* 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.
* Unlike a {@link android.view.View}, a Drawable does not have any facility to
* receive events or otherwise interact with the user.
 

public abstract class Drawable {
 ......
}

也就是說 Drawable 只是一個抽象概念, 表示"something that can be drawn".

Though usually not visible to the application, Drawables may take a variety of forms:
1. Bitmap: the simplest Drawable, a PNG or JPEG image.
2. Nine Patch: an extension
 
 to the PNG format allows it to specify information about how to stretch it and place things inside of it.
3. Shape: contains simple drawing commands instead of a raw bitmap, allowing it to resize better in some cases.
4. Layers: a compound drawable, which draws multiple underlying drawables on top of
 
 each other.
5. States: a compound drawable that selects one of a set of drawables based on its state.
6. Levels: a compound drawable that selects one of a set of drawables based on its level.
7. Scale : a compound drawable with a single child drawable, whose overall size is modified based on the current level.

那麼形式就比較明朗了, Drawable 是一個抽象的概念, 而 Bitmap 是其存在的實體之一.

public final class Bitmap implements Parcelable {
......
}

public class BitmapDrawable extends Drawable {
......
}

    private static Drawable drawableFromBitmap(Resources res, Bitmap bm, byte[] np,
            Rect pad, Rect layoutBounds, String srcName) {

        if (np != null) {
            return new NinePatchDrawable(res, bm, np, pad, layoutBounds, srcName);
        }

        return new BitmapDrawable(res, bm);
    }

通過 BitmapDrawable 的建構函式,使得 Bitmap 可以轉換為 Drawable.

    /**
     * Returns the bitmap used by this drawable to render. May be null.
     */
    public final Bitmap getBitmap() {
        return mBitmapState.mBitmap;
    }