QT入门语法——signal，slot

谁借莪１个温暖的怀抱￠ 2022-06-13 13:25 224阅读 0赞

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Qt 5 Alpha 已经发布。我们会在后面的文章中看到 Qt 5 的新变化。今天，我们先来看一下 Qt 5 带来的一个最主要的变化：signal/slot 机制的改变。

#  Qt 5 之前的语法 #

在 Qt 5 之前，我们需要使用下面的语句来链接 signal 和 slot：

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      <span>connect</span> 
      <span>(</span> 
      <span>sender</span> 
      <span>,</span> 
      <span> </span> 
      <span>SIGNAL</span> 
      <span>(</span> 
      <span>valueChanged</span> 
      <span>(</span> 
      <span>QString</span> 
      <span>,</span> 
      <span> </span> 
      <span>QString</span> 
      <span>)</span> 
      <span>)</span> 
      <span>,</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>receiver</span> 
      <span>,</span> 
      <span> </span> 
      <span>SLOT</span> 
      <span>(</span> 
      <span>updateValue</span> 
      <span>(</span> 
      <span>QString</span> 
      <span>)</span> 
      <span>)</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
    </div></td> 
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Qt 实际上利用`SIGNAL`和`SLOT`这两个宏，把其后的函数名转换成一个字符串。随后，moc 将会扫描全部文件，将所有的 signal 和 slot 提取出来做成一个映射表。`QObject::connect()`函数则会从这个映射表里面找到该字符串，从 signal 的名字就可以找到 slot 的名字，因此也就知道了在 signal emit 的时候，该去调用哪一个 slot 函数。

# Qt 5 之前的 signal/slot 语法的问题 #

从上面的解释可以看出，Qt 5 之前版本提供的这种语法其实有一些问题：

*  没有编译期检查：因为函数名被处理成字符串，所有的检查都是在运行时完成的。这就是为什么有时会发生编译通过了，但 slot 并没有被调用。此时，你就应该去检查 console 的输出，看看有没有什么 warning 说明 connect 并没有成功。
 *  因为处理的是字符串，所以 slot 中的类型名字必须用 signal 的完全一致，而且在头文件中的和实际 connect 语句中的也必须一致。也就是说，如果你使用了 typedef 或者 namespace，connect 就可能不成功（在 Qt 5 之前的版本中，我们当然也可以使用 namespace，但是必须保证头文件中的和 connect 语句中的文本完全一致）。

# 新语法：使用函数指针 #

在 Qt5 提供了一套新的语法。之前的语法依然可以使用，但是现在，我们有了更好的选择：

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      <span>connect</span> 
      <span>(</span> 
      <span>sender</span> 
      <span>,</span> 
      <span></span> 
      <span>&amp;</span> 
      <span>Sender</span> 
      <span>::</span> 
      <span>valueChanged</span> 
      <span>,</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>receiver</span> 
      <span>,</span> 
      <span></span> 
      <span>&amp;</span> 
      <span>Receiver</span> 
      <span>::</span> 
      <span>updateValue</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
    </div> <p></p></td> 
  </tr> 
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</table>

这个看起来和之前的版本很类似，因此很容易迁移到新的语法。下面我们看看新语法有什么好处：

##  编译器检查 ##

如果把 signal 或者 slot 名字编写错误，或者 slot 的参数同 signal 不一致，你会在编译期就获得一个错误。这肯定会在重构、或者修改 signal 或 slot 的名字时节省很多时间。

另一个影响是，Qt 可以利用`static_cast`返回更友好的错误信息。例如，如果我们少了`Q_OBJECT`宏，则会有：

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      <span>#include &lt;QtCore/QtCore&gt;</span> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <span>class</span> 
      <span> </span> 
      <span>Goo</span> 
      <span> </span> 
      <span>:</span> 
      <span> </span> 
      <span>public</span> 
      <span> </span> 
      <span>QObject</span> 
      <span> </span> 
      <span>{ </span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>Goo</span> 
      <span>(</span> 
      <span>)</span> 
      <span> </span> 
      <span>{ </span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>connect</span> 
      <span>(</span> 
      <span>this</span> 
      <span>,</span> 
      <span> </span> 
      <span>&amp;</span> 
      <span>Goo</span> 
      <span>::</span> 
      <span>someSignal</span> 
      <span>,</span> 
      <span> </span> 
      <span>this</span> 
      <span>,</span> 
      <span> </span> 
      <span>&amp;</span> 
      <span>QObject</span> 
      <span>::</span> 
      <span>deleteLater</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>}</span> 
     </div> 
     <div> 
      <span>signals</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>void</span> 
      <span> </span> 
      <span>someSignal</span> 
      <span>(</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
     <div> 
      <span>}</span> 
      <span>;</span> 
     </div> 
    </div></td> 
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其错误信息是：

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      <span>qobject</span> 
      <span>.</span> 
      <span>h</span> 
      <span>:</span> 
      <span></span> 
      <span>In</span> 
      <span></span> 
      <span>member</span> 
      <span>function</span> 
      <span></span> 
      <span>‘void QObject::qt_check_for_QOBJECT_macro(const T&amp;) const [with T = Goo]’</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>qobject</span> 
      <span>.</span> 
      <span>h</span> 
      <span>:</span> 
      <span>535</span> 
      <span>:</span> 
      <span>9</span> 
      <span>:</span> 
      <span>&nbsp;&nbsp;</span> 
      <span>instantiated</span> 
      <span>from</span> 
      <span></span> 
      <span>‘static typename QtPrivate::QEnableIf::ArgumentCount) &gt;= (int)(QtPrivate::FunctionPointer::ArgumentCount)), void*&gt;::Type QObject::connect(const typename QtPrivate::FunctionPointer::Object*, Func1, const typename QtPrivate::FunctionPointer::Object*, Func2, Qt::ConnectionType) [with Func1 = void (Goo::*)(), Func2 = void (QObject::*)(), typename QtPrivate::QEnableIf::ArgumentCount) &gt;= (int)(QtPrivate::FunctionPointer::ArgumentCount)), void*&gt;::Type = void*, typename QtPrivate::FunctionPointer::Object = Goo, typename QtPrivate::FunctionPointer::Object = QObject]’</span> 
     </div> 
     <div> 
      <span>main</span> 
      <span>.</span> 
      <span>cc</span> 
      <span>:</span> 
      <span>4</span> 
      <span>:</span> 
      <span>68</span> 
      <span>:</span> 
      <span>&nbsp;&nbsp;</span> 
      <span>instantiated</span> 
      <span>from</span> 
      <span>here</span> 
     </div> 
     <div> 
      <span>qobject</span> 
      <span>.</span> 
      <span>h</span> 
      <span>:</span> 
      <span>353</span> 
      <span>:</span> 
      <span>5</span> 
      <span>:</span> 
      <span></span> 
      <span>error</span> 
      <span>:</span> 
      <span></span> 
      <span>void</span> 
      <span></span> 
      <span>value</span> 
      <span>not</span> 
      <span></span> 
      <span>ignored</span> 
      <span>as</span> 
      <span></span> 
      <span>it</span> 
      <span>ought</span> 
      <span>to</span> 
      <span></span> 
      <span>be</span> 
     </div> 
     <div> 
      <span>make</span> 
      <span>:</span> 
      <span></span> 
      <span>*</span> 
      <span>*</span> 
      <span>*</span> 
      <span></span> 
      <span>[</span> 
      <span>main</span> 
      <span>.</span> 
      <span>o</span> 
      <span>]</span> 
      <span></span> 
      <span>Error</span> 
      <span></span> 
      <span>1</span> 
     </div> 
    </div> </td> 
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##  参数的自动类型转换 ##

现在，我们不仅可以更好地使用 typedef 或 namespace，而且可以利用隐式类型转换。在下面的例子中，我们的 signal 有一个`QString`参数，而 slot 需要的是`QVariant`。在新语法中，`QString`将被自动转换成`QVariant`：

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      <span>class</span> 
      <span> </span> 
      <span>Test</span> 
      <span> </span> 
      <span>:</span> 
      <span> </span> 
      <span>public</span> 
      <span> </span> 
      <span>QObject</span> 
     </div> 
     <div> 
      <span>{ </span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>Q_OBJECT</span> 
     </div> 
     <div> 
      <span>public</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>Test</span> 
      <span>(</span> 
      <span>)</span> 
      <span> </span> 
      <span>{ </span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>connect</span> 
      <span>(</span> 
      <span>this</span> 
      <span>,</span> 
      <span> </span> 
      <span>&amp;</span> 
      <span>Test</span> 
      <span>::</span> 
      <span>someSignal</span> 
      <span>,</span> 
      <span> </span> 
      <span>this</span> 
      <span>,</span> 
      <span> </span> 
      <span>&amp;</span> 
      <span>Test</span> 
      <span>::</span> 
      <span>someSlot</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>}</span> 
     </div> 
     <div> 
      <span>signals</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>void</span> 
      <span> </span> 
      <span>someSignal</span> 
      <span>(</span> 
      <span>const</span> 
      <span> </span> 
      <span>QString</span> 
      <span> </span> 
      <span>&amp;</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
     <div> 
      <span>public</span> 
      <span>:</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>void</span> 
      <span> </span> 
      <span>someSlot</span> 
      <span>(</span> 
      <span>const</span> 
      <span> </span> 
      <span>QVariant</span> 
      <span> </span> 
      <span>&amp;</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
     <div> 
      <span>}</span> 
      <span>;</span> 
     </div> 
    </div></td> 
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## 连接到任意函数 ##

如果你留心上面的例子，就会发现，我们的 signal 被连接到了一个 public 函数，但这个函数并不是 slot。Qt 的新语法通过函数指针直接调用函数，而不需要 moc 的特殊处理（但是 signal 依然需要）。

更进一步，我们可以将 signal 连接到任意函数：

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      <span>static</span> 
      <span></span> 
      <span>void</span> 
      <span></span> 
      <span>someFunction</span> 
      <span>(</span> 
      <span>)</span> 
      <span></span> 
      <span>{ </span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>qDebug</span> 
      <span>(</span> 
      <span>)</span> 
      <span></span> 
      <span>&lt;&lt;</span> 
      <span></span> 
      <span>“pressed”</span> 
      <span>;</span> 
     </div> 
     <div> 
      <span>}</span> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <span>// … somewhere else</span> 
     </div> 
     <div> 
      <span>QObject</span> 
      <span>::</span> 
      <span>connect</span> 
      <span>(</span> 
      <span>button</span> 
      <span>,</span> 
      <span></span> 
      <span>&amp;</span> 
      <span>QPushButton</span> 
      <span>::</span> 
      <span>clicked</span> 
      <span>,</span> 
      <span></span> 
      <span>someFunction</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
    </div> <p></p></td> 
  </tr> 
 </tbody> 
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这样处理，就可以让你很方便的同 boost 或者 tr1::bind 这样的第三方库协作。

##  C++11 lambda 表达式 ##

至此之前，我们所有的示例都是基于 [C++][C]98. 标准的。但是，如果你的编译器支持 [c++][C]11，我相信你一看到“函数指针”这几个字，就一定会想到 C++11 的新特性：Lambda 表达式。现在，Lambda 表达式至少被 MSVC 2010，GCC 4.5，clang 3.1 这几个编译器支持。不过对于后面两个编译器，你需要在编译时加上 -std=c++0x 参数。

现在我们可以用 Lambda 表达式重写了：

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      <span>void</span> 
      <span></span> 
      <span>MyWindow</span> 
      <span>::</span> 
      <span>saveDocumentAs</span> 
      <span>(</span> 
      <span>)</span> 
      <span></span> 
      <span>{ </span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>QFileDialog *</span> 
      <span>dlg</span> 
      <span></span> 
      <span>=</span> 
      <span></span> 
      <span>new</span> 
      <span></span> 
      <span>QFileDialog</span> 
      <span>(</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>dlg</span> 
      <span>-&gt;</span> 
      <span>open</span> 
      <span>(</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>QObject</span> 
      <span>::</span> 
      <span>connect</span> 
      <span>(</span> 
      <span>dlg</span> 
      <span>,</span> 
      <span></span> 
      <span>&amp;</span> 
      <span>QDialog</span> 
      <span>::</span> 
      <span>finished</span> 
      <span>,</span> 
      <span></span> 
      <span>[</span> 
      <span>=</span> 
      <span>]</span> 
      <span>(</span> 
      <span>int</span> 
      <span></span> 
      <span>result</span> 
      <span>)</span> 
      <span></span> 
      <span>{ </span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>if</span> 
      <span></span> 
      <span>(</span> 
      <span>result</span> 
      <span>)</span> 
      <span></span> 
      <span>{ </span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>QFile</span> 
      <span>file</span> 
      <span>(</span> 
      <span>dlg</span> 
      <span>-&gt;</span> 
      <span>selectedFiles</span> 
      <span>(</span> 
      <span>)</span> 
      <span>.</span> 
      <span>first</span> 
      <span>(</span> 
      <span>)</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>// … save document here …</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>}</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>dlg</span> 
      <span>-&gt;</span> 
      <span>deleteLater</span> 
      <span>(</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
     <div> 
      <span>&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span>}</span> 
      <span>)</span> 
      <span>;</span> 
     </div> 
     <div> 
      <span>}</span> 
     </div> 
    </div> <p></p></td> 
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这种语法允许我们更方便地编写异步代码。

[C]: http://lib.csdn.net/base/cplusplus