Bitmap的extractAlpha方法使用举例

  前几天使用一款android手机测试的时候， 
发现了应用的 shortcut 九宫格页面有一个点击效果，
就是当点击一个应用的icon图标的时候，会在icon的周围有荧光效果，
无论icon的形状是什么样子的都会有这样的效果,然后又想到Apidemo里面有个alphaDrawable例子
大家可以去在回顾一下，之后我就想到了会不会是使用这个extractAlpha实现的，自己就动手写了个例子
发现效果确实不错，分享给大家
主要关键点
1、设置imageview的src drawable
2、从src drawable中抽取 bitmap的alpha（只有透明度没有颜色）
3、使用bitmap的alpha填充一种颜色后生产新的bitmap
4、使用新生成的bitmap做一个statelistdrawable作为imageview的backgroud
5、这需要注意的是要跟imageview设置几个像素的padding这样才不会让src和backgroud重合
具体效果和应用如下，好学的童鞋可以研究一下。

package com.study;

import android.app.Activity;
import android.os.Bundle;


public class BitmapAlphaActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
    }

}

package com.study;


import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Bitmap.Config;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.StateListDrawable;
import android.util.AttributeSet;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;


/**
 * A layout that arranges its children in a grid.  The size of the
 * cells is set by the {@link #setCellSize} method and the
 * android:cell_width and android:cell_height attributes in XML.
 * The number of rows and columns is determined at runtime.  Each
 * cell contains exactly one view, and they flow in the natural
 * child order (the order in which they were added, or the index
 * in {@link #addViewAt}.  Views can not span multiple cells.
 */
public class FixedGridLayout extends ViewGroup {
    int mCellWidth;
    int mCellHeight;


    public FixedGridLayout(Context context) {
        super(context);
    }


    public FixedGridLayout(Context context, AttributeSet attrs) {
        super(context, attrs);


        // Read the resource attributes.
        TypedArray a = context.obtainStyledAttributes(
                attrs, R.styleable.FixedGridLayout);
        mCellWidth = a.getDimensionPixelSize(
                R.styleable.FixedGridLayout_cellWidth, -1);
        mCellHeight = a.getDimensionPixelSize(
                R.styleable.FixedGridLayout_cellHeight, -1);
        a.recycle();
        
    }


    public void setCellWidth(int px) {
        mCellWidth = px;
        requestLayout();
    }


    public void setCellHeight(int px) {
        mCellHeight = px;
        requestLayout();
    }


    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int cellWidthSpec = MeasureSpec.makeMeasureSpec(mCellWidth,
                MeasureSpec.AT_MOST);
        int cellHeightSpec = MeasureSpec.makeMeasureSpec(mCellHeight,
                MeasureSpec.AT_MOST);


        int count = getChildCount();
        for (int index=0; index<count; index++) {
            final View child = getChildAt(index);
            child.measure(cellWidthSpec, cellHeightSpec);
        }
        // Use the size our parents gave us
        setMeasuredDimension(resolveSize(mCellWidth*count, widthMeasureSpec),
                resolveSize(mCellHeight*count, heightMeasureSpec));
    }


    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        int cellWidth = mCellWidth;
        int cellHeight = mCellHeight;
        int columns = (r - l) / cellWidth;
        if (columns < 0) {
            columns = 1;
        }
        int x = 0;
        int y = 0;
        int i = 0;
        int count = getChildCount();
        for (int index=0; index<count; index++) {
            final View child = getChildAt(index);


            int w = child.getMeasuredWidth();
            int h = child.getMeasuredHeight();


            int left = x + ((cellWidth-w)/2);
            int top = y + ((cellHeight-h)/2);


            child.layout(left, top, left+w, top+h);
            if (i >= (columns-1)) {
                // advance to next row
                i = 0;
                x = 0;
                y += cellHeight;
            } else {
                i++;
                x += cellWidth;
            }
        }
    }
    
    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        int cnt = getChildCount();
        Paint p = new Paint();
        p.setColor(Color.CYAN);
        
        for (int i=0; i<cnt; i++) {
            View v = getChildAt(i);
            setStateDrawable((ImageView)v, p);
        }
    }

private void setStateDrawable(ImageView v, Paint p) {
BitmapDrawable bd = (BitmapDrawable) v.getDrawable();
Bitmap b = bd.getBitmap();
Bitmap bitmap = Bitmap.createBitmap(bd.getIntrinsicWidth(), bd.getIntrinsicHeight(), Config.ARGB_8888);
Canvas canvas = new Canvas(bitmap);
canvas.drawBitmap(b.extractAlpha(), 0, 0, p);

StateListDrawable sld = new StateListDrawable();
sld.addState(new int[]{android.R.attr.state_pressed}, new BitmapDrawable(bitmap));

v.setBackgroundDrawable(sld);
}
}