Refactor FolderIcon to separate the preview effect into it's own class

-> Created com.android.launcher3.folder package to house most folder-related files
   (aside from the FolderInfo) which is more related to the model than the UI.

Change-Id: I767063e1e4c775c01a799a3bede30cd94ac48ade

src/com/android/launcher3/folder/StackFolderIconLayoutRule.java

@@ -0,0 +1,86 @@
+/**
+ * Copyright (C) 2015 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.launcher3.folder;
+
+import com.android.launcher3.folder.FolderIcon.PreviewItemDrawingParams;
+
+public class StackFolderIconLayoutRule implements FolderIcon.PreviewLayoutRule {
+
+    public static final int NUM_ITEMS_IN_PREVIEW = 3;
+
+    // The degree to which the item in the back of the stack is scaled [0...1]
+    // (0 means it's not scaled at all, 1 means it's scaled to nothing)
+    private static final float PERSPECTIVE_SCALE_FACTOR = 0.35f;
+
+    // The amount of vertical spread between items in the stack [0...1]
+    private static final float PERSPECTIVE_SHIFT_FACTOR = 0.18f;
+
+    //private int mIntrinsicIconSize;
+    private float mBaselineIconScale;
+    private int mBaselineIconSize;
+    private int mAvailableSpaceInPreview;
+    private float mMaxPerspectiveShift;
+
+    public void init(int availableSpace, int intrinsicIconSize) {
+        mAvailableSpaceInPreview = availableSpace;
+
+        // cos(45) = 0.707  + ~= 0.1) = 0.8f
+        int adjustedAvailableSpace = (int) ((mAvailableSpaceInPreview / 2) * (1 + 0.8f));
+
+        int unscaledHeight = (int) (intrinsicIconSize * (1 + PERSPECTIVE_SHIFT_FACTOR));
+
+        mBaselineIconScale = (1.0f * adjustedAvailableSpace / unscaledHeight);
+
+        mBaselineIconSize = (int) (intrinsicIconSize * mBaselineIconScale);
+        mMaxPerspectiveShift = mBaselineIconSize * PERSPECTIVE_SHIFT_FACTOR;
+    }
+
+    @Override
+    public PreviewItemDrawingParams computePreviewItemDrawingParams(int index,
+            PreviewItemDrawingParams params) {
+
+        index = NUM_ITEMS_IN_PREVIEW - index - 1;
+        float r = (index * 1.0f) / (NUM_ITEMS_IN_PREVIEW - 1);
+        float scale = (1 - PERSPECTIVE_SCALE_FACTOR * (1 - r));
+
+        float offset = (1 - r) * mMaxPerspectiveShift;
+        float scaledSize = scale * mBaselineIconSize;
+        float scaleOffsetCorrection = (1 - scale) * mBaselineIconSize;
+
+        // We want to imagine our coordinates from the bottom left, growing up and to the
+        // right. This is natural for the x-axis, but for the y-axis, we have to invert things.
+        float transY = mAvailableSpaceInPreview - (offset + scaledSize + scaleOffsetCorrection);
+        float transX = (mAvailableSpaceInPreview - scaledSize) / 2;
+        float totalScale = mBaselineIconScale * scale;
+        final float overlayAlpha = (80 * (1 - r)) / 255f;
+
+        if (params == null) {
+            params = new PreviewItemDrawingParams(transX, transY, totalScale, overlayAlpha);
+        } else {
+            params.transX = transX;
+            params.transY = transY;
+            params.scale = totalScale;
+            params.overlayAlpha = overlayAlpha;
+        }
+        return params;
+    }
+
+    @Override
+    public int numItems() {
+        return NUM_ITEMS_IN_PREVIEW;
+    }
+}