1. CustomView 1.1 Creating a custom view 1.2 Override onDraw() 1.3 Create Drawing Objects 1.4 Handle Layout Events 1.5 Draw! 2. SurfaceView 2.1 Difference between SurfaceView and View? 3. Exercise 12

1. Android Programming
Lesson 12
2D Graphics
NGUYEN The Linh

2. Android Programming
Contents
1 CustomView
2 SurfaceView
3 Exercise 12
2

3. Android Programming
2D Graphics
2D Graphics
3

4. Android Programming
CustomView
 Creating a custom view
4

5. Android Programming
CustomView
 Creating a custom view
5

6. Android Programming
CustomView
 Override onDraw()
 The most important step in drawing a custom view is to override
the onDraw() method.
 The parameter to onDraw() is a Canvas object that the view can
use to draw itself.
 The Canvas class defines methods for drawing text, lines,
bitmaps, and many other graphics primitives. You can use these
methods in onDraw() to create your custom user interface (UI).
6

7. Android Programming
CustomView
 Create Drawing Objects
 The android.graphics framework divides drawing into two areas:
• What to draw, handled by Canvas
• How to draw, handled by Paint.
 For instance, Canvas provides a method to draw a line,
while Paint provides methods to define that line's color.
 Canvas has a method to draw a rectangle, while Paint defines
whether to fill that rectangle with a color or leave it empty.
7

8. Android Programming
CustomView
 Create Drawing Objects
 Simply put, Canvas defines shapes that you can draw on the
screen, while Paint defines the color, style, font, and so forth of
each shape you draw.
 Creating objects ahead of time is an important optimization. Views
are redrawn very frequently, and many drawing objects require
expensive initialization. Creating drawing objects within
your onDraw() method significantly reduces performance and can
make your UI appear sluggish.
8

9. Android Programming
CustomView
 Handle Layout Events
 In order to properly draw your custom view, you need to know
what size it is.
 You should never make assumptions about the size of your view
on the screen.
 App needs to handle different screen sizes, multiple screen
densities, and various aspect ratios in both portrait and landscape
mode.
9

10. Android Programming
CustomView
 Handle Layout Events
 In order to properly draw your custom view, you need to know
what size it is.
 You should never make assumptions about the size of your view
on the screen.
 App needs to handle different screen sizes, multiple screen
densities, and various aspect ratios in both portrait and landscape
mode.
10

11. Android Programming
CustomView
 Handle Layout Events
 Although View has many methods for handling measurement,
most of them do not need to be overridden. If your view doesn't
need special control over its size, you only need to override one
method: onSizeChanged().
 onSizeChanged() is called when your view is first assigned a
size, and again if the size of your view changes for any reason.
11

12. Android Programming
CustomView
 Draw!
 Once you have your object creation and measuring code defined,
you can implement onDraw().
 Every view implements onDraw() differently, but there are some
common operations that most views share:
12

13. Android Programming
CustomView
 Draw!
 Draw text using drawText(). Specify the typeface by
calling setTypeface(), and the text color by calling setColor().
 Draw primitive shapes using drawRect(), drawOval(),
and drawArc(). Change whether the shapes are filled, outlined, or
both by calling setStyle().
13

14. Android Programming
2D Graphics
 Draw!
 Draw more complex shapes using the Path class. Define a shape
by adding lines and curves to a Path object, then draw the shape
using drawPath(). Just as with primitive shapes, paths can be
outlined, filled, or both, depending on the setStyle().
 Define gradient fills by creating LinearGradient objects.
Call setShader() to use your LinearGradient on filled shapes.
 Draw bitmaps using drawBitmap().
14

15. Android Programming
CustomView
 Example 12.1
15

16. Android Programming
2D Graphics
SurfaceView
16

17. Android Programming
SurfaceView
 Difference between SurfaceView and View?
 Views are all drawn on the same GUI thread which is also used
for all user interaction.
 So if you need to update GUI rapidly or if the rendering takes too
much time and affects user experience then use SurfaceView.
 The main difference is that SurfaceView can be drawn on by
background threads but Views can't.
17

18. Android Programming
SurfaceView
 Example 12.2
18

19. Android Programming
Exercise 12
 Example 12.2
 Kill Ants
19

20. Android Programming