Photo Picker Dialog: Add caching for the decoded images.

chrome/android/java/src/org/chromium/chrome/browser/photo_picker/PickerBitmapViewHolder.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.support.v7.widget.RecyclerView.ViewHolder;
 import android.text.TextUtils;
 
 import java.util.List;
 
 /**
  * Holds on to a {@link PickerBitmapView} that displays information about a picker bitmap.
  */
 public class PickerBitmapViewHolder
         extends ViewHolder implements DecoderServiceHost.ImageDecodedCallback {
+    // The size (in pixels) of the low-res thumbnails.
+    private static final int GRAINY_THUMBNAIL_SIZE_PX = 40;

[Theresa, 2017/05/04 16:53:13]

40px on an xxxhdpi is going to be a very different experience than 40px on an
mdpi device. We should consider using dp rather than pixels.

[Finnur, 2017/05/05 20:41:45]

Good idea. Done.

+
     // Our parent category.
     private PickerCategoryView mCategoryView;
 
     // The bitmap view we are holding on to.
     private final PickerBitmapView mItemView;
 
     // The request we are showing the bitmap for.
     private PickerBitmap mBitmapDetails;
 
     /**
      * The PickerBitmapViewHolder.
      * @param itemView The {@link PickerBitmapView} view for showing the image.
      */
     public PickerBitmapViewHolder(PickerBitmapView itemView) {
         super(itemView);
         mItemView = itemView;
     }
 
     // DecoderServiceHost.ImageDecodedCallback
 
     @Override
     public void imageDecodedCallback(String filePath, Bitmap bitmap) {
         if (bitmap == null || bitmap.getWidth() == 0 || bitmap.getHeight() == 0) {
             return;
         }
 
+        if (mCategoryView.getHighResBitmaps().get(filePath) == null) {
+            mCategoryView.getHighResBitmaps().put(filePath, bitmap);
+        }
+
+        if (mCategoryView.getLowResBitmaps().get(filePath) == null) {
+            // Scaling the image down takes between 0-1 ms on average (Nexus 6 phone debug build).

[Theresa, 2017/05/04 16:53:13]

The Nexus 6 is a relatively good device. Did we take measurements on a slower
(e.g. svelte or old KitKat) device?

[Finnur, 2017/05/05 20:41:45]

0-2 ms on my Evercross Svelte device.

+            Bitmap lowres = BitmapUtils.scale(bitmap, GRAINY_THUMBNAIL_SIZE_PX, false);

[Theresa, 2017/05/04 16:53:13]

Can this be done on an async background thread rather than the UI thread?

[Finnur, 2017/05/05 20:41:45]

Done.

+            mCategoryView.getLowResBitmaps().put(filePath, lowres);
+        }
+
         if (!TextUtils.equals(mBitmapDetails.getFilePath(), filePath)) {
             return;
         }
 
         mItemView.setThumbnailBitmap(bitmap);
     }
 
     /**
      * Display a single item from |position| in the PickerCategoryView.
      * @param categoryView The PickerCategoryView to use to fetch the image.
      * @param position The position of the item to fetch.
      */
     public void displayItem(PickerCategoryView categoryView, int position) {
         mCategoryView = categoryView;
 
         List<PickerBitmap> pickerBitmaps = mCategoryView.getPickerBitmaps();
         mBitmapDetails = pickerBitmaps.get(position);
 
         if (mBitmapDetails.type() == PickerBitmap.CAMERA
                 || mBitmapDetails.type() == PickerBitmap.GALLERY) {
             mItemView.initialize(mBitmapDetails, null, false);
             return;
         }
 
-        // TODO(finnur): Use cached image, if available.
-
-        mItemView.initialize(mBitmapDetails, null, true);
+        String filePath = mBitmapDetails.getFilePath();
+        Bitmap original = mCategoryView.getHighResBitmaps().get(filePath);
+        if (original != null) {
+            mItemView.initialize(mBitmapDetails, original, false);
+            return;
+        }
 
         int size = mCategoryView.getImageSize();
+        Bitmap placeholder = mCategoryView.getLowResBitmaps().get(filePath);
+        if (placeholder != null) {
+            // Scaling the image up takes between 3-4 ms on average (Nexus 6 phone debug build).
+            placeholder = BitmapUtils.scale(placeholder, size, false);
+            mItemView.initialize(mBitmapDetails, placeholder, true);
+        } else {
+            mItemView.initialize(mBitmapDetails, null, true);
+        }
+
         mCategoryView.getDecoderServiceHost().decodeImage(mBitmapDetails.getFilePath(), size, this);
     }
 
     /**
      * Returns the file path of the current request.
      */
     public String getFilePath() {
         return mBitmapDetails == null ? null : mBitmapDetails.getFilePath();
     }
 }