倒影 特效 控件 View ReflectionLayout

最近做倒影特效，网上找了很多资料，都是通过View的getDrawingCache或去Bitmap然后再画上去，我用的时候问题多多啊。UI初始化未完成时，怎么改写获取DrawingCache都报空异常啊。哎哟我的天，网上罗列了一大堆的传说保证能用的神秘解决方案，我都试了，一样的问题。我同时做了一个延时处理，也就是初始化等个几秒再获取DrawingCache去画倒影。思来想去，还是觉得不爽。潜心研究，搞了如下一个ReflectionLayout控件处理倒影，代码如下，有需要的可以试试，包你爽到底。

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.LinearGradient;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.graphics.Shader;
import android.graphics.PorterDuff.Mode;
import android.graphics.Shader.TileMode;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.widget.FrameLayout;

/**
 * Generic layout which creates a glass reflection look. Measuring has many
 * side-effects and works best with a single ImageView child but should be
 * flexible enough to display any other type of view.
 * 
 * @author keane
 * @version 1.0
 */
public class ReflectionLayout extends FrameLayout {
	private static final String TAG = "ReflectionLayout";

	private static final boolean DEBUG_DRAWING_TIME = false;

	/**
	 * Desired reflection layout size (including the child). This should be
	 * configurable at runtime somehow. It may be smaller than this depending on
	 * layout constraints.
	 */
	private static final float REFLECTION_SIZE = 2.20f;

	/* Drawing tools used to create the reflection pool effect. */
	private final Paint mDarkPaint = new Paint();
	private final Paint mReflectionPaint = new Paint();
	private final Matrix mMatrix = new Matrix();
	private final Shader mShader;

	public ReflectionLayout(Context context) {
		this(context, null);
	}

	public ReflectionLayout(Context context, AttributeSet attrs) {
		this(context, attrs, 0);
	}

	public ReflectionLayout(Context context, AttributeSet attrs, int defStyle) {
		super(context, attrs, defStyle);

		setWillNotDraw(false);

		mDarkPaint.setColor(0x98000000);
		mShader = new LinearGradient(0, 0, 0, 1, 0x70ffffff, 0x00ffffff, TileMode.MIRROR);
//		mShader = new LinearGradient(0, 0, 0, 1, 0x70ffffff, 0x00ffffff, TileMode.MIRROR);
		mReflectionPaint.setShader(mShader);
//		mReflectionPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));
		mReflectionPaint.setXfermode(new PorterDuffXfermode(Mode.DST_IN));
		
	}

	@Override
	protected void onMeasure(int wspec, int hspec) {
		super.onMeasure(wspec, hspec);

		if (getChildCount() > 0) {
			View child = getChildAt(0);
			int childw = child.getMeasuredWidth();
			int childh = child.getMeasuredHeight();

			/* Enlarge the child's height by 33% for the reflection. */
			setMeasuredDimension(resolveSize(childw, wspec), resolveSize((int) (childh * REFLECTION_SIZE), hspec));
		}
	}

	@Override
	protected void onDraw(Canvas canvas) {
		long now;
		if (DEBUG_DRAWING_TIME) {
			now = System.currentTimeMillis();
		}

		/* Magic magic magic... */
		if (getChildCount() > 0) {
			drawReflection(canvas);
		}

		if (DEBUG_DRAWING_TIME) {
			long elapsed = System.currentTimeMillis() - now;
			Log.d(TAG, "Drawing took " + elapsed + " ms");
		}
	}

	private void drawReflection(Canvas canvas) {
		View child = getChildAt(0);
		int childw = child.getWidth();
		int childh = child.getHeight();
		int selfh = getHeight();
		int poolh = selfh - childh;

		/*
		 * Save a layer so that we can render off screen initially in order to
		 * achieve the DST_OUT xfer mode. This allows us to have a non-solid
		 * background.
		 */
		canvas.saveLayer(child.getLeft(), child.getBottom(), child.getRight(), child.getBottom() + getBottom(), null, Canvas.HAS_ALPHA_LAYER_SAVE_FLAG);

		/* Draw the flipped child. */
		canvas.save();
		canvas.scale(1, -1);
		canvas.translate(0, -(childh * 2));
		child.draw(canvas);
		canvas.restore();

		/* Saturate the flipped image with a dark color. */
		canvas.drawRect(0, childh, childw, selfh, mDarkPaint);

		/* Carve out the reflection area's alpha channel. */
		mMatrix.setScale(1, poolh);
		mMatrix.postTranslate(0, childh);
		mShader.setLocalMatrix(mMatrix);
		canvas.drawRect(0, childh, childw, selfh, mReflectionPaint);

		/* Apply the canvas layer. */
		canvas.restore();
	}
}