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..28fa09d2153fecc10e2aff1a320469034aa9c89a 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
@@ -5,6 +5,7 @@
 package org.chromium.chrome.browser.photo_picker;
 
 import android.graphics.Bitmap;
+import android.os.AsyncTask;
 import android.support.v7.widget.RecyclerView.ViewHolder;
 import android.text.TextUtils;
 
@@ -15,12 +16,18 @@ import java.util.List;
  */
 public class PickerBitmapViewHolder
         extends ViewHolder implements DecoderServiceHost.ImageDecodedCallback {
+    // The size (in dp) of the low-res thumbnails.
+    private static final int GRAINY_THUMBNAIL_SIZE_DP = 12;

[Theresa, 2017/05/08 19:34:42]

Nit: Put this in dimens.xml and use Resources.getDimensionPixelSize() to
retrieve the value in px.

[Finnur, 2017/05/09 11:53:56]

Done.

+
     // Our parent category.
     private PickerCategoryView mCategoryView;
 
     // The bitmap view we are holding on to.
     private final PickerBitmapView mItemView;
 
+    // The screen density.
+    private final float mDensity;
+
     // The request we are showing the bitmap for.
     private PickerBitmap mBitmapDetails;
 
@@ -31,6 +38,7 @@ public class PickerBitmapViewHolder
     public PickerBitmapViewHolder(PickerBitmapView itemView) {
         super(itemView);
         mItemView = itemView;
+        mDensity = itemView.getContext().getResources().getDisplayMetrics().density;
     }
 
     // DecoderServiceHost.ImageDecodedCallback
@@ -41,6 +49,16 @@ public class PickerBitmapViewHolder
             return;
         }
 
+        if (mCategoryView.getHighResBitmaps().get(filePath) == null) {
+            mCategoryView.getHighResBitmaps().put(filePath, bitmap);
+        }
+
+        if (mCategoryView.getLowResBitmaps().get(filePath) == null) {
+            new BitmapScalerTask(mCategoryView.getLowResBitmaps(), filePath,
+                    (int) (GRAINY_THUMBNAIL_SIZE_DP * mDensity))
+                    .executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, bitmap);
+        }
+
         if (!TextUtils.equals(mBitmapDetails.getFilePath(), filePath)) {
             return;
         }
@@ -65,12 +83,24 @@ 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();
-        mCategoryView.getDecoderServiceHost().decodeImage(mBitmapDetails.getFilePath(), size, this);
+        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).

[Theresa, 2017/05/08 19:34:42]

nit: This type of information is likely to get out of date. I think it would be
useful to include a bug # here that has a comment with the stats and relevant
information (e.g. device type, OS version, Chrome version), so that we can look
back a couple of years from now and know what analysis was done.

[Finnur, 2017/05/09 11:53:56]

Done.

+            placeholder = BitmapUtils.scale(placeholder, size, false);
+            mItemView.initialize(mBitmapDetails, placeholder, true);
+        } else {
+            mItemView.initialize(mBitmapDetails, null, true);
+        }
+
+        mCategoryView.getDecoderServiceHost().decodeImage(filePath, size, this);
     }
 
     /**