Photo Picker Dialog: Add caching for the decoded images.

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

Index: chrome/android/java/src/org/chromium/chrome/browser/photo_picker/PickerBitmapViewHolder.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/photo_picker/PickerBitmapViewHolder.java b/chrome/android/java/src/org/chromium/chrome/browser/photo_picker/PickerBitmapViewHolder.java
index bd8dba0de3b4683aba1e342f1edd57a12cc7ec72..b43506aeab6ea49799f025addef278f7e8e3b001 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/photo_picker/PickerBitmapViewHolder.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/photo_picker/PickerBitmapViewHolder.java
@@ -15,6 +15,9 @@ import java.util.List;
  */
 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;
 
@@ -41,6 +44,16 @@ public class PickerBitmapViewHolder
             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;
         }
@@ -65,11 +78,23 @@ public class PickerBitmapViewHolder
             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);
     }