AlertDialog 背景透明

AlertDialog alertDialog;
    void showMagicDialog(int id){
        AlertDialog.Builder builder = new AlertDialog.Builder(this, R.style.magicDialog);
        builder.setView(R.layout.dialog_layout);
        alertDialog = builder.create();
        alertDialog.show();
        Window w = alertDialog.getWindow();
        Drawable bgDrawable = new ColorDrawable(0x00000000);
//找到主布局控件, 并修改它的背景       
w.getDecorView().findViewById(getIdFromInternalR("customPanel")).setBackground(bgDrawable);
       //窗体背景修改为透明
        w.setBackgroundDrawable(bgDrawable);
    }

public static int getIdFromInternalR(String idName){
        try {
            Class clasz = Class.forName("com.android.internal.R$id");
            Field field = clasz.getDeclaredField(idName);
            field.setAccessible(true);
            int id= (int)field.get(null);
            return id;
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
        return 0;
    }

ViewGroup vg = (ViewGroup)getDecorView();
for(int i = 0; i < vg.getChildCount(); i++)