Allow customizing how the content of BottomSheet is dimmed

Fixes #2

bottomsheet/src/main/java/com/flipboard/bottomsheet/BaseViewTransformer.java

@@ -0,0 +1,18 @@
+package com.flipboard.bottomsheet;
+
+import android.view.View;
+
+/**
+ * A very simple view transformer which only handles applying a correct dim.
+ */
+public abstract class BaseViewTransformer implements ViewTransformer {
+
+    public static final float MAX_DIM_ALPHA = 0.7f;
+
+    @Override
+    public float getDimAlpha(float translation, float maxTranslation, float peekedTranslation, BottomSheetLayout parent, View view) {
+        float progress = translation / maxTranslation;
+        return progress * MAX_DIM_ALPHA;
+    }
+
+}

bottomsheet/src/main/java/com/flipboard/bottomsheet/BottomSheetLayout.java

@@ -49,6 +49,13 @@ public void onAnimationCancel(Animator animation) {
 
     }
 
+    private class IdentityViewTransformer extends BaseViewTransformer {
+        @Override
+        public void transformView(float translation, float maxTranslation, float peekedTranslation, BottomSheetLayout parent, View view) {
+            // no-op
+        }
+    }
+
     public enum State {
         HIDDEN,
         PEEKED,
@@ -59,7 +66,6 @@ public interface OnSheetStateChangeListener {
         void onSheetStateChanged(State state);
     }
 
-    private static final float MAX_DIM_ALPHA = 0.7f;
     private static final long ANIMATION_DURATION = 300;
 
     private Rect contentClipRect = new Rect();
@@ -71,7 +77,7 @@ public interface OnSheetStateChangeListener {
     private VelocityTracker velocityTracker;
     private float minFlingVelocity;
     private float touchSlop;
-    private ViewTransformer defaultViewTransformer;
+    private ViewTransformer defaultViewTransformer = new IdentityViewTransformer();
     private ViewTransformer viewTransformer;
     private OnSheetDismissedListener onSheetDismissedListener;
     private boolean shouldDimContentView = true;
@@ -178,9 +184,7 @@ private void setSheetTranslation(float sheetTranslation) {
         this.contentClipRect.set(0, 0, getWidth(), bottomClip);
         getSheetView().setTranslationY(getHeight() - sheetTranslation);
         transformView(sheetTranslation);
-
-        float progress = Math.max(sheetTranslation - getPeekSheetTranslation(), 0) / (getHeight() - getPeekSheetTranslation());
-        dimView.setAlpha(shouldDimContentView ? progress * MAX_DIM_ALPHA : 0);
+        dimView.setAlpha(shouldDimContentView ? getDimAlpha(sheetTranslation) : 0);
     }
 
     private void transformView(float sheetTranslation) {
@@ -191,6 +195,15 @@ private void transformView(float sheetTranslation) {
         }
     }
 
+    private float getDimAlpha(float sheetTranslation) {
+        if (viewTransformer != null) {
+            return viewTransformer.getDimAlpha(sheetTranslation, getMaxSheetTranslation(), getPeekSheetTranslation(), this, getContentView());
+        } else if (defaultViewTransformer != null) {
+            return defaultViewTransformer.getDimAlpha(sheetTranslation, getMaxSheetTranslation(), getPeekSheetTranslation(), this, getContentView());
+        }
+        return 0;
+    }
+
     private float getSheetTranslation() {
         return sheetTranslation;
     }

bottomsheet/src/main/java/com/flipboard/bottomsheet/ViewTransformer.java

@@ -16,4 +16,18 @@ public interface ViewTransformer {
      */
     void transformView(float translation, float maxTranslation, float peekedTranslation, BottomSheetLayout parent, View view);
 
+    /**
+     * Called on when the translation of the sheet view changes allowing you to customize the amount of dimming which
+     * is applied to the content view.
+     *
+     * @param translation The current translation of the presented sheet view.
+     * @param maxTranslation The max translation of the presented sheet view.
+     * @param peekedTranslation The peeked state translation of the presented sheet view.
+     * @param parent The BottomSheet presenting the sheet view.
+     * @param view The content view to transform.
+     *
+     * @return The alpha to apply to the dim overlay.
+     */
+    float getDimAlpha(float translation, float maxTranslation, float peekedTranslation, BottomSheetLayout parent, View view);
+
 }