src/android/src/com/google/android/inputmethod/japanese/view/CandidateBackgroundFocusedDrawable.java

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package org.mozc.android.inputmethod.japanese.view;

import android.graphics.Canvas;
import android.graphics.LinearGradient;
import android.graphics.Paint;
import android.graphics.Rect;
import android.graphics.Shader;
import android.graphics.Shader.TileMode;

import java.util.ArrayList;
import java.util.List;

/**
 * Drawable to render a candidate background.
 *
 */
public class CandidateBackgroundFocusedDrawable extends BaseBackgroundDrawable {

  private static final int SHADOW_PIXELS = 3;

  private final int topColor;
  private final int bottomColor;
  private final int shadowColor;

  private int left;
  private int top;
  private int right;
  private int bottom;

  /** Cached Paint instance to reuse for performance reason. */
  private final Paint paint = new Paint();
  private final List<Shader> shaderList = new ArrayList<Shader>(3);

  public CandidateBackgroundFocusedDrawable(
      int leftPadding, int topPadding, int rightPadding, int bottomPadding,
      int topColor, int bottomColor, int shadowColor) {
    super(leftPadding, topPadding, rightPadding, bottomPadding);
    this.topColor = topColor;
    this.bottomColor = bottomColor;
    this.shadowColor = shadowColor;
  }

  @Override
  public void draw(Canvas canvas) {
    if (isCanvasRectEmpty()) {
      return;
    }

    // Local variable cache for better performance.
    Paint paint = this.paint;
    int left = this.left;
    int top = this.top;
    int right = this.right;
    int bottom = this.bottom;

    for (Shader shader : shaderList) {
      paint.reset();
      paint.setAntiAlias(true);
      paint.setShader(shader);
      canvas.drawRect(left, top, right, bottom, paint);
    }
  }

  @Override
  protected void onBoundsChange(Rect rect) {
    super.onBoundsChange(rect);

    shaderList.clear();

    if (isCanvasRectEmpty()) {
      return;
    }

    Rect canvasRect = getCanvasRect();
    left = canvasRect.left;
    top = canvasRect.top;
    right = canvasRect.right;
    bottom = canvasRect.bottom;
    // Shader filling with simple gradient color.
    shaderList.add(new LinearGradient(0, top, 0, bottom, topColor, bottomColor, TileMode.CLAMP));

    // Shader rendering top/bottom edge gradient shadows.
    float verticalShadowStep0 = (SHADOW_PIXELS + 1) / (float) (bottom - top);
    float verticalShadowStep1 = 1.0f - verticalShadowStep0;
    shaderList.add(new LinearGradient(0, top, 0, bottom,
        new int[]{shadowColor, 0, 0, shadowColor},
        new float[]{0.0f, verticalShadowStep0, verticalShadowStep1, 1.0f}, TileMode.CLAMP));

    // Shader rendering left/right edge gradient shadows.
    float horizontalShadowStep0 = (SHADOW_PIXELS + 1) / (float) (right - left);
    float horizontalShadowStep1 = 1.0f - horizontalShadowStep0;
    shaderList.add(new LinearGradient(left, 0, right, 0,
        new int[]{shadowColor, 0, 0, shadowColor},
        new float[]{0.0f, horizontalShadowStep0, horizontalShadowStep1, 1.0f}, TileMode.CLAMP));
  }
}