Though usually not visible to the application, Drawables may take a variety of forms: 尽管通常情况下对于应用是不可见的,Drawables 可以采取很多形式:

Bitmap: the simplest Drawable, a PNG or JPEG image. Bitmap: 简单化的Drawable, PNG 或JPEG图像. 
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.

//setImageResource()
public void setImageResource (int resId)//占用UI thread;
// setImageBitmap()
ImageView iv;
String fileName = "/data/data/com.test/aa.png";
Bitmap bm = BitmapFactory.decodeFile(fileName); 
iv.setImageBitmap(bm); //占用内存
// setImageBitmap()
Bitmap image = BitmapFactory.decodeFile(imgFile.getAbsolutePath());
imageView.setImageBitmap(image);
// Bitmap转换成Drawable
Bitmap image = BitmapFactory.decodeFile(imgFile.getAbsolutePath());
BitmapDrawable bitmapDrawable = new BitmapDrawable(image);
imageView.setImageDrawable(bitmapDrawable);
// 结论：Bitmap是Drawable . Drawable不一定是Bitmap