android-Bitmap知识

2.1、BitmapFactory.Options的属性解析

BitmapFactory.Options中有以下属性：

inBitmap——在解析Bitmap时重用该Bitmap，不过必须等大的Bitmap而且inMutable须为true
inMutable——配置Bitmap是否可以更改，比如：在Bitmap上隔几个像素加一条线段
inJustDecodeBounds——为true仅返回Bitmap的宽高等属性
inSampleSize——须>=1,表示Bitmap的压缩比例，如：inSampleSize=4，将返回一个是原始图的1/16大小的Bitmap
inPreferredConfig——Bitmap.Config.ARGB_8888等
inDither——是否抖动，默认为false
inPremultiplied——默认为true，一般不改变它的值
inDensity——Bitmap的像素密度
inTargetDensity——Bitmap最终的像素密度
inScreenDensity——当前屏幕的像素密度
inScaled——是否支持缩放，默认为true，当设置了这个，Bitmap将会以inTargetDensity的值进行缩放
inPurgeable——当存储Pixel的内存空间在系统内存不足时是否可以被回收
inInputShareable——inPurgeable为true情况下才生效，是否可以共享一个InputStream
inPreferQualityOverSpeed——为true则优先保证Bitmap质量其次是解码速度
outWidth——返回的Bitmap的宽
outHeight——返回的Bitmap的高

inTempStorage——解码时的临时空间，建议16*1024

2.2、优化策略

1、BitmapConfig的配置 
2、使用decodeFile、decodeResource、decodeStream进行解析Bitmap时，配置inDensity和inTargetDensity，两者应该相等,值可以等于屏幕像素密度*0.75f 
3、使用inJustDecodeBounds预判断Bitmap的大小及使用inSampleSize进行压缩 
4、对Density>240的设备进行Bitmap的适配（缩放Density） 
5、2.3版本inNativeAlloc的使用 
6、4.4以下版本inPurgeable、inInputShareable的使用 

7、Bitmap的回收

针对上面方案，把Bitmap解码的代码封装成了一个工具类，如下：

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();
    }
}

https://www.cnblogs.com/chenliyang/p/6559477.html