Photo Picker Dialog: Add caching for the decoded images.

chrome/android/java/src/org/chromium/chrome/browser/photo_picker/BitmapScalerTask.java

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.chrome.browser.photo_picker;
+
+import android.graphics.Bitmap;
+import android.os.AsyncTask;
+import android.util.LruCache;
+
+import org.chromium.base.ThreadUtils;
+
+/**
+ * A worker task to enumerate image files on disk.
+ */
+class BitmapScalerTask extends AsyncTask<Bitmap, Void, Bitmap> {
+    private final LruCache<String, Bitmap> mCache;
+    private final String mFilePath;
+    private final int mSize;
+
+    /**
+     * A BitmapScalerTask constructor.
+     */
+    public BitmapScalerTask(LruCache<String, Bitmap> cache, String filePath, int size) {
+        mCache = cache;
+        mFilePath = filePath;
+        mSize = size;
+    }
+
+    /**
+     * Enumerates (in the background) the image files on disk. Called on a non-UI thread

[Michael van Ouwerkerk, 2017/05/08 14:44:22]

nit: please update the description

[Finnur, 2017/05/08 14:59:17]

Done.

+     * @param params Ignored, do not use.
+     * @return A sorted list of images (by last-modified first).
+     */
+    @Override
+    protected Bitmap doInBackground(Bitmap... bitmaps) {
+        assert !ThreadUtils.runningOnUiThread();
+
+        if (isCancelled()) return null;
+
+        return BitmapUtils.scale(bitmaps[0], mSize, false);
+    }
+
+    /**
+     * Communicates the results back to the client. Called on the UI thread.
+     * @param result The resulting scaled bitmap
+     */
+    @Override
+    protected void onPostExecute(Bitmap result) {
+        if (isCancelled()) {
+            return;
+        }
+
+        mCache.put(mFilePath, result);
+    }
+}