源碼分析

private static class TilesInitTask extends AsyncTask<Void, Void, int[]> { 
        @Override
        protected int[] doInBackground(Void... params) {
            try {
                String sourceUri = source.toString();
                Context context = contextRef.get();
                DecoderFactory<? extends ImageRegionDecoder> decoderFactory = decoderFactoryRef.get();
                SubsamplingScaleImageView view = viewRef.get();
                if (context != null && decoderFactory != null && view != null) {
                    view.debug("TilesInitTask.doInBackground");
　　　　　　　　　　　　// 獲取decoder 
                    decoder = decoderFactory.make();
                    Point dimensions = decoder.init(context, source);
                    int sWidth = dimensions.x;
                    int sHeight = dimensions.y;
                    int exifOrientation = view.getExifOrientation(context, sourceUri);
　　　　　　　　　　　　// 獲取 region，或者說修正 region
                    if (view.sRegion != null) {
                        view.sRegion.left = Math.max(0, view.sRegion.left);
                        view.sRegion.top = Math.max(0, view.sRegion.top);
                        view.sRegion.right = Math.min(sWidth, view.sRegion.right);
                        view.sRegion.bottom = Math.min(sHeight, view.sRegion.bottom);
                        sWidth = view.sRegion.width();
                        sHeight = view.sRegion.height();
                    }
                    return new int[] { sWidth, sHeight, exifOrientation };
                }
            } catch (Exception e) {
                Log.e(TAG, "Failed to initialise bitmap decoder", e);
                this.exception = e;
            }
            return null;
        }

        @Override
        protected void onPostExecute(int[] xyo) {
            final SubsamplingScaleImageView view = viewRef.get();
            if (view != null) {
                if (decoder != null && xyo != null && xyo.length == 3) {
                    view.onTilesInited(decoder, xyo[0], xyo[1], xyo[2]);
                } else if (exception != null && view.onImageEventListener != null) {
                    view.onImageEventListener.onImageLoadError(exception);
                }
            }
        }
    }

    private DecoderFactory<? extends ImageRegionDecoder> regionDecoderFactory = new CompatDecoderFactory<ImageRegionDecoder>(SkiaImageRegionDecoder.class);

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;

    @Keep
    @SuppressWarnings("unused")
    public SkiaImageRegionDecoder() {
        this(null);
    }

    @SuppressWarnings({"WeakerAccess", "SameParameterValue"})
    public SkiaImageRegionDecoder(@Nullable Bitmap.Config bitmapConfig) {
        Bitmap.Config globalBitmapConfig = SubsamplingScaleImageView.getPreferredBitmapConfig();
        if (bitmapConfig != null) {
            this.bitmapConfig = bitmapConfig;
        } else if (globalBitmapConfig != null) {
            this.bitmapConfig = globalBitmapConfig;
        } else {
　　　　　　　// 如果沒有傳配置，就會使用 565 的方式，這樣一個像素佔有2個位元組，16位 = 5+6+5
            this.bitmapConfig = Bitmap.Config.RGB_565;
        }
    }

    @Override
    @NonNull  
    // 總結起來就是根據不同的圖片資源類型來選擇合適的 regiondecoder 進行解析，最終返回的是圖片的寬高。
    public Point init(Context context, @NonNull 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);
                if (inputStream == null) {
                    throw new Exception("Content resolver returned null stream. Unable to initialise with uri.");
                }
                decoder = BitmapRegionDecoder.newInstance(inputStream, false);
            } finally {
                if (inputStream != null) {
                    try { inputStream.close(); } catch (Exception e) { /* Ignore */ }
                }
            }
        }
        return new Point(decoder.getWidth(), decoder.getHeight());
    }

// overrides for the dimensions of the generated tiles 省略無關的程式碼
    public static final int TILE_SIZE_AUTO = Integer.MAX_VALUE;
    private int maxTileWidth = TILE_SIZE_AUTO;
    private int maxTileHeight = TILE_SIZE_AUTO;

        this.decoder = decoder;
        this.sWidth = sWidth;
        this.sHeight = sHeight;
        this.sOrientation = sOrientation;
        checkReady();
        if (!checkImageLoaded() && maxTileWidth > 0 && maxTileWidth != TILE_SIZE_AUTO && maxTileHeight > 0 && maxTileHeight != TILE_SIZE_AUTO && getWidth() > 0 && getHeight() > 0) {
            initialiseBaseLayer(new Point(maxTileWidth, maxTileHeight));
        }
        invalidate();
        requestLayout();

  protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        createPaints();

        // When using tiles, on first render with no tile map ready, initialise it and kick off async base image loading.
        if (tileMap == null && decoder != null) {
            initialiseBaseLayer(getMaxBitmapDimensions(canvas));
        }
      
        preDraw();

        if (tileMap != null && isBaseLayerReady()) {
            // Optimum sample size for current scale
            int sampleSize = Math.min(fullImageSampleSize, calculateInSampleSize(scale));
            // First check for missing tiles - if there are any we need the base layer underneath to avoid gaps
            boolean hasMissingTiles = false;
            for (Map.Entry<Integer, List<Tile>> tileMapEntry : tileMap.entrySet()) {
                if (tileMapEntry.getKey() == sampleSize) {
                    for (Tile tile : tileMapEntry.getValue()) {
                        if (tile.visible && (tile.loading || tile.bitmap == null)) {
                            hasMissingTiles = true;
                        }
                    }
                }
            }

            // Render all loaded tiles. LinkedHashMap used for bottom up rendering - lower res tiles underneath.
            for (Map.Entry<Integer, List<Tile>> tileMapEntry : tileMap.entrySet()) {
                if (tileMapEntry.getKey() == sampleSize || hasMissingTiles) {
                    for (Tile tile : tileMapEntry.getValue()) {
                        sourceToViewRect(tile.sRect, tile.vRect);
                        if (!tile.loading && tile.bitmap != null) {
                            if (tileBgPaint != null) {
                                canvas.drawRect(tile.vRect, tileBgPaint);
                            }
                            if (matrix == null) { matrix = new Matrix(); }
                            matrix.reset();
                            setMatrixArray(srcArray, 0, 0, tile.bitmap.getWidth(), 0, tile.bitmap.getWidth(), tile.bitmap.getHeight(), 0, tile.bitmap.getHeight());
                            matrix.setPolyToPoly(srcArray, 0, dstArray, 0, 4);
                            canvas.drawBitmap(tile.bitmap, matrix, bitmapPaint);
  
                        } 
                }
            }
        } else if (bitmap != null) {

            float xScale = scale, yScale = scale;
            if (bitmapIsPreview) {
                xScale = scale * ((float)sWidth/bitmap.getWidth());
                yScale = scale * ((float)sHeight/bitmap.getHeight());
            }

            if (matrix == null) { matrix = new Matrix(); }
            matrix.reset();
            matrix.postScale(xScale, yScale);
            matrix.postRotate(getRequiredRotation());
            matrix.postTranslate(vTranslate.x, vTranslate.y);
            if (tileBgPaint != null) {
                if (sRect == null) { sRect = new RectF(); }
                sRect.set(0f, 0f, bitmapIsPreview ? bitmap.getWidth() : sWidth, bitmapIsPreview ? bitmap.getHeight() : sHeight);
                matrix.mapRect(sRect);
                canvas.drawRect(sRect, tileBgPaint);
            }
            canvas.drawBitmap(bitmap, matrix, bitmapPaint);

        }
   }