Drawable - 作為Android平下通用的圖形對象，它可以裝載常用格式的圖像，比如GIF、PNG、JPG，當然也支持BMP，當然還提供壹些高級的可視化對象，比如漸變、圖形等。
A bitmap is a Drawable. A Drawable is not necessarily a bitmap. Like all thumbs are fingers but not all fingers are thumbs.
Bitmap是Drawable . Drawable不壹定是Bitmap .就像拇指是指頭,但不是所有的指頭都是拇指壹樣.
The API dictates: API規定:
Though usually not visible to the application, Drawables may take a variety of forms: 盡管通常情況下對於應用是不可見的,Drawables 可以采取很多形式:
Bitmap: the simplest Drawable, a PNG or JPEG image. Bitmap: 簡單化的Drawable, PNG 或JPEG圖像.
Nine Patch: an extension to the PNG format allows it to specify
information about how to stretch it and place things inside of it.
Shape: contains simple drawing commands instead of a raw bitmap, allowing it to resize better in some cases.
Layers: a compound drawable, which draws multiple underlying drawables on top of each other.
States: a compound drawable that selects one of a set of drawables based on its state.
Levels: a compound drawable that selects one of a set of drawables based on its level.
Scale: a compound drawable with a single child drawable, whose overall size is modified based on the current level.