Android Bitmap和Drawable互转及使用BitmapFactory解析图片流

一、Bitmap转Drawable

Bitmap bmp=xxx;
BitmapDrawable bd=new BitmapDrawable(bmp);

因为BtimapDrawable是Drawable的子类，最终直接使用bd对象即可。

二、 Drawable转Bitmap
转成Bitmap对象后，可以将Drawable对象通过Android的SK库存成一个字节输出流，最终还可以保存成为jpg和png的文件。

Drawable d=xxx;
BitmapDrawable bd = (BitmapDrawable) d;
Bitmap bm = bd.getBitmap();

最终bm就是我们需要的Bitmap对象了。

Drawable->Bitmap

public static Bitmap convertDrawable2BitmapByCanvas(Drawable drawable) {
　　Bitmap bitmap = Bitmap.createBitmap(drawable.getIntrinsicWidth(),drawable.getIntrinsicHeight(),
　　　　drawable.getOpacity() != PixelFormat.OPAQUE ? Bitmap.Config.ARGB_8888: Bitmap.Config.RGB_565);
　　Canvas canvas = new Canvas(bitmap);
　　// canvas.setBitmap(bitmap);
　　drawable.setBounds(0, 0, drawable.getIntrinsicWidth(),
　　drawable.getIntrinsicHeight());
　　drawable.draw(canvas);
　　return bitmap;
}

三、Bitmap转byte[]

public static byte[] convertBitmap2Bytes(Bitmap bm) {
　　ByteArrayOutputStream baos = new ByteArrayOutputStream();
　　bm.compress(Bitmap.CompressFormat.PNG, 100, baos);
　　return baos.toByteArray();
}

四、BitmapFactory图片解析工具类

　　BitmapFactory自带了一套decodeXXX(params...)方法，用于解析获取图片；其中一个需要注意的细节是opts参数，因为比较常见的是大图，或者一系列的图片，而过多的图片就会有内存溢出的潜在可能性，这个opts参数即是用于设置解析图片流的一些参数的，比如设置宽高，图片缩放等等；
　　下面是google官方关于BitmapFactory.Options的字段的解释；

Fields
public Bitmap	inBitmap	If set, decode methods that take the Options object will attempt to reuse this bitmap when loading content.
public int	inDensity	The pixel density to use for the bitmap.
public boolean	inDither	If dither is true, the decoder will attempt to dither the decoded image.
public boolean	inInputShareable	This field works in conjuction with inPurgeable.
public boolean	inJustDecodeBounds	If set to true, the decoder will return null (no bitmap), but the out...
public boolean	inMutable	If set, decode methods will always return a mutable Bitmap instead of an immutable one.
public boolean	inPreferQualityOverSpeed	If inPreferQualityOverSpeed is set to true, the decoder will try to decode the reconstructed image to a higher quality even at the expense of the decoding speed.
public Bitmap.Config	inPreferredConfig	If this is non-null, the decoder will try to decode into this internal configuration.
public boolean	inPremultiplied	If true (which is the default), the resulting bitmap will have its color channels pre-multipled by the alpha channel.
public boolean	inPurgeable	If this is set to true, then the resulting bitmap will allocate its pixels such that they can be purged if the system needs to reclaim memory.
public int	inSampleSize	If set to a value > 1, requests the decoder to subsample the original image, returning a smaller image to save memory.
public boolean	inScaled	When this flag is set, if inDensity and inTargetDensity are not 0, the bitmap will be scaled to matchinTargetDensity when loaded, rather than relying on the graphics system scaling it each time it is drawn to a Canvas.
public int	inScreenDensity	The pixel density of the actual screen that is being used.
public int	inTargetDensity	The pixel density of the destination this bitmap will be drawn to.
public byte[]	inTempStorage	Temp storage to use for decoding.
public boolean	mCancel	Flag to indicate that cancel has been called on this object.
public int	outHeight	The resulting height of the bitmap, set independent of the state of inJustDecodeBounds.
public String	outMimeType	If known, this string is set to the mimetype of the decoded image.
public int	outWidth	The resulting width of the bitmap, set independent of the state of inJustDecodeBounds.

挑几个通常情况下需要用到的字段
inJustDecodeBounds:boolean类型变量，设置为true时表示只获取图片的宽高；而不获取实际图片；
outHeight:此参数可以进行设置，值为图片的高度，可以通过设置inJustDecodeBounds为true获取图片的高度，然后再对这个高度进行设置，以获取理想高度的图片；
outWidght:获取图片宽度，值的含义同上；
inSampleSize:如果设置值大小1，表示获取一个小一点的图片，用于节省内存，比如：inSampleSize = 4 返回一个原图1/4宽高的图像(图片像素个数为原图1/16)；

获取一个大图的步骤通常如下:
1。传入一个参数opts，设置opts.inJustDecodeBounds为true；
2。获取宽高信息，再通过outHeight、outWidth设置理想图片宽高；
3。重新设置opts.inJustDecodeBounds为false，表示需要获取图片；
4。设置inSampleSize，用于节省内存；

有时候可能还需要设置其它的参数，这里列举两个
inPreferredConfig = Bitmap.Config.ARGB_4444;默认是Bitmap.Config.ARGB_8888;
inPurgeablein、InputShareable 这两个参数需要同时使用；