qt 控件不能自适应窗口问题

新建的窗口增加子控件后，无法自适应窗口大小。

用记事本打开mainwindow.ui文件如下：

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralWidget">
   <widget class="QWidget" name="horizontalLayoutWidget">
    <property name="geometry">
     <rect>
      <x>60</x>
      <y>80</y>
      <width>181</width>
      <height>101</height>
     </rect>
    </property>
    <layout class="QHBoxLayout" name="horizontalLayout">
     <item>
      <widget class="QPushButton" name="pushButton">
       <property name="text">
        <string>按钮2</string>
       </property>
      </widget>
     </item>
     <item>
      <widget class="QPushButton" name="pushButton_2">
       <property name="text">
        <string>按钮1</string>
       </property>
      </widget>
     </item>
    </layout>
   </widget>
  </widget>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

发现centralWidget后面还有一个horizontalLayoutWidget，这时我们需要centralWidget后面直接跟布局，而不是Widget。因此删除多于的horizontalLayoutWidget，保留horizontalLayout：

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralWidget">
    <!--多余的horizontalLayoutWidget控件已被删除 -->
   <layout class="QHBoxLayout" name="horizontalLayout">
    <item>
     <widget class="QPushButton" name="pushButton">
      <property name="text">
       <string>按钮2</string>
      </property>
     </widget>
    </item>
    <item>
     <widget class="QPushButton" name="pushButton_2">
      <property name="text">
       <string>按钮1</string>
      </property>
     </widget>
    </item>
   </layout>
  </widget>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

运行后，效果如图：