Glide4.8源码拆解（四）Bitmap解析之"下采样"浅析

前言

Glide归根结底是一个图片加载框架，它一定会涉及到BitmapFactory相关API把Bitmap读取到内存；可能大家已经很熟悉如何高效的加载Bitmap(比如使用inSample等)，这一章还是要看一看Glide是如何玩转的；

本文主要分析这两个类：

• DownsampleStrategy
• Downsampler

从"Glide会对原图进行放大"案例开始

假设我们有一张宽高100x200的网络图片，需要加载到300x300像素的ImageView上(scaleType属性为CenterCrop)，用Glide加载，不做任何处理会得到多大的Bitmap?

Glide.with(MainActivity.this).load(URL).listener(new RequestListener<Drawable>() {
        @Override
        public boolean onLoadFailed(@Nullable GlideException e, Object model, Target<Drawable> target, boolean isFirstResource) {
          return false;
        }

        @Override
        public boolean onResourceReady(Drawable resource, Object model, Target<Drawable> target, DataSource dataSource, boolean isFirstResource) {
          if (resource instanceof BitmapDrawable){
            Bitmap bitmap = ((BitmapDrawable) resource).getBitmap();
            Log.d("onResourceReady",bitmap.toString());
          }
          return false;
        }
      }).into(vh.imageView);

不出意外会得到300*300的Bitmap，Bitmap比例显然被放大了；

我怀疑是进行了Transformation操作，所以我们要禁用Transformation;

假设我们对RequestOptions加上noTransformation的属性，比如这个样子:

 RequestOptions requestOptions = RequestOptions.noTransformation();
 Glide.with(MainActivity.this).load(URL).apply(requestOptions).listener(xxx).into(vh.imageView);

不出意外会得到300*600的Bitmap，Bitmap比例依然在被放大，甚至更大了；

如果我们不想让Bitmap被放大，可以用加载原图尺寸的方式，比如设置override(Target.SIZE_ORIGINAL,Target.SIZE_ORIGINAL)或者用SimpleTarget这样的Target,但是加载原图终究不是解决方案，为什么？内存的原因，大部分情况下我们还是希望Glide来对图片进行缩小，加载原图的操作等于是直接把APP往OOM送近了一步；

Bitmap被放大不是Glide发明的，是Android官方带的头，比如同一张图片，放在res中不同drawable文件夹下，得到的尺寸不一样；既然如此，Bitmap被放大肯定是有积极意义的，也难怪Glide很少提及这事；

但是，总会有强迫症患者不想Bitmap被放大，Glide肯定也想到了这一点，具体怎么限制Bitmap不会放大，答案的使用RequestOptions.downsample()方法;

RequestOptions.downsample()方法接受参数类型为：DownsampleStrategy，那么我们从DownsampleStrategy开始分析；

DownsampleStrategy

DownsampleStrategy顾名思义就是下采样策略，下采样是图像进行缩小的一种方式；

DownsampleStrategy.java

public abstract class DownsampleStrategy {
//获取缩放比例
public abstract float getScaleFactor(int sourceWidth, int sourceHeight, int requestedWidth,
      int requestedHeight);
      
//获取SampleSize策略
public abstract SampleSizeRounding getSampleSizeRounding(int sourceWidth, int sourceHeight,
      int requestedWidth, int requestedHeight);
}

public enum SampleSizeRounding {
    //内存优先 
    MEMORY,
    //图片质量优先
    QUALITY,
  }

DownsampleStrategy是抽象类，提供两个抽象方法，getScaleFactor()顾名思义是获取缩放比例的，getSampleSizeRounding()可能是获取SampleSize的，DownsampleStrategy真是的实现类在其内部，几个嵌套内部类FitCenter,CenterOutside,AtLeast,AtMost,None,CenterInside,除此之外，DownsampleStrategy还定义对应类的静态变量；

DownsampleStrategy.java

 public static final DownsampleStrategy FIT_CENTER = new FitCenter();

 public static final DownsampleStrategy CENTER_OUTSIDE = new CenterOutside();

 public static final DownsampleStrategy AT_LEAST = new AtLeast();
 
 public static final DownsampleStrategy AT_MOST = new AtMost();
 
 public static final DownsampleStrategy CENTER_INSIDE = new CenterInside();
 
 public static final DownsampleStrategy NONE = new None();
 
 public static final DownsampleStrategy DEFAULT = CENTER_OUTSIDE;

其中DEFAULT应该是Glide默认的策略，它指向的是CENTER_OUTSIDE,我们简单看一下CenterOutside的逻辑；

CenterOutside.java

  private static class CenterOutside extends DownsampleStrategy {

    @Synthetic
    CenterOutside() { }

    @Override
    public float getScaleFactor(int sourceWidth, int sourceHeight, int requestedWidth,
        int requestedHeight) {
        //宽度比例(控件/图片)
      float widthPercentage = requestedWidth / (float) sourceWidth;
      //高度比例(控件/图片)
      float heightPercentage = requestedHeight / (float) sourceHeight;
      //谁大取谁
      return Math.max(widthPercentage, heightPercentage);
    }

    @Override
    public SampleSizeRounding getSampleSizeRounding(int sourceWidth, int sourceHeight,
        int requestedWidth, int requestedHeight) {
      return SampleSizeRounding.QUALITY;
    }
  }

主要关注是CenterOutside的getScaleFactor()逻辑,该逻辑非常简单，主要是拿控件宽高/图片宽高，得到尺寸取最大值，我们回到文章开头的那个例子，假设此时控件宽高是300x300，图片宽高100x200：

widthPercentage = 300/100 = 3.0f;

heightPercentage = 300/200 = 1.5f;

Math.max(widthPercentage, heightPercentage) = 3.0f；

从这个推理来看，文章开头那个图片确实会被放大3倍；

文章开头试图找控制缩放比例不超过1的，就是只缩小不放大的，有没有这样的策略，其实是有的，比如AtLeast,AtMost；

AtLeast.java

private static class AtLeast extends DownsampleStrategy {

    @Synthetic
    AtLeast() { }

    @Override
    public float getScaleFactor(int sourceWidth, int sourceHeight, int requestedWidth,
        int requestedHeight) {
      int minIntegerFactor = Math.min(sourceHeight / requestedHeight, sourceWidth / requestedWidth);
      return minIntegerFactor == 0 ? 1f : 1f / Integer.highestOneBit(minIntegerFactor);
    }

    @Override
    public SampleSizeRounding getSampleSizeRounding(int sourceWidth, int sourceHeight,
        int requestedWidth, int requestedHeight) {
      return SampleSizeRounding.QUALITY;
    }
  }

AtLeast对getScaleFactor的逻辑简要分析：

获取图片宽高/布局宽高的比例的最小值，转成int值；

如果这个值等于0，直接返回1，等于0其实就意味着图片宽高/布局宽高至少有一个是小于1的，也就屏蔽了需要放大的情况；

如果minIntegerFactor不等一0，肯定是需要缩小的，最后返回1f / Integer.highestOneBit(minIntegerFactor),其中Integer.highestOneBit(minIntegerFactor)是取minIntegerFactor的二进制形式最左边的最高一位且高位后面全部补零，最后返回int型的结果；

其余的DownsampleStrategy实现类就不讲解了，DownsampleStrategy是负责计算缩放比例和SampleSize策略，那么真正去进行Bitmap计算的是Downsampler;

重头戏Downsampler

Downsampler.java

Downsampler这个类在我们之前分析Decode流程时已经遇到过它，该类的主要职责是从输入流中解析出Bitmap，核心功能的入口方法在decode();

  public Resource<Bitmap> decode(InputStream is, int requestedWidth, int requestedHeight,
      Options options, DecodeCallbacks callbacks) throws IOException {
    Preconditions.checkArgument(is.markSupported(), "You must provide an InputStream that supports"
        + " mark()");

    byte[] bytesForOptions = byteArrayPool.get(ArrayPool.STANDARD_BUFFER_SIZE_BYTES, byte[].class);
    //获取默认的BitmapFactory
    BitmapFactory.Options bitmapFactoryOptions = getDefaultOptions();
    bitmapFactoryOptions.inTempStorage = bytesForOptions;
    //decodeFormat主要是RGB_8888||RGB_565
    DecodeFormat decodeFormat = options.get(DECODE_FORMAT);
    //获取DownsampleStrategy
    DownsampleStrategy downsampleStrategy = options.get(DownsampleStrategy.OPTION);
    //是否fixedBitmapSize
    boolean fixBitmapToRequestedDimensions = options.get(FIX_BITMAP_SIZE_TO_REQUESTED_DIMENSIONS);
    //是否支持硬件位图
    boolean isHardwareConfigAllowed =
      options.get(ALLOW_HARDWARE_CONFIG) != null && options.get(ALLOW_HARDWARE_CONFIG);

    try {
        //解析出Bitmap
      Bitmap result = decodeFromWrappedStreams(is, bitmapFactoryOptions,
          downsampleStrategy, decodeFormat, isHardwareConfigAllowed, requestedWidth,
          requestedHeight, fixBitmapToRequestedDimensions, callbacks);
     //BitmapResource包装Bitmap
      return BitmapResource.obtain(result, bitmapPool);
    } finally {
      releaseOptions(bitmapFactoryOptions);
      byteArrayPool.put(bytesForOptions);
    }
  }

首先调用getDefaultOptions()获取默认的BitmapFactory;

默认的BitmapFactory

默认的BitmapFactory使用一个队列缓存，初始化设置在resetOptions()方法；

  private static void resetOptions(BitmapFactory.Options decodeBitmapOptions) {
    decodeBitmapOptions.inTempStorage = null;
    decodeBitmapOptions.inDither = false;
    decodeBitmapOptions.inScaled = false;
    decodeBitmapOptions.inSampleSize = 1;
    decodeBitmapOptions.inPreferredConfig = null;
    decodeBitmapOptions.inJustDecodeBounds = false;
    decodeBitmapOptions.inDensity = 0;
    decodeBitmapOptions.inTargetDensity = 0;
    decodeBitmapOptions.outWidth = 0;
    decodeBitmapOptions.outHeight = 0;
    decodeBitmapOptions.outMimeType = null;
    decodeBitmapOptions.inBitmap = null;
    decodeBitmapOptions.inMutable = true;
  }

继续decode()方法分析，首先从Option中获取decodeFormat，fixBitmapToRequestedDimensions，isHardwareConfigAllowed等，这个Option是从RequestOptions传递过来，代表用户的配置;

• decodeFormat表示解析格式，RGB_565或ARGB_8888;
• fixBitmapToRequestedDimensions表示是否填充尺寸，不进行缩放
• isHardwareConfigAllowed是否允许硬件位图，详细了解可以看Glide官方文档：muyangmin.github.io/glide-docs-…
• 用BitmapResource包装Bitmap返回； 真正的加载Bitmap逻辑在decodeFromWrappedStreams()方法：

decodeFromWrappedStreams

decodeFromWrappedStreams()

  private Bitmap decodeFromWrappedStreams(InputStream is,
      BitmapFactory.Options options, DownsampleStrategy downsampleStrategy,
      DecodeFormat decodeFormat, boolean isHardwareConfigAllowed, int requestedWidth,
      int requestedHeight, boolean fixBitmapToRequestedDimensions,
      DecodeCallbacks callbacks) throws IOException {
    long startTime = LogTime.getLogTime();
    //解析出输入流图片尺寸
    int[] sourceDimensions = getDimensions(is, options, callbacks, bitmapPool);
    int sourceWidth = sourceDimensions[0];
    int sourceHeight = sourceDimensions[1];
    //得到mimeType
    String sourceMimeType = options.outMimeType;
    if (sourceWidth == -1 || sourceHeight == -1) {
      isHardwareConfigAllowed = false;
    }
    //获取图片旋转方向
    int orientation = ImageHeaderParserUtils.getOrientation(parsers, is, byteArrayPool);
    int degreesToRotate = TransformationUtils.getExifOrientationDegrees(orientation);
    boolean isExifOrientationRequired = TransformationUtils.isExifOrientationRequired(orientation);
    //判断目标控件尺寸
    int targetWidth = requestedWidth == Target.SIZE_ORIGINAL ? sourceWidth : requestedWidth;
    int targetHeight = requestedHeight == Target.SIZE_ORIGINAL ? sourceHeight : requestedHeight;
    //获取image类型
    ImageType imageType = ImageHeaderParserUtils.getType(parsers, is, byteArrayPool);
    //计算缩放
    calculateScaling();
    //计算其他config
    calculateConfig();

    boolean isKitKatOrGreater = Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT;
    //处理inBitmap
    if ((options.inSampleSize == 1 || isKitKatOrGreater) && shouldUsePool(imageType)) {
      int expectedWidth;
      int expectedHeight;
      if (sourceWidth >= 0 && sourceHeight >= 0
          && fixBitmapToRequestedDimensions && isKitKatOrGreater) {
        expectedWidth = targetWidth;
        expectedHeight = targetHeight;
      } else {
        float densityMultiplier = isScaling(options)
            ? (float) options.inTargetDensity / options.inDensity : 1f;
        int sampleSize = options.inSampleSize;
        int downsampledWidth = (int) Math.ceil(sourceWidth / (float) sampleSize);
        int downsampledHeight = (int) Math.ceil(sourceHeight / (float) sampleSize);
        expectedWidth = Math.round(downsampledWidth * densityMultiplier);
        expectedHeight = Math.round(downsampledHeight * densityMultiplier);
      }
      //设置inBitmap
      if (expectedWidth > 0 && expectedHeight > 0) {
        setInBitmap(options, bitmapPool, expectedWidth, expectedHeight);
      }
    }
    //BitmapFactory.Option配置完毕，调用decodeStream解析
    Bitmap downsampled = decodeStream(is, options, callbacks, bitmapPool);
    callbacks.onDecodeComplete(bitmapPool, downsampled);
    
    Bitmap rotated = null;
    if (downsampled != null) {
      //重新设置density
      downsampled.setDensity(displayMetrics.densityDpi);
      //处理旋转信息
      rotated = TransformationUtils.rotateImageExif(bitmapPool, downsampled, orientation);
      if (!downsampled.equals(rotated)) {
      //添加进BitmapPool
        bitmapPool.put(downsampled);
      }
    }

    return rotated;
  }

• decodeFromWrappedStreams()方法首先调用getDimensions()获取输入流图片尺寸和mimeType；
• 获取图片的方向和旋转角度；
• 确认目标控件的宽高，获取图片的ImageType;
• 调用calculateScaling()计算缩放信息；
• 调用calculateConfig()计算其他配置信息；
• 根据配置，调用setInBitmap()设置inBitmap;
• 上诉流程完成BitmapFactory.Option配置，调用decodeStream()解析输入流；
• 得到Bitmap，对Bitmap做最后的操作；

getDimensions()

  private static int[] getDimensions(InputStream is, BitmapFactory.Options options,
      DecodeCallbacks decodeCallbacks, BitmapPool bitmapPool) throws IOException {
    //只解析Bounds
    options.inJustDecodeBounds = true;
    decodeStream(is, options, decodeCallbacks, bitmapPool);
     //重置为false;
    options.inJustDecodeBounds = false;
    return new int[] { options.outWidth, options.outHeight };
  }

getDimensions()使用options.inJustDecodeBounds = true读取输入流图片信息；

计算缩放

calculateScaling()

  private static void calculateScaling(
      ImageType imageType,
      InputStream is,
      DecodeCallbacks decodeCallbacks,
      BitmapPool bitmapPool,
      DownsampleStrategy downsampleStrategy,
      int degreesToRotate,
      int sourceWidth,
      int sourceHeight,
      int targetWidth,
      int targetHeight,
      BitmapFactory.Options options) throws IOException {
    //尺寸不能为0
    if (sourceWidth <= 0 || sourceHeight <= 0) {
      return;
    }

    final float exactScaleFactor;
    //根据方向和角度获取exactScaleFactor
    if (degreesToRotate == 90 || degreesToRotate == 270) {
      exactScaleFactor = downsampleStrategy.getScaleFactor(sourceHeight, sourceWidth,
          targetWidth, targetHeight);
    } else {
      exactScaleFactor =
          downsampleStrategy.getScaleFactor(sourceWidth, sourceHeight, targetWidth, targetHeight);
    }
    //exactScaleFactor不能小于等于0
    if (exactScaleFactor <= 0f) {
      throw new IllegalArgumentException("");
    }
    //获取SampleSizeRounding
    SampleSizeRounding rounding = downsampleStrategy.getSampleSizeRounding(sourceWidth,
        sourceHeight, targetWidth, targetHeight);
    if (rounding == null) {
      throw new IllegalArgumentException("Cannot round with null rounding");
    }
    //获取Bitmap输出宽高
    int outWidth = round(exactScaleFactor * sourceWidth);
    int outHeight = round(exactScaleFactor * sourceHeight);
    //转成int类型的factor
    int widthScaleFactor = sourceWidth / outWidth;
    int heightScaleFactor = sourceHeight / outHeight;
    //根据SampleSizeRounding得到scaleFactor
    int scaleFactor = rounding == SampleSizeRounding.MEMORY
        ? Math.max(widthScaleFactor, heightScaleFactor)
        : Math.min(widthScaleFactor, heightScaleFactor);

    int powerOfTwoSampleSize;
    //不支持下采样
    if (Build.VERSION.SDK_INT <= 23
        && NO_DOWNSAMPLE_PRE_N_MIME_TYPES.contains(options.outMimeType)) {
      powerOfTwoSampleSize = 1;
    } else {
    //在scaleFactor再进行处理，保证是2的指数幂
      powerOfTwoSampleSize = Math.max(1, Integer.highestOneBit(scaleFactor));
      if (rounding == SampleSizeRounding.MEMORY
          && powerOfTwoSampleSize < (1.f / exactScaleFactor)) {
        powerOfTwoSampleSize = powerOfTwoSampleSize << 1;
      }
    }
    //设置到inSampleSize
    options.inSampleSize = powerOfTwoSampleSize;
    int powerOfTwoWidth;
    int powerOfTwoHeight;
    //针对不同图片格式，重新计算采样后的宽高
    if (imageType == ImageType.JPEG) {
    //libjpeg引擎最大采样size=8，skia会进行二次采样
      int nativeScaling = Math.min(powerOfTwoSampleSize, 8);
      powerOfTwoWidth = (int) Math.ceil(sourceWidth / (float) nativeScaling);
      powerOfTwoHeight = (int) Math.ceil(sourceHeight / (float) nativeScaling);
      //如大大于8，skia会对剩下的进行二次采样计算逻辑
      int secondaryScaling = powerOfTwoSampleSize / 8;
      if (secondaryScaling > 0) {
        powerOfTwoWidth = powerOfTwoWidth / secondaryScaling;
        powerOfTwoHeight = powerOfTwoHeight / secondaryScaling;
      }
    } else if (imageType == ImageType.PNG || imageType == ImageType.PNG_A) {
      powerOfTwoWidth = (int) Math.floor(sourceWidth / (float) powerOfTwoSampleSize);
      powerOfTwoHeight = (int) Math.floor(sourceHeight / (float) powerOfTwoSampleSize);
    } else if (imageType == ImageType.WEBP || imageType == ImageType.WEBP_A) {
    //不同版本采样不同的计算方式round或者floor
      if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
        powerOfTwoWidth = Math.round(sourceWidth / (float) powerOfTwoSampleSize);
        powerOfTwoHeight = Math.round(sourceHeight / (float) powerOfTwoSampleSize);
      } else {
        powerOfTwoWidth = (int) Math.floor(sourceWidth / (float) powerOfTwoSampleSize);
        powerOfTwoHeight = (int) Math.floor(sourceHeight / (float) powerOfTwoSampleSize);
      }
    } else if (
        sourceWidth % powerOfTwoSampleSize != 0 || sourceHeight % powerOfTwoSampleSize != 0) {
      int[] dimensions = getDimensions(is, options, decodeCallbacks, bitmapPool);
      powerOfTwoWidth = dimensions[0];
      powerOfTwoHeight = dimensions[1];
    } else {
      powerOfTwoWidth = sourceWidth / powerOfTwoSampleSize;
      powerOfTwoHeight = sourceHeight / powerOfTwoSampleSize;
    }
    //计算采样后进行缩放的比例
    double adjustedScaleFactor = downsampleStrategy.getScaleFactor(
        powerOfTwoWidth, powerOfTwoHeight, targetWidth, targetHeight);
    //计算inTargetDensity和inDensity进行缩放
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
    //这一块计算方法没有弄明白
      options.inTargetDensity = adjustTargetDensityForError(adjustedScaleFactor);
      options.inDensity = getDensityMultiplier(adjustedScaleFactor);
    }
    //设置inScaled
    if (isScaling(options)){
      options.inScaled = true;
    } else {
      options.inDensity = options.inTargetDensity = 0;
    }
  }

calculateScaling代码看似复杂，但是仔细分析流程还是很清晰的，该方法主要计算BitmapFactory.Factory的inSampleSize和inTargetDensity、inDensity，inSampleSize主要是进行向下采样，采样后自然会对图片进行缩小，但是可能不满足目标缩放比例，所以再配合inTargetDensity、inDensity进行二次缩放；主要针对两个缩放变量powerOfTwoSampleSize和adjustedScaleFactor；具体流程分析:

流程一

1. 计算powerOfTwoSampleSize过程:powerOfTwoSampleSize是最终赋值给options.inSampleSize的；

• 根据图片角度不同，分别调用downSampleStategy.getScaleFactor()方法，得到exactScaleFactor;
• 通过exactScaleFactor计算出outWidth和outHeight,再重新计算widthScaleFactor和heightScaleFactor(ps:widthScaleFactor和heightScaleFactor是int类型，而exactScaleFactor是浮点型，结果是不一样的);
• 通过策略得到int类型的比例scaleFactor
• 判断是否支持下采样，计算出最终的缩放比例powerOfTwoSampleSize
• 赋值给options.inSampleSize

流程二

2. 计算adjustedScaleFactor过程：adjustedScaleFactor是最终计算options.inTargetDensity和options.inDensity的；

• 不同格式和版本的下采样逻辑不同，Glide分别实现采样逻辑的计算，然后生成采样后的宽高尺寸powerOfTwoWidth和powerOfTwoHeight;
• 再此调用downsampleStrategy.getScaleFactor()得到缩放比adjustedScaleFactor;
• 调用adjustTargetDensityForError()和getDensityMultiplier()得到inTargetDensity和inDensity并赋值；
• 上面inDensity依赖options.inScaled = true，最终还得判断是否能够进行scale;

计算硬件位图/Bitmap.Config

calculateConfig()

  private void calculateConfig(
      InputStream is,
      DecodeFormat format,
      boolean isHardwareConfigAllowed,
      boolean isExifOrientationRequired,
      BitmapFactory.Options optionsWithScaling,
      int targetWidth,
      int targetHeight) {
      //如果支持硬件位图
    if (hardwareConfigState.setHardwareConfigIfAllowed(
        targetWidth,
        targetHeight,
        optionsWithScaling,
        format,
        isHardwareConfigAllowed,
        isExifOrientationRequired)) {
      return;
    }
    //这种情况，直接配置ARGB_8888
    if (format == DecodeFormat.PREFER_ARGB_8888
        || Build.VERSION.SDK_INT == Build.VERSION_CODES.JELLY_BEAN) {
      optionsWithScaling.inPreferredConfig = Bitmap.Config.ARGB_8888;
      return;
    }
    //是否有透明的通道
    boolean hasAlpha = false;
    try {
      hasAlpha = ImageHeaderParserUtils.getType(parsers, is, byteArrayPool).hasAlpha();
    } catch (IOException e) {
    
    }
    //有透明的通道，需要配置成ARGB_8888
    optionsWithScaling.inPreferredConfig =
        hasAlpha ? Bitmap.Config.ARGB_8888 : Bitmap.Config.RGB_565;
    if (optionsWithScaling.inPreferredConfig == Config.RGB_565) {
      optionsWithScaling.inDither = true;//565支持抖动界面
    }
  }

calculateConfig()主要是对BitmapFactory.Option的硬件位图和inPreferredConfig进行计算；如果用户配置8888直接配置，如果用户配置565，还需要判断是否有透明度通道，如果有透明度通道，依然采用8888解码器；

硬件位图

HardwareConfigState.java

  boolean setHardwareConfigIfAllowed(
      int targetWidth,
      int targetHeight,
      BitmapFactory.Options optionsWithScaling,
      DecodeFormat decodeFormat,
      boolean isHardwareConfigAllowed,
      boolean isExifOrientationRequired) {
    if (!isHardwareConfigAllowed
        || Build.VERSION.SDK_INT < Build.VERSION_CODES.O
        || isExifOrientationRequired) {
        Android O以上支持
      return false;
    }
    //对尺寸有要求；
    boolean result =
        targetWidth >= MIN_HARDWARE_DIMENSION
            && targetHeight >= MIN_HARDWARE_DIMENSION
            && isFdSizeBelowHardwareLimit();

    if (result) {
      optionsWithScaling.inPreferredConfig = Bitmap.Config.HARDWARE;
      optionsWithScaling.inMutable = false;
    }
    return result;
  }

硬件位图Bitmap.Config.HARDWARE 是一种 Android O 添加的新的位图格式。硬件位图仅在显存 (graphic memory) 里存储像素数据，并对图片仅在屏幕上绘制的场景做了优化。

###设置InBitmap

setInBitmap()

  private static void setInBitmap(
      BitmapFactory.Options options, BitmapPool bitmapPool, int width, int height) {
    @Nullable Bitmap.Config expectedConfig = null;
    // Avoid short circuiting, it appears to break on some devices.
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
    //硬件位图不设置
      if (options.inPreferredConfig == Config.HARDWARE) {
        return;
      }
      //inJustDecodeBoudes时候有可能可以获取到
      expectedConfig = options.outConfig;
    }

    if (expectedConfig == null) {
      expectedConfig = options.inPreferredConfig;
    }
   
    options.inBitmap = bitmapPool.getDirty(width, height, expectedConfig);
  }

设置成硬件位图的不能进行inBitmap操作，最终会设置options.inBitmap，其中缓存的位图从bitmapPool中获取;

解析输出流

decodeStream()

万事具体只差东风，设置完BitmapFactory.Options之后，正在解析输入流的代码就在decodeStream():

  private static Bitmap decodeStream(InputStream is, BitmapFactory.Options options,
      DecodeCallbacks callbacks, BitmapPool bitmapPool) throws IOException {
    if (options.inJustDecodeBounds) {
      is.mark(MARK_POSITION);
    } else {
      callbacks.onObtainBounds();
    }
    int sourceWidth = options.outWidth;
    int sourceHeight = options.outHeight;
    String outMimeType = options.outMimeType;
    final Bitmap result;
    TransformationUtils.getBitmapDrawableLock().lock();
    try {
    //真正的解析调用
      result = BitmapFactory.decodeStream(is, null, options);
    } catch (IllegalArgumentException e) {
      if (options.inBitmap != null) {
        try {
        //发生异常要做inBitmap回收
          is.reset();
          bitmapPool.put(options.inBitmap);
          options.inBitmap = null;//不适用inBitmap
          return decodeStream(is, options, callbacks, bitmapPool);
        } catch (IOException resetException) {
          throw bitmapAssertionException;
        }
      }
      throw bitmapAssertionException;
    } finally {
      TransformationUtils.getBitmapDrawableLock().unlock();
    }
    if (options.inJustDecodeBounds) {
      is.reset();
    }
    return result;
  }

decodeStream()核心的方法是调用BitmapFactory.decodeStream(is, null, options)做解析工作，剩下的代码都是对异常解析，如果发现异常时inBitmap不为空，设置inBitmap为null并重试一次；

总结

本文主要是对Glide下采样做了简单介绍，从代码流程上分析，可以分为对BitmapFactory.Options的配置、调用BitmapFactory解析输入流以及对Bitmap最后的处理三个步骤，其中Options配置阶段在计算缩放上最为重要，其中在缩放因子计算阶段，涉及native以及libjpeg底层的逻辑尤为重要，值得学习；