检测应用程序是否已经安装

private boolean isAvilible( Context context, String packageName )    {        final PackageManager packageManager = context.getPackageManager();        // 获取所有已安装程序的包信息        List<PackageInfo> pinfo = packageManager.getInstalledPackages(0);        for ( int i = 0; i < pinfo.size(); i++ )        {            if(pinfo.get(i).packageName.equalsIgnoreCase(packageName))                return true;        }        return false;    }