Android Bitmap和Drawable的对比

android bitmap和drawable的对比

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.

对比项	显示清晰度	支持透明色	支持色相色差调整	支持像素操作
bitmap	相同	是	是	是
drawable	相同	是	否	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

- drawable和bitmap之间可以互相转换，drawable占用内存远小于bitmap。

- setimagedrawable使用资源文件；setimagebitmap使用bitmap图片，该图片可能是读取本地相册，或者从资源文件转换而来。

- setimageresource()和setimagebitmap()

//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

小结

bitmap: 简单化的drawable, png 或jpeg图像. 

drawable在内存占用和绘制速度这两个非常关键的点上胜过bitmap

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