Android画图学习总结

首先，如何获取 res 中的资源

数据包package：android.content.res
主要类：Resources
Android SDK中的简介：Class for accessing an application’s resources.Class for accessing an application’s resources. This sits on top of the asset manager of the application (accessible through getAssets()) and provides a higher-level API for getting typed data from the assets.
其主要接口按照功能，划分为以下三部分：

getXXXX() 例如： int getColor(int id) Drawable getDrawable(int id) String getString(int id)	直接获取res中存放的资源
InputStream openRawResource(int id)	获取资源的数据流，读取资源数据
void parseBundleExtras( XmlResourceParser parser, Bundle outBundle)	从XML文件中获取数据

Resource为每种资源提供了相应的接口来获取这种资源，除了可以直接获取资源外，还额外提供了以数据流的方式获取资源，这在以后的应用程序开发中会经常使用，那么如何获取Resources了，如下：Resources r = this.getContext().getResources();

其次，如何获取资源中的画图对象

数据包package：android.graphics.drawable
主要类：Drawable
Android SDK中的简介：A Drawable is a general abstraction for “something that can be drawn.” Most often you will deal with Drawable as the type of resource retrieved for drawing things to the screen; the Drawable class provides a generic API for dealing with an underlying visual resource that may take a variety of forms.
看了以上简介，发现Drawable是个virtual class，具体如何画图，需要具体分析Drawable的子类，例如：BitmapDrawable
Android SDK中的简介：A Drawable that wraps a bitmap and can be tiled, stretched, or aligned. You can create a BitmapDrawable from a file path, an input stream, through XML inflation, or from a Bitmap object. It can be defined in an XML file with the <bitmap> element.
其主要接口如下：

BitmapDrawable() BitmapDrawable(Bitmap bitmap) BitmapDrawable(String filepath) BitmapDrawable(InputStream is)
void draw(Canvas canvas)	Draw in its bounds (set via setBounds) respecting optional effects such as alpha (set via setAlpha) and color filter (set via setColorFilter).
final Bitmap getBitmap()
final Paint getPaint()

Drawable是个抽象类，在BitmapDrawable中我们就看到位图的具体操作，在仔细看下BitmapDrawable的构造函数，我们就会发现与Resource中的openRawResource()接口是相对应的,就可以通过以下方法来获取位图：
Resources r = this.getContext().getResources();
Inputstream is = r.openRawResource(R.drawable.my_background_image);
BitmapDrawable bmpDraw = new BitmapDrawable(is);
Bitmap bmp = bmpDraw.getBitmap();

然后，看几个常用的辅助类

1. Paint
   数据包package：android.graphics
   Android SDK中的简介：The Paint class holds the style and color information about how to draw geometries, text and bitmaps. 主要就是定义：画刷的样式，画笔的大小/颜色等。
2. Typeface
   数据包 package：android.graphics
   Android SDK中的简介：The Typeface class specifies the typeface and intrinsic style of a font. 主要就是定义：字体。

最后，核心类显示资源

数据包package：android.graphics
主要类：Canvas
Android SDK中的简介：The Canvas class holds the “draw” calls. To draw something, you need 4 basic components: A Bitmap to hold the pixels, a Canvas to host the draw calls (writing into the bitmap), a drawing primitive (e.g. Rect, Path, text, Bitmap), and a paint (to describe the colors and styles for the drawing).
按照结构的功能，将主要接口分为以下3部分：

boolean clipXXXX()	Region区域操作： DIFFERENCE INTERSECT REPLACE REVERSE_DIFFERENCE UNION XOR
void drawXXXX()	画图函数
void rotate() void scale() void skew() void translate()	画布操作函数

Region在这里需要特殊说明下：Region就是一个区域，也就是画布（Canvas）中的有效区域，在无效区域上draw，对画布没有任何改变

另外一种方法,可以切割源图

bt = Bitmap.createBitmap(40, 40, Bitmap.Config.ARGB_8888);

Canvas canvas = new Canvas(bt);
Resources r = this.getContext().getResources();
Drawable pic=r.getDrawable(R.drawable.icon);
pic.setBounds(0, 0, 40, 40);
pic.draw(canvas);