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);
}

}