Android 仿PhotoShop调色板应用(二) 透明度绘制之AlphaPatternDrawable

Android 仿PhotoShop调色板应用(二) 透明度绘制之AlphaPatternDrawable

这里讲一下如何实现PS调色板中的透明度选择条.首先说一下要点:

1. 透明度选择条实际上是基于白色(0xffffffff)和灰色(0xffcbcbcb)之间的颜色区间选取, 由此我们可以实现一个半透明颜色的选取

2.该应用不仅可以做透明度颜色选取，也可以在应用中实现半透明的图像效果

下面看一下代码，主要是基于Drawable的重写：

/*
 * Copyright (C) 2010 Daniel Nilsson
 *
 * 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 net.margaritov.preference.colorpicker;

import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
import android.graphics.Canvas;
import android.graphics.ColorFilter;
import android.graphics.Paint;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;

/**
 * This drawable that draws a simple white and gray chessboard pattern.
 * It's pattern you will often see as a background behind a
 * partly transparent image in many applications.
 * @author Daniel Nilsson
 */
public class AlphaPatternDrawable extends Drawable {

	private int mRectangleSize = 10;

	private Paint mPaint = new Paint();
	private Paint mPaintWhite = new Paint();
	private Paint mPaintGray = new Paint();

	private int numRectanglesHorizontal;
	private int numRectanglesVertical;

	/**
	 * Bitmap in which the pattern will be cahched.
	 */
	private Bitmap		mBitmap;

	public AlphaPatternDrawable(int rectangleSize) {
		mRectangleSize = rectangleSize;
		mPaintWhite.setColor(0xffffffff);
		mPaintGray.setColor(0xffcbcbcb);
	}

	@Override
	public void draw(Canvas canvas) {
		canvas.drawBitmap(mBitmap, null, getBounds(), mPaint);
	}

	@Override
	public int getOpacity() {
		return 0;
	}

	@Override
	public void setAlpha(int alpha) {
		throw new UnsupportedOperationException("Alpha is not supported by this drawwable.");
	}

	@Override
	public void setColorFilter(ColorFilter cf) {
		throw new UnsupportedOperationException("ColorFilter is not supported by this drawwable.");
	}

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

		int height = bounds.height();
		int width = bounds.width();

		numRectanglesHorizontal = (int) Math.ceil((width / mRectangleSize));
		numRectanglesVertical = (int) Math.ceil(height / mRectangleSize);

		generatePatternBitmap();

	}

	/**
	 * This will generate a bitmap with the pattern
	 * as big as the rectangle we were allow to draw on.
	 * We do this to chache the bitmap so we don't need to
	 * recreate it each time draw() is called since it
	 * takes a few milliseconds.
	 */
	private void generatePatternBitmap(){

		if(getBounds().width() <= 0 || getBounds().height() <= 0){
			return;
		}
		
		mBitmap = Bitmap.createBitmap(getBounds().width(), getBounds().height(), Config.ARGB_8888);
		Canvas canvas = new Canvas(mBitmap);

		Rect r = new Rect();
		boolean verticalStartWhite = true;
		for (int i = 0; i <= numRectanglesVertical; i++) {

			boolean isWhite = verticalStartWhite;
			for (int j = 0; j <= numRectanglesHorizontal; j++) {

				r.top = i * mRectangleSize;
				r.left = j * mRectangleSize;
				r.bottom = r.top + mRectangleSize;
				r.right = r.left + mRectangleSize;

				canvas.drawRect(r, isWhite ? mPaintWhite : mPaintGray);

				isWhite = !isWhite;
			}

			verticalStartWhite = !verticalStartWhite;

		}

	}

}