Make FadingEdgeScrollView more generic

chrome/android/java/src/org/chromium/chrome/browser/widget/FadingEdgeScrollView.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.widget;
+
+import android.content.Context;
+import android.graphics.Canvas;
+import android.graphics.Color;
+import android.graphics.Paint;
+import android.support.annotation.IntDef;
+import android.util.AttributeSet;
+import android.widget.ScrollView;
+
+import org.chromium.base.ApiCompatibilityUtils;
+import org.chromium.chrome.R;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+/**
+ * An extension of the ScrollView that supports edge boundaries coming in.
+ */
+public class FadingEdgeScrollView extends ScrollView {
+    /** Draw no lines at all. */
+    public static final int DRAW_NO_EDGE = 0;
+
+    /** Draw an edge that fades in, depending on how much is left to scroll. */
+    public static final int DRAW_FADING_EDGE = 1;
+
+    /** Draw either no line (if there is nothing to scroll) or a fully opaque line. */
+    public static final int DRAW_HARD_EDGE = 2;
+
+    @Retention(RetentionPolicy.SOURCE)
+    @IntDef({DRAW_NO_EDGE, DRAW_FADING_EDGE, DRAW_HARD_EDGE})
+    public @interface EdgeType {}
+
+    private static final int POSITION_TOP = 0;
+    private static final int POSITION_BOTTOM = 1;
+
+    private final Paint mSeparatorPaint = new Paint();
+    private final int mSeparatorColor;
+    private final int mSeparatorHeight;
+
+    @EdgeType
+    private int mDrawTopEdge = DRAW_FADING_EDGE;
+    @EdgeType
+    private int mDrawBottomEdge = DRAW_FADING_EDGE;
+
+    public FadingEdgeScrollView(Context context, AttributeSet attrs) {
+        super(context, attrs);
+
+        mSeparatorColor =
+                ApiCompatibilityUtils.getColor(getResources(), R.color.toolbar_shadow_color);
+        mSeparatorHeight = getResources().getDimensionPixelSize(R.dimen.separator_height);
+    }
+
+    @Override
+    protected void dispatchDraw(Canvas canvas) {
+        super.dispatchDraw(canvas);
+        setVerticalFadingEdgeEnabled(true);
+        float topEdgeStrength = getTopFadingEdgeStrength();
+        float bottomEdgeStrength = getBottomFadingEdgeStrength();
+        setVerticalFadingEdgeEnabled(false);
+
+        drawBoundaryLine(canvas, POSITION_TOP, topEdgeStrength, mDrawTopEdge);
+        drawBoundaryLine(canvas, POSITION_BOTTOM, bottomEdgeStrength, mDrawBottomEdge);
+    }
+
+    /**
+     * Sets which edge should be drawn.
+     * @param topEdgeType    Whether to draw the edge on the top part of the view.
+     * @param bottomEdgeType Whether to draw the edge on the bottom part of the view.
+     */
+    public void setEdgeVisibility(@EdgeType int topEdgeType, @EdgeType int bottomEdgeType) {
+        mDrawTopEdge = topEdgeType;
+        mDrawBottomEdge = bottomEdgeType;
+        invalidate();
+    }
+
+    /**
+     * Draws a line at the top or bottom of the view. This should be called from dispatchDraw() so
+     * it gets drawn on top of the View's children.
+     *
+     * @param canvas       The canvas on which to draw.
+     * @param position     Where to draw the line: either POSITION_TOP or POSITION_BOTTOM.
+     * @param edgeStrength A value between 0 and 1 indicating the relative size of the line. 0
+     *                     means no line at all. 1 means a fully opaque line.
+     * @param edgeType     How to draw the line.
+     */
+    private void drawBoundaryLine(
+            Canvas canvas, int position, float edgeStrength, @EdgeType int edgeType) {
+        if (edgeType == DRAW_NO_EDGE) {
+            return;
+        } else if (edgeType == DRAW_FADING_EDGE) {
+            edgeStrength = Math.max(0.0f, Math.min(1.0f, edgeStrength));
+        } else {
+            edgeStrength = 1.0f;
+        }
+        if (edgeStrength <= 0.0f) return;
+
+        int adjustedA = (int) (Color.alpha(mSeparatorColor) * edgeStrength);
+        int adjustedR = (int) (Color.red(mSeparatorColor) * edgeStrength);
+        int adjustedG = (int) (Color.green(mSeparatorColor) * edgeStrength);
+        int adjustedB = (int) (Color.blue(mSeparatorColor) * edgeStrength);
+        mSeparatorPaint.setColor(Color.argb(adjustedA, adjustedR, adjustedG, adjustedB));
+
+        int left = getScrollX();
+        int right = left + getRight();
+
+        if (position == POSITION_BOTTOM) {
+            int bottom = getScrollY() + getBottom() - getTop();
+            canvas.drawRect(left, bottom - mSeparatorHeight, right, bottom, mSeparatorPaint);
+        } else if (position == POSITION_TOP) {
+            int top = getScrollY();
+            canvas.drawRect(left, top, right, top + mSeparatorHeight, mSeparatorPaint);
+        }
+    }
+}