Qt 之消息机制和事件讲解

1、图形界面应用程序的消息处理模型

特点：
基于操作系统才能运行；
GUI应用程序提供的功能必须由用户触发；
用户操作界面时操作系统是第一个感知的；
系统内核的消息通过事件处理转变成QT的信号。

2. Qt中的事件处理

（1）在Qt中，事件被封装成一个个对象，所有的事件均继承自抽象类QEvent. 事件处理的核心包括事件产生、分发、接受和处理。

①事件的产生
问题：谁来产生事件？
最容易想到的是我们的输入设备，比如键盘、鼠标产生的keyPressEvent，keyReleaseEvent，mousePressEvent，mouseReleaseEvent事件（他们被封装成QMouseEvent和QKeyEvent）。

②Qt中事件的分发

问题：谁来负责分发事件？
对于non-GUI的Qt程序，是由QCoreApplication负责将QEvent分发给QObject的子类-Receiver.；对于Qt GUI程序，由QApplication来负责。

③事件的接受和处理
问题：谁来接受和处理事件？
答案是QObject。类是整个Qt对象模型的心脏，事件处理机制是QObject三大职责（内存管理、内省(intropection)与事件处理制）之一。任何一个想要接受并处理事件的对象均须继承自QObject，可以选择重载QObject::event()函数或事件的处理权转给父类。

（2）Qt平台将系统产生的消息转变成Qt事件

A. Qt事件是一个QEvent（或子类）的对象；
B. 有时一个事件包含多个事件类型，比如鼠标事件又可以分为鼠标按下、双击、和移动多种操作事件类型由QEvent类的枚举型QEvent::Type来表示，可由帮助文档进行查询;
C. Qt事件用于描述程序内部或外部发生的对应动作（描述的是操作系统发生来的消息，一个系统消息对应着一个消息事件）;
D. 任意QObject对象都具备时间处理的能力.

note：QEvent子类可以表示一个事件，但并不能处理这个事件

Qt 程序需要在main()函数创建一个QApplication对象，然后调用它的exec()函数。这个函数就是开始 Qt 的事件循环。在执行exec()函数之后，程序将进入事件循环来监听应用程序的事件。当事件发生时，Qt 将创建一个事件对象。Qt 中所有事件类都继承于QEvent。在事件对象创建完毕后，Qt 将这个事件对象传递给QObject的event()函数。event()函数并不直接处理事件，而是将这些事件对象按照它们不同的类型，分发给不同的事件处理器（event handler）。如上所述，event()函数主要用于事件的分发。

（3）GUI应用程序的事件处理方式

A. Qt事件产生后会立即被分发到QWidget对象（QObject的子类，如按键QPushButton对象等）;
B. QWidget对象其内部会有一个event(QEVent*)函数被调用，进行事件处理;
C. event()根据事件类型调用不同的事件处理函数（默认的子函数）;
D. 在事件处理函数中发送Qt中预定义的信号;
E. 调用信号关联的槽函数。

（4） QPushButton事件处理分析

①接收到鼠标事件

②QApplication调用QObject::event(QEvent*)成员函数来处理，进行事件的分派。

③调用QPushButton的mouseReleaseEvent(QMouseEvent*)成员函数

④QPushButton调用click()成员函数

⑤触发信号SIGNAL(clicked())

（5）实例

实例一：自定义事件处理函数

鼠标左键右键

wigth.h

<span style="color:#000000"><code class="language-cpp"><span style="color:#f1fa8c">#<span style="color:#ff79c6">ifndef</span> WIDGET_H</span>
<span style="color:#f1fa8c">#<span style="color:#ff79c6">define</span> WIDGET_H</span>

<span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c"><QWidget></span></span>

<span style="color:#ff79c6">class</span> Widget <span style="color:#6272a4">:</span> <span style="color:#ff79c6">public</span> QWidget
<span style="color:#999999">{</span>Q_OBJECT
<span style="color:#ff79c6">protected</span><span style="color:#6272a4">:</span><span style="color:#ff79c6">void</span> <span style="color:#8be9fd">mousePressEvent</span><span style="color:#999999">(</span>QMouseEvent <span style="color:#6272a4">*</span>event<span style="color:#999999">)</span><span style="color:#999999">;</span>
<span style="color:#ff79c6">public</span><span style="color:#6272a4">:</span><span style="color:#8be9fd">Widget</span><span style="color:#999999">(</span>QWidget <span style="color:#6272a4">*</span>parent <span style="color:#6272a4">=</span> <span style="color:#f1fa8c">0</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
<span style="color:#6272a4">~</span><span style="color:#8be9fd">Widget</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
<span style="color:#999999">}</span><span style="color:#999999">;</span>

<span style="color:#f1fa8c">#<span style="color:#ff79c6">endif</span> </span><span style="color:#6272a4">// WIDGET_H</span>

</code></span>

widgth.cpp

<span style="color:#000000"><code class="language-cpp"><span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c">"widget.h"</span></span>
<span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c"><QMouseEvent></span></span>
<span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c"><QDebug></span></span>
<span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c"><QMenu></span></span>
Widget<span style="color:#6272a4">::</span><span style="color:#8be9fd">Widget</span><span style="color:#999999">(</span>QWidget <span style="color:#6272a4">*</span>parent<span style="color:#999999">)</span><span style="color:#6272a4">:</span> <span style="color:#8be9fd">QWidget</span><span style="color:#999999">(</span>parent<span style="color:#999999">)</span>
<span style="color:#999999">{</span>
<span style="color:#999999">}</span>

<span style="color:#ff79c6">void</span> Widget<span style="color:#6272a4">::</span><span style="color:#8be9fd">mousePressEvent</span><span style="color:#999999">(</span>QMouseEvent <span style="color:#6272a4">*</span>event<span style="color:#999999">)</span>
<span style="color:#999999">{</span><span style="color:#ff79c6">if</span><span style="color:#999999">(</span>event<span style="color:#6272a4">-</span><span style="color:#6272a4">></span><span style="color:#8be9fd">button</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#6272a4">==</span>Qt<span style="color:#6272a4">::</span>LeftButton<span style="color:#999999">)</span><span style="color:#999999">{</span><span style="color:#8be9fd">qDebug</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#6272a4"><<</span><span style="color:#f1fa8c">"LeftButton clicked!"</span><span style="color:#999999">;</span><span style="color:#999999">}</span><span style="color:#ff79c6">else</span> <span style="color:#ff79c6">if</span><span style="color:#999999">(</span>event<span style="color:#6272a4">-</span><span style="color:#6272a4">></span><span style="color:#8be9fd">button</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#6272a4">==</span>Qt<span style="color:#6272a4">::</span>RightButton<span style="color:#999999">)</span><span style="color:#999999">{</span><span style="color:#8be9fd">qDebug</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#6272a4"><<</span><span style="color:#f1fa8c">"RightButton clicked!"</span><span style="color:#999999">;</span><span style="color:#999999">}</span>

<span style="color:#999999">}</span>

Widget<span style="color:#6272a4">::</span><span style="color:#6272a4">~</span><span style="color:#8be9fd">Widget</span><span style="color:#999999">(</span><span style="color:#999999">)</span>
<span style="color:#999999">{</span><span style="color:#999999">}</span>

</code></span>

main.cpp

<span style="color:#000000"><code class="language-cpp"><span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c"><QApplication></span></span>
<span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c">"widget.h"</span></span>

<span style="color:#ff79c6">int</span> <span style="color:#8be9fd">main</span><span style="color:#999999">(</span><span style="color:#ff79c6">int</span> argc<span style="color:#999999">,</span> <span style="color:#ff79c6">char</span> <span style="color:#6272a4">*</span>argv<span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">)</span>
<span style="color:#999999">{</span>QApplication <span style="color:#8be9fd">a</span><span style="color:#999999">(</span>argc<span style="color:#999999">,</span> argv<span style="color:#999999">)</span><span style="color:#999999">;</span>Widget w<span style="color:#999999">;</span>w<span style="color:#999999">.</span><span style="color:#8be9fd">show</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#ff79c6">return</span> a<span style="color:#999999">.</span><span style="color:#8be9fd">exec</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
<span style="color:#999999">}</span>

</code></span>

实例二：自定义事件处理函数

QMyPushButton.h

<span style="color:#000000"><code class="language-cpp"><span style="color:#f1fa8c">#<span style="color:#ff79c6">ifndef</span> _QMYPUSHBUTTON_H_</span>
<span style="color:#f1fa8c">#<span style="color:#ff79c6">define</span> _QMYPUSHBUTTON_H_</span><span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c"><QPushButton></span></span><span style="color:#ff79c6">typedef</span> <span style="color:#ff79c6">void</span> <span style="color:#999999">(</span>QButtonListener<span style="color:#999999">)</span><span style="color:#999999">(</span>QObject<span style="color:#6272a4">*</span><span style="color:#999999">,</span>QMouseEvent<span style="color:#6272a4">*</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#ff79c6">class</span> QMyPushButton <span style="color:#6272a4">:</span> <span style="color:#ff79c6">public</span> QPushButton
<span style="color:#999999">{</span>Q_OBJECT<span style="color:#ff79c6">protected</span><span style="color:#6272a4">:</span>QButtonListener<span style="color:#6272a4">*</span> m_listener<span style="color:#999999">;</span><span style="color:#6272a4">//重写QPushButton的事件处理函数 就有可能不会产生clicked信号</span><span style="color:#ff79c6">void</span> <span style="color:#8be9fd">mouseReleaseEvent</span><span style="color:#999999">(</span>QMouseEvent <span style="color:#6272a4">*</span>e<span style="color:#999999">)</span><span style="color:#999999">;</span>
<span style="color:#ff79c6">public</span><span style="color:#6272a4">:</span><span style="color:#ff79c6">explicit</span> <span style="color:#8be9fd">QMyPushButton</span><span style="color:#999999">(</span>QWidget<span style="color:#6272a4">*</span> parent <span style="color:#6272a4">=</span> <span style="color:#f1fa8c">0</span><span style="color:#999999">,</span> QButtonListener<span style="color:#6272a4">*</span> listener <span style="color:#6272a4">=</span> <span style="color:#f1fa8c">0</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
<span style="color:#999999">}</span><span style="color:#999999">;</span><span style="color:#f1fa8c">#<span style="color:#ff79c6">endif</span> </span><span style="color:#6272a4">// _QMYPUSHBUTTON_H_</span>
</code></span>

QMyPushButton.cpp

<span style="color:#000000"><code class="language-cpp"><span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c">"QMyPushButton.h"</span></span>
<span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c"><QMouseEvent></span></span>QMyPushButton<span style="color:#6272a4">::</span><span style="color:#8be9fd">QMyPushButton</span><span style="color:#999999">(</span>QWidget<span style="color:#6272a4">*</span> parent<span style="color:#999999">,</span> QButtonListener<span style="color:#6272a4">*</span> listener<span style="color:#999999">)</span><span style="color:#6272a4">:</span><span style="color:#8be9fd">QPushButton</span><span style="color:#999999">(</span>parent<span style="color:#999999">)</span>
<span style="color:#999999">{</span>m_listener <span style="color:#6272a4">=</span> listener<span style="color:#999999">;</span>
<span style="color:#999999">}</span><span style="color:#6272a4">//重写改写事件处理函数，会改变程序的行为。</span>
<span style="color:#ff79c6">void</span> QMyPushButton<span style="color:#6272a4">::</span><span style="color:#8be9fd">mouseReleaseEvent</span><span style="color:#999999">(</span>QMouseEvent <span style="color:#6272a4">*</span>e<span style="color:#999999">)</span>
<span style="color:#999999">{</span><span style="color:#ff79c6">if</span><span style="color:#999999">(</span>m_listener <span style="color:#6272a4">!=</span> <span style="color:#f1fa8c">NULL</span><span style="color:#999999">)</span><span style="color:#999999">{</span><span style="color:#6272a4">//调用自定义的事件处理函数，尽管按钮的clicked信号被连接到onMyButtonClicked槽函数，</span><span style="color:#6272a4">//但因自定义的m_listener函数里并不触发clicked信号，从而槽函数不会被调用。</span><span style="color:#8be9fd">m_listener</span><span style="color:#999999">(</span><span style="color:#ff79c6">this</span><span style="color:#999999">,</span> e<span style="color:#999999">)</span><span style="color:#999999">;</span>e<span style="color:#6272a4">-</span><span style="color:#6272a4">></span><span style="color:#8be9fd">accept</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#6272a4">//事件被接收，就不再传递到父QWidget</span><span style="color:#8be9fd">setDown</span><span style="color:#999999">(</span><span style="color:#8be9fd">false</span><span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#6272a4">//按钮设置为“弹起”状态</span><span style="color:#999999">}</span><span style="color:#ff79c6">else</span><span style="color:#999999">{</span><span style="color:#6272a4">//父类的mouseReleaseEvent会去调用clicked()，并触发SIGNAL(clicked())</span><span style="color:#6272a4">//从而调用到连接到该信号的槽函数（本例为onMyButtonClicked())</span>QPushButton<span style="color:#6272a4">::</span><span style="color:#8be9fd">mouseReleaseEvent</span><span style="color:#999999">(</span>e<span style="color:#999999">)</span><span style="color:#999999">;</span> <span style="color:#6272a4">//调用父类</span><span style="color:#999999">}</span>
<span style="color:#999999">}</span>
</code></span>

Widget.h

<span style="color:#000000"><code class="language-cpp"><span style="color:#f1fa8c">#<span style="color:#ff79c6">ifndef</span> _WIDGET_H_</span>
<span style="color:#f1fa8c">#<span style="color:#ff79c6">define</span> _WIDGET_H_</span><span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c"><QWidget></span></span>
<span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c">"QMyPushButton.h"</span></span><span style="color:#ff79c6">class</span> Widget <span style="color:#6272a4">:</span> <span style="color:#ff79c6">public</span> QWidget
<span style="color:#999999">{</span>Q_OBJECTQMyPushButton myButton<span style="color:#999999">;</span><span style="color:#ff79c6">protected</span> slots<span style="color:#6272a4">:</span><span style="color:#ff79c6">void</span> <span style="color:#8be9fd">onMyButtonClicked</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#ff79c6">public</span><span style="color:#6272a4">:</span><span style="color:#8be9fd">Widget</span><span style="color:#999999">(</span>QWidget <span style="color:#6272a4">*</span>parent <span style="color:#6272a4">=</span> <span style="color:#f1fa8c">0</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#6272a4">~</span><span style="color:#8be9fd">Widget</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
<span style="color:#999999">}</span><span style="color:#999999">;</span><span style="color:#f1fa8c">#<span style="color:#ff79c6">endif</span> </span><span style="color:#6272a4">// _WIDGET_H_</span>
</code></span>

Widget.cpp

<span style="color:#000000"><code class="language-cpp"><span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c">"Widget.h"</span></span>
<span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c"><qDebug></span></span><span style="color:#6272a4">//自定义事件处理函数</span>
<span style="color:#ff79c6">void</span> <span style="color:#8be9fd">onMyButtonMouseRelease</span><span style="color:#999999">(</span>QObject<span style="color:#6272a4">*</span> sender<span style="color:#999999">,</span> QMouseEvent<span style="color:#6272a4">*</span> e<span style="color:#999999">)</span>
<span style="color:#999999">{</span><span style="color:#8be9fd">qDebug</span><span style="color:#999999">(</span><span style="color:#999999">)</span> <span style="color:#6272a4"><<</span> <span style="color:#f1fa8c">"onMyButtonMouseRelease(QObject* sender, QMouseEvent* e)"</span><span style="color:#999999">;</span>
<span style="color:#999999">}</span>Widget<span style="color:#6272a4">::</span><span style="color:#8be9fd">Widget</span><span style="color:#999999">(</span>QWidget <span style="color:#6272a4">*</span>parent<span style="color:#999999">)</span><span style="color:#6272a4">:</span> <span style="color:#8be9fd">QWidget</span><span style="color:#999999">(</span>parent<span style="color:#999999">)</span><span style="color:#999999">,</span><span style="color:#8be9fd">myButton</span><span style="color:#999999">(</span><span style="color:#ff79c6">this</span><span style="color:#999999">,</span> onMyButtonMouseRelease<span style="color:#999999">)</span> <span style="color:#6272a4">//实验2：myButton(this, 0)</span>
<span style="color:#999999">{</span>myButton<span style="color:#999999">.</span><span style="color:#8be9fd">setText</span><span style="color:#999999">(</span><span style="color:#f1fa8c">"QMyPushButton"</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#8be9fd">connect</span><span style="color:#999999">(</span><span style="color:#6272a4">&</span>myButton<span style="color:#999999">,</span> <span style="color:#8be9fd">SIGNAL</span><span style="color:#999999">(</span><span style="color:#8be9fd">clicked</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">,</span> <span style="color:#ff79c6">this</span><span style="color:#999999">,</span> <span style="color:#8be9fd">SLOT</span><span style="color:#999999">(</span><span style="color:#8be9fd">onMyButtonClicked</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
<span style="color:#999999">}</span><span style="color:#6272a4">//槽函数，用于接收按钮的clicked信号</span>
<span style="color:#ff79c6">void</span> Widget<span style="color:#6272a4">::</span><span style="color:#8be9fd">onMyButtonClicked</span><span style="color:#999999">(</span><span style="color:#999999">)</span>
<span style="color:#999999">{</span><span style="color:#8be9fd">qDebug</span><span style="color:#999999">(</span><span style="color:#999999">)</span> <span style="color:#6272a4"><<</span> <span style="color:#f1fa8c">"onMyButtonClicked()"</span> <span style="color:#999999">;</span>
<span style="color:#999999">}</span>Widget<span style="color:#6272a4">::</span><span style="color:#6272a4">~</span><span style="color:#8be9fd">Widget</span><span style="color:#999999">(</span><span style="color:#999999">)</span>
<span style="color:#999999">{</span>
<span style="color:#999999">}</span>
</code></span>

main.cpp

<span style="color:#000000"><code class="language-cpp"><span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c">"Widget.h"</span></span>
<span style="color:#f1fa8c">#<span style="color:#ff79c6">include</span> <span style="color:#f1fa8c"><QApplication></span></span><span style="color:#ff79c6">int</span> <span style="color:#8be9fd">main</span><span style="color:#999999">(</span><span style="color:#ff79c6">int</span> argc<span style="color:#999999">,</span> <span style="color:#ff79c6">char</span> <span style="color:#6272a4">*</span>argv<span style="color:#999999">[</span><span style="color:#999999">]</span><span style="color:#999999">)</span>
<span style="color:#999999">{</span>QApplication <span style="color:#8be9fd">a</span><span style="color:#999999">(</span>argc<span style="color:#999999">,</span> argv<span style="color:#999999">)</span><span style="color:#999999">;</span>Widget w<span style="color:#999999">;</span>w<span style="color:#999999">.</span><span style="color:#8be9fd">show</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span><span style="color:#ff79c6">return</span> a<span style="color:#999999">.</span><span style="color:#8be9fd">exec</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
<span style="color:#999999">}</span>
</code></span>

（6）事件（QEvent）和信号（SIGNAL）的不同

例如：单击界面上的按钮，那么就会产生鼠标事件QMouseEvent（不是按钮产生的），由于按钮被按下了，所以他会发出一个单击信号clicked()信号（是按钮产生的），这里只考虑单击信号而不用考虑鼠标事件，但如果要设计一个按钮，或者当单击按钮时让它产生别的效果，此时就要考虑鼠标事件了，由此，信号和事件是两个不同层面的东西，发出者不同，作用不同。Qt中，所有的QObject的子类实例均可对事件接收和处理！

3. 小结

（1）Qt中的事件和信号不同

（2）事件由QObject对象进行处理

（3）信号由QObject对象触发

（4）重写事件处理函数可能改变程序行为

（5）信号的触发不会对程序行为造成影响

（6）事件处理是在实际工程开发中应用非常普遍的