library/src/main/java/com/davemorrissey/labs/subscaleview/decoder/SkiaImageRegionDecoder.java

package com.davemorrissey.labs.subscaleview.decoder;

import android.content.ContentResolver;
import android.content.Context;
import android.content.pm.PackageManager;
import android.content.res.AssetManager;
import android.content.res.Resources;
import android.graphics.*;
import android.net.Uri;
import android.os.Build;
import android.text.TextUtils;

import java.io.InputStream;
import java.util.List;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * Default implementation of {@link com.davemorrissey.labs.subscaleview.decoder.ImageRegionDecoder}
 * using Android's {@link android.graphics.BitmapRegionDecoder}, based on the Skia library. This
 * works well in most circumstances and has reasonable performance due to the cached decoder instance,
 * however it has some problems with grayscale, indexed and CMYK images.
 *
 * A {@link ReadWriteLock} is used to delegate responsibility for multi threading behaviour to the
 * {@link BitmapRegionDecoder} instance on SDK >= 21, whilst allowing this class to block until no
 * tiles are being loaded before recycling the decoder. In practice, {@link BitmapRegionDecoder} is
 * synchronized internally so this has no real impact on performance.
 */
public class SkiaImageRegionDecoder implements ImageRegionDecoder {

    private BitmapRegionDecoder decoder;
    private final ReadWriteLock decoderLock = new ReentrantReadWriteLock(true);

    private static final String FILE_PREFIX = "file://";
    private static final String ASSET_PREFIX = FILE_PREFIX + "/android_asset/";
    private static final String RESOURCE_PREFIX = ContentResolver.SCHEME_ANDROID_RESOURCE + "://";

    private final Bitmap.Config bitmapConfig;

    public SkiaImageRegionDecoder() {
        this(null);
    }

    public SkiaImageRegionDecoder(Bitmap.Config bitmapConfig) {
        if (bitmapConfig == null) {
            this.bitmapConfig = Bitmap.Config.RGB_565;
        } else {
            this.bitmapConfig = bitmapConfig;
        }
    }

    @Override
    public Point init(Context context, Uri uri) throws Exception {
        String uriString = uri.toString();
        if (uriString.startsWith(RESOURCE_PREFIX)) {
            Resources res;
            String packageName = uri.getAuthority();
            if (context.getPackageName().equals(packageName)) {
                res = context.getResources();
            } else {
                PackageManager pm = context.getPackageManager();
                res = pm.getResourcesForApplication(packageName);
            }

            int id = 0;
            List<String> segments = uri.getPathSegments();
            int size = segments.size();
            if (size == 2 && segments.get(0).equals("drawable")) {
                String resName = segments.get(1);
                id = res.getIdentifier(resName, "drawable", packageName);
            } else if (size == 1 && TextUtils.isDigitsOnly(segments.get(0))) {
                try {
                    id = Integer.parseInt(segments.get(0));
                } catch (NumberFormatException ignored) {
                }
            }

            decoder = BitmapRegionDecoder.newInstance(context.getResources().openRawResource(id), false);
        } else if (uriString.startsWith(ASSET_PREFIX)) {
            String assetName = uriString.substring(ASSET_PREFIX.length());
            decoder = BitmapRegionDecoder.newInstance(context.getAssets().open(assetName, AssetManager.ACCESS_RANDOM), false);
        } else if (uriString.startsWith(FILE_PREFIX)) {
            decoder = BitmapRegionDecoder.newInstance(uriString.substring(FILE_PREFIX.length()), false);
        } else {
            InputStream inputStream = null;
            try {
                ContentResolver contentResolver = context.getContentResolver();
                inputStream = contentResolver.openInputStream(uri);
                decoder = BitmapRegionDecoder.newInstance(inputStream, false);
            } finally {
                if (inputStream != null) {
                    try { inputStream.close(); } catch (Exception e) { }
                }
            }
        }
        return new Point(decoder.getWidth(), decoder.getHeight());
    }

    @Override
    public Bitmap decodeRegion(Rect sRect, int sampleSize) {
        getDecodeLock().lock();
        try {
            if (decoder != null && !decoder.isRecycled()) {
                BitmapFactory.Options options = new BitmapFactory.Options();
                options.inSampleSize = sampleSize;
                options.inPreferredConfig = bitmapConfig;
                Bitmap bitmap = decoder.decodeRegion(sRect, options);
                if (bitmap == null) {
                    throw new RuntimeException("Skia image decoder returned null bitmap - image format may not be supported");
                }
                return bitmap;
            } else {
                throw new IllegalStateException("Cannot decode region after decoder has been recycled");
            }
        } finally {
            getDecodeLock().unlock();
        }
    }

    @Override
    public synchronized boolean isReady() {
        return decoder != null && !decoder.isRecycled();
    }

    @Override
    public synchronized void recycle() {
        decoderLock.writeLock().lock();
        try {
            decoder.recycle();
            decoder = null;
        } finally {
            decoderLock.writeLock().unlock();
        }
    }

    /**
     * Before SDK 21, BitmapRegionDecoder was not synchronized internally. Any attempt to decode
     * regions from multiple threads with one decoder instance causes a segfault. For old versions
     * use the write lock to enforce single threaded decoding.
     */
    private Lock getDecodeLock() {
        if (Build.VERSION.SDK_INT < 21) {
            return decoderLock.writeLock();
        } else {
            return decoderLock.readLock();
        }
    }
}