvas
• Transparent SurfaceView?
• SurfaceView.setZOrderOnTop()
• SurfaceView.getHolder().setFormat(PixelForm
at.TRANSPARENT);

18. 固定聚會：每週三＠果子咖啡(近捷運南京復興站)
技術分享：每月底週六14:30 - 17:30
以及眾多不定期活動(ex: Workshop)

19. Questions?

20. Thanks!