代码：
private ProgressDialog preProgressDialog;
if(preProgressDialog == null || !preProgressDialog.isShowing()){
preProgressDialog = ProgressDialog.show(getContext(), null, "加载中，请稍候......");
} else{
//如果dialog正在显示，说明一个界面正在加载
return;
}
if (preProgressDialog != null) {
preProgressDialog.dismiss();
}
