//我的问题是：progressDialog.show()为什么不显示？我是想让progressDialog两秒后自动消失


public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //创建一个ProgressDialog
        setContentView(R.layout.activity_main);
        ProgressDialog progressDialog = new ProgressDialog(this);
        progressDialog.setTitle("This is a test");
        progressDialog.setCancelable(false);
        progressDialog.setMessage("Loading...");
        
        
        progressDialog.show();

//为什么不是等待2秒后自动消失，而是根本没有显示这个UI呢？
        long timeStart = System.currentTimeMillis();
        while(System.currentTimeMillis() - timeStart < 2000){};
        progressDialog.dismiss();
    }
}