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A packaging of the IntelliJ Community Edition platform-impl library. This is release number 1 of trunk branch 142.
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/*
 * Copyright 2000-2014 JetBrains s.r.o.
 *
 * 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.intellij.ui.components;

import com.intellij.openapi.util.registry.Registry;
import com.intellij.ui.ColorUtil;

import javax.swing.*;
import java.awt.*;

/**
 * @author Sergey.Malenkov
 */
public class GradientViewport extends JViewport {
  private final Insets myInsets;
  private final boolean myAlways;

  public GradientViewport(Component view, Insets insets, boolean forScrollBars) {
    myInsets = new Insets(insets.top, insets.left, insets.bottom, insets.right);
    myAlways = forScrollBars;
    setView(view);
  }

  protected Component getHeader() {
    return null;
  }

  @Override
  public void paint(Graphics g) {
    super.paint(g);
    g = g.create();
    try {
      Component view = getView();
      Color background = view == null ? null : view.getBackground();
      Component header = getHeader();
      if (header != null) {
        header.setBounds(0, 0, getWidth(), header.getPreferredSize().height);
        if (background != null) {
          g.setColor(background);
          g.fillRect(header.getX(), header.getY(), header.getWidth(), header.getHeight());
        }
      }
      if (g instanceof Graphics2D && background != null && !Registry.is("ui.no.bangs.and.whistles")) {
        paintGradient((Graphics2D)g, background, 0, header == null ? 0 : header.getHeight());
      }
      if (header != null) {
        header.paint(g);
      }
    }
    finally {
      g.dispose();
    }
  }

  private void paintGradient(Graphics2D g2d, Color background, int x1, int y1) {
    Component view = getView();
    if (background != null && view != null) {
      int x2 = x1, x3 = getWidth() - x2, x4 = x3;
      int y2 = y1, y3 = getHeight() - y2, y4 = y3;

      if (myInsets.left > 0 && view.getX() < 0) {
        x2 += myInsets.left;
      }
      if (myInsets.top > 0 && view.getY() < 0) {
        y2 += myInsets.top;
      }
      if (myInsets.right > 0 && view.getX() > getWidth() - view.getWidth()) {
        x3 -= myInsets.right;
      }
      if (myInsets.bottom > 0 && view.getY() > getHeight() - view.getHeight()) {
        y3 -= myInsets.bottom;
      }
      Component parent = myAlways ? null : getParent();
      if (parent instanceof JScrollPane) {
        JScrollPane pane = (JScrollPane)parent;
        JScrollBar vBar = pane.getVerticalScrollBar();
        if (vBar != null && vBar.isVisible()) {
          if (vBar.getX() < getX()) {
            x2 = x1;
          }
          else {
            x3 = x4;
          }
        }
        JScrollBar hBar = pane.getHorizontalScrollBar();
        if (hBar != null && hBar.isVisible()) {
          if (hBar.getY() < getY()) {
            y2 = y1;
          }
          else {
            y3 = y4;
          }
        }
      }
      Color transparent = ColorUtil.toAlpha(background, 0);
      if (x1 != x2) {
        g2d.setPaint(new GradientPaint(x1, y1, background, x2, y1, transparent));
        g2d.fillPolygon(new int[]{x1, x2, x2, x1}, new int[]{y1, y2, y3, y4}, 4);
      }
      if (x3 != x4) {
        g2d.setPaint(new GradientPaint(x3, y1, transparent, x4, y1, background));
        g2d.fillPolygon(new int[]{x4, x3, x3, x4}, new int[]{y1, y2, y3, y4}, 4);
      }
      if (y1 != y2) {
        g2d.setPaint(new GradientPaint(x1, y1, background, x1, y2, transparent));
        g2d.fillPolygon(new int[]{x1, x2, x3, x4}, new int[]{y1, y2, y2, y1}, 4);
      }
      if (y3 != y4) {
        g2d.setPaint(new GradientPaint(x1, y3, transparent, x1, y4, background));
        g2d.fillPolygon(new int[]{x1, x2, x3, x4}, new int[]{y4, y3, y3, y4}, 4);
      }
    }
  }
}