2.Bitmap優化

主要有兩類方法：
一、decodeBitmap:對Bitmap不壓縮，但是會根據屏幕的密度合適的進行縮放壓縮
二、compressBimtap:對Bitmap進行超過最大寬高的壓縮，同時也會根據屏幕的密度合適的進行縮放壓縮。





public class BitmapDecodeUtil {

    private static final int DEFAULT_DENSITY = 240;

    private static final float SCALE_FACTOR = 0.75f;

    private static final Bitmap.Config DEFAULT_BITMAP_CONFIG = Bitmap.Config.RGB_565;

    private static BitmapFactory.Options getBitmapOptions(Context context) {

        BitmapFactory.Options options = new BitmapFactory.Options();

        options.inScaled = true;

        options.inPreferredConfig = DEFAULT_BITMAP_CONFIG;

        options.inPurgeable = true;

        options.inInputShareable = true;

        options.inJustDecodeBounds = false;

        if (Build.VERSION.SDK_INT <= Build.VERSION_CODES.GINGERBREAD_MR1) {

            Field field = null;

            try {

                field = BitmapFactory.Options.class.getDeclaredField("inNativeAlloc");

                field.setAccessible(true);

                field.setBoolean(options, true);

            } catch (NoSuchFieldException e) {

                e.printStackTrace();

            } catch (IllegalAccessException e) {

                e.printStackTrace();

            }

        }

        int displayDensityDpi = context.getResources().getDisplayMetrics().densityDpi;

        float displayDensity = context.getResources().getDisplayMetrics().density;

        if (displayDensityDpi > DEFAULT_DENSITY && displayDensity > 1.5f) {

            int density = (int) (displayDensityDpi * SCALE_FACTOR);

            options.inDensity = density;

            options.inTargetDensity = density;

        }

        return options;

    }

    public static Bitmap decodeBitmap(Context context, int resId) {

        checkParam(context);

        return BitmapFactory.decodeResource(context.getResources(), resId, getBitmapOptions(context));

    }

    public static Bitmap decodeBitmap(Context context, String pathName) {

        checkParam(context);

        return BitmapFactory.decodeFile(pathName, getBitmapOptions(context));

    }

    public static Bitmap decodeBitmap(Context context, InputStream is) {

        checkParam(context);

        checkParam(is);

        return BitmapFactory.decodeStream(is, null, getBitmapOptions(context));

    }

    public static Bitmap compressBitmap(Context context,int resId, int maxWidth, int maxHeight) {

        checkParam(context);

        final TypedValue value = new TypedValue();

        InputStream is = null;

        try {

            is = context.getResources().openRawResource(resId, value);

            return compressBitmap(context, is, maxWidth, maxHeight);

        } catch (Exception e) {

            e.printStackTrace();

        } finally {

            if (is != null) {

                try {

                    is.close();

                } catch (IOException e) {

                    e.printStackTrace();

                }

            }

        }

        return null;

    }

    public static Bitmap compressBitmap(Context context, String pathName, int maxWidth, int maxHeight) {

        checkParam(context);

        InputStream is = null;

        try {

            is = new FileInputStream(pathName);

            return compressBitmap(context, is, maxWidth, maxHeight);

        } catch (FileNotFoundException e) {

            e.printStackTrace();

        } finally {

            if (is != null) {

                try {

                    is.close();

                } catch (IOException e) {

                    e.printStackTrace();

                }

            }

        }

        return null;

    }

    public static Bitmap compressBitmap(Context context, InputStream is, int maxWidth, int maxHeight) {

        checkParam(context);

        checkParam(is);

        BitmapFactory.Options opt = new BitmapFactory.Options();

        opt.inJustDecodeBounds = true;

        BitmapFactory.decodeStream(is, null, opt);

        int height = opt.outHeight;

        int width = opt.outWidth;

        int sampleSize = computeSampleSize(width, height, maxWidth, maxHeight);

        BitmapFactory.Options options = getBitmapOptions(context);

        options.inSampleSize = sampleSize;

        return BitmapFactory.decodeStream(is, null, options);

    }

    private static int computeSampleSize(int width, int height, int maxWidth, int maxHeight) {

        int inSampleSize = 1;

        if (height > maxHeight || width > maxWidth) {

            final int heightRate = Math.round((float) height / (float) maxHeight);

            final int widthRate = Math.round((float) width / (float) maxWidth);

            inSampleSize = heightRate < widthRate ? heightRate : widthRate;

        }

        if (inSampleSize % 2 != 0) {

            inSampleSize -= 1;

        }

        return inSampleSize <= 1 ? 1 : inSampleSize;

    }

    private static <T> void checkParam(T param){

        if(param == null)

            throw new NullPointerException();

    }

}