Android开发中，View的图片是动态生成的，我们需要增加点击效果。像iReader 的书架中的书籍，点击变暗。
使用很简单：
ImageView iv = (ImageView) this.findViewById(R.id.image_view1);
Drawable d = Drawable.createFromPath("picPath");
StateListDrawable sld = createSLD(this, d);
iv.setImageDrawable(sld);
///增加如下两个方法：
public Drawable createDrawable(Drawable d, Paint p) {
BitmapDrawable bd = (BitmapDrawable) d;
Bitmap b = bd.getBitmap();
Bitmap bitmap = Bitmap.createBitmap(bd.getIntrinsicWidth(),
bd.getIntrinsicHeight(), Config.ARGB_8888);
Canvas canvas = new Canvas(bitmap);
canvas.drawBitmap(b, 0, 0, p); // 关键代码，使用新的Paint画原图，
return new BitmapDrawable(bitmap);
}
/** 设置Selector。 本次只增加点击变暗的效果，注释的代码为更多的效果*/
public StateListDrawable createSLD(Context context, Drawable drawable) {
StateListDrawable bg = new StateListDrawable();
Paint p = new Paint();
p.setColor(0x40222222); //Paint ARGB色值，A = 0x40 不透明。RGB222222 暗色
Drawable normal = drawable;
Drawable pressed = createDrawable(drawable, p);
// p = new Paint();
// p.setColor(0x8000FF00);
// Drawable focused = createDrawable(drawable, p);
// p = new Paint();
// p.setColor(0x800000FF);
// Drawable unable = createDrawable(drawable, p);
// View.PRESSED_ENABLED_STATE_SET
bg.addState(new int[] { android.R.attr.state_pressed,
android.R.attr.state_enabled }, pressed);
// View.ENABLED_FOCUSED_STATE_SET
// bg.addState(new int[] { android.R.attr.state_enabled,
// android.R.attr.state_focused }, focused);
// View.ENABLED_STATE_SET
bg.addState(new int[] { android.R.attr.state_enabled }, normal);
// View.FOCUSED_STATE_SET
// bg.addState(new int[] { android.R.attr.state_focused }, focused);
// // View.WINDOW_FOCUSED_STATE_SET
// bg.addState(new int[] { android.R.attr.state_window_focused },
// unable);
// View.EMPTY_STATE_SET
bg.addState(new int[] {}, normal);
return bg;
}