Drawable与Bitmap的联系与区别

Bitmap - 称作位图，一般位图的文件格式后缀为bmp，当然编码器也有很多如RGB565、RGB888。作为一种逐像素的显示对象执行效率高，但是缺点也很明显存储效率低。我们理解为一种存储对象比较好。
Drawable - 作为Android平下通用的图形对象，它可以装载常用格式的图像，比如GIF、PNG、JPG，当然也支持BMP，当然还提供一些高级的可视化对象，比如渐变、图形等。

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不一定是Bitmap .就像拇指是指头,但不是所有的指头都是拇指一样.

The API dictates: API规定:

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.
小结：

对比项	显示清晰度	占用内存	支持缩放	支持色相色差调整	支持旋转	支持透明色	绘制速度	支持像素操作
Bitmap	相同	大	是	是	是	是	慢	是
Drawable	相同	小	是	否	是	是	快	否

Drawable在内存占用和绘制速度这两个非常关键的点上胜过Bitmap

    /**
     * 添加一个方法
     * bitmap转drawable
     */
    private Drawable bitmapToDrawable(Bitmap bitmap) {
        Drawable drawable = new BitmapDrawable(null, bitmap);
        return drawable;
    }

    /**
     * drawable转bitmap
     * 
     * @param drawable 
     * @return
     */
    private Bitmap drawableToBitamp(Drawable drawable) {
        if (drawable instanceof BitmapDrawable) {
            BitmapDrawable bd = (BitmapDrawable) drawable;
            return bd.getBitmap();
        }
        //以下为drawable转bitmap的通用方法
        int w = drawable.getIntrinsicWidth();
        int h = drawable.getIntrinsicHeight();
        Bitmap bitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bitmap);
        drawable.setBounds(0, 0, w, h);
        drawable.draw(canvas);
        return bitmap;
    }

此时想不想看Android中关于BitmapDrawable的api文档,没错他是drabwable的其中一个子类,
参考文章