1.概述

在选择图片的功能实现中，发现某些图片存在缩略图过于小，因而展示模糊的问题。经分析确认确实查询到的图片尺寸特别小。

2.代码

获取缩略图

fun getImageThumbnail(context: Context, id: Int, int width, int height): Bitmap? {return try {var bitmap: Bitmap? = nullif (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {try {bitmap = context.contentResolver.loadThumbnail(ContentUris.withAppendedId(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, id.toLong()),Size(width, height),CancellationSignal())} catch (e: Exception) {}} else {bitmap = MediaStore.Images.Thumbnails.getThumbnail(context.contentResolver,id.toLong(),MediaStore.Images.Thumbnails.MINI_KIND,null)}bitmap} catch (e: Exception) {null}

阅读源码：

ContentResolver.java中

 public static Bitmap loadThumbnail(@NonNull ContentInterface content, @NonNull Uri uri,@NonNull Size size, @Nullable CancellationSignal signal, int allocator)throws IOException {Objects.requireNonNull(content);Objects.requireNonNull(uri);Objects.requireNonNull(size);// Convert to Point, since that's what the API is defined asfinal Bundle opts = new Bundle();opts.putParcelable(EXTRA_SIZE, new Point(size.getWidth(), size.getHeight()));final Int64Ref orientation = new Int64Ref(0);Bitmap bitmap = ImageDecoder.decodeBitmap(ImageDecoder.createSource(() -> {final AssetFileDescriptor afd = content.openTypedAssetFile(uri, "image/*", opts,signal);final Bundle extras = afd.getExtras();orientation.value = (extras != null) ? extras.getInt(EXTRA_ORIENTATION, 0) : 0;return afd;}), (ImageDecoder decoder, ImageInfo info, Source source) -> {decoder.setAllocator(allocator);// One last-ditch check to see if we've been canceled.if (signal != null) signal.throwIfCanceled();// We requested a rough thumbnail size, but the remote size may have// returned something giant, so defensively scale down as needed.final int widthSample = info.getSize().getWidth() / size.getWidth();final int heightSample = info.getSize().getHeight() / size.getHeight();final int sample = Math.max(widthSample, heightSample);if (sample > 1) {decoder.setTargetSampleSize(sample);}});// Transform the bitmap if requested. We use a side-channel to// communicate the orientation, since EXIF thumbnails don't contain// the rotation flags of the original image.if (orientation.value != 0) {final int width = bitmap.getWidth();final int height = bitmap.getHeight();final Matrix m = new Matrix();m.setRotate(orientation.value, width / 2, height / 2);bitmap = Bitmap.createBitmap(bitmap, 0, 0, width, height, m, false);}return bitmap;}

此处info的尺寸就非常小，后续的操作是如果尺寸大于传入的size的时候，进行缩小。但是由于info的尺寸本身就非常小了，所以不会进行后续处理，返回的bitmap的宽高就非常小。

而且我发现，就算是其他的正常尺寸的图片，读到的bitmap的尺寸也是小于传入的尺寸的。至于这个的原因，未找到。

代码0中另一个版本方法去获取的尺寸也是一样，怎么修改都不会增大。

3.解决方案

想到的是异常的解决方案，当目标尺寸和实际尺寸差异太大，比如5倍的时候，采用其他方式去获取缩略图。此处的5倍是临时决定的，缺乏科学依据。