Java实现Qt的SIGNAL-SLOT机制

SIGNAL-SLOT是Qt的一大特色，使用起来十分方便。在传统的AWT和Swing编程中，我们都是为要在

监听的对象上添加Listener监听器。被监听对象中保存有Listener的列表，当相关事件发生时，被监听

对象会通知所有Listener。而在Qt中，我们只需通过connect方法连接两个对象上的方法就可以了，非常

方便、优雅地实现了传统的观察者Observer模式。

Qt是如何办到的呢？对于发出SIGNAL的对象，我们需要在其头文件定义中声明Q_Object宏，之后Qt的

预处理器MOC会为我们自动添加上相应的代码来实现SIGNAL-SLOT机制。这与AspectJ自定义了Javac

编译器很类似，都是通过增强编译器来自动添加相应的代码。

增强编译或增加预处理太复杂，怎样能够简单的实现这种机制呢？首先我们实现一个类似的QObject类，

需要发射SIGNAL的类都要继承它。在QObject类中，我们自动为其子类提供监听器列表，查找SLOT方法，

信号发射等功能。

QObject.java

1.在连接方法中，我们将信号和新建的ReceiverSlot类保存到Map中，从而将它们关联起来。

public static void connect(QObject sender, String signal, Object receiver, String slot) { if (sender.signalSlotMap == null) sender.signalSlotMap = new HashMap<String, List<ReceiverSlot>>(); List<ReceiverSlot> slotList = sender.signalSlotMap.get(signal); if (slotList == null) { slotList = new LinkedList<ReceiverSlot>(); sender.signalSlotMap.put(signal, slotList); } slotList.add(createReceiverSlot(receiver, slot)); } static class ReceiverSlot { Object receiver; Method slot; Object[] args; }

2.在创建ReceiverSlot时，我们解析SLOT方法名，如将slot(String,String)解析为方法slot，参数两个String。

如果解析失败我们就认为该SLOT仍是一个信号，也就是SIGNAL-SIGNAL的连接。这种情况下，我们需要

传递调用的不是receiver的SLOT方法，而是emit方法继续发射信号。

private static ReceiverSlot createReceiverSlot(Object receiver, String slot) { ReceiverSlot receiverSlot = new ReceiverSlot(); receiverSlot.receiver = receiver; Pattern pattern = Pattern.compile("(\\w+)\\(([\\w+,]*)\\)"); Matcher matcher = pattern.matcher(slot); if (matcher.matches() && matcher.groupCount() == 2) { // 1.Connect SIGNAL to SLOT try { String methodName = matcher.group(1); String argStr = matcher.group(2); ArrayList<String> argList = new ArrayList<String>(); pattern = Pattern.compile("\\w+"); matcher = pattern.matcher(argStr); while (matcher.find()) argList.add(matcher.group()); String[] arguments = argList.toArray(new String[0]); receiverSlot.slot = findMethod(receiver, methodName, arguments); receiverSlot.args = new Object[0]; } catch (Exception e) { e.printStackTrace(); } } else { // 2.Connect SIGNAL to SIGNAL if (receiver instanceof QObject) { receiverSlot.slot = emitMethod; receiverSlot.args = new Object[] { slot }; } } return receiverSlot; }

private static Method emitMethod; protected Map<String, List<ReceiverSlot>> signalSlotMap; static { try { emitMethod = QObject.class.getDeclaredMethod("emit", String.class, Object[].class); } catch (Exception e) { e.printStackTrace(); } }

3.解析后，如果是SIGNAL-SLOT的连接，那我我们根据方法名和参数找到该方法，准备反射调用。

private static Method findMethod(Object receiver, String methodName, String[] arguments) throws NoSuchMethodException { Method slotMethod = null; if (arguments.length == 0) slotMethod = receiver.getClass().getMethod(methodName, new Class[0]); else { for (Method method : receiver.getClass().getMethods()) { // 1.Check method name if (!method.getName().equals(methodName)) continue; // 2.Check parameter number Class<?>[] paramTypes = method.getParameterTypes(); if (paramTypes.length != arguments.length) continue; // 3.Check parameter type boolean isMatch = true; for (int i = 0; i < paramTypes.length; i++) { if (!paramTypes[i].getSimpleName().equals(arguments[i])) { isMatch = false; break; } } if (isMatch) { slotMethod = method; break; } } if (slotMethod == null) throw new NoSuchMethodException("Cannot find method[" + methodName + "] with parameters: " + Arrays.toString(arguments)); } return slotMethod; }

4.发射信号时，我们取到所有与该SIGNAL关联的ReceiverSlot类，逐个发射信号。

protected void emit(String signal, Object... args) { System.out.println(getClass().getSimpleName() + " emit signal " + signal); if (signalSlotMap == null) return; List<ReceiverSlot> slotList = signalSlotMap.get(signal); if (slotList == null || slotList.isEmpty()) return; for (ReceiverSlot objSlot : slotList) { try { if (objSlot.slot == emitMethod) objSlot.slot.invoke(objSlot.receiver, objSlot.args[0], args); else objSlot.slot.invoke(objSlot.receiver, args); } catch (Exception e) { e.printStackTrace(); } } }

之后，我们实现一个它的子类QWidget，将常用的Swing控件都封装在QWidget的子类中，为这些控件提供

常见的预定义的SIGNAL，像Qt中的clicked和returnPressed。

QWidget.java

public class QWidget<T extends JComponent> extends QObject implements QSwing<T> { protected T widget; public QWidget(Class<T> clazz) { try { widget = clazz.newInstance(); } catch (Exception e) { e.printStackTrace(); } } @Override public T getSwingWidget() { return this.widget; } }

以下是封装了JButton和JTextField的QWidget子类。

QPushButton.java

public class QPushButton extends QWidget<JButton> { public static final String CLICKED = "clicked"; public QPushButton(String text) { super(JButton.class); widget.setText(text); widget.addActionListener(new ActionListener() { @Override public void actionPerformed(ActionEvent e) { emit(CLICKED); } }); } }

QLineEdit.java

public class QLineEdit extends QWidget<JTextField> { public static final String RETURN_PRESSED = "returnPressed"; public QLineEdit() { super(JTextField.class); widget.addActionListener(new ActionListener() { @Override public void actionPerformed(ActionEvent e) { emit(RETURN_PRESSED); } }); } }

下面我们来写个测试类实验下Java版的SIGNAL-SLOT机制，依旧是之前的浏览器的例子。

AddressBar.java

public class AddressBar extends QWidget<JPanel> { /** * SIGNAL */ public static final String NEW_BUTTON_CLICKED = "newButtonClicked"; public static final String GO_TO_ADDRESS = "goToAddress(String,String)"; /** * SLOT */ public static final String HANDLE_GO_TO_ADDRESS = "handleGoToAddress()"; private QPushButton newButton; private QLineEdit addressEdit; private QPushButton goButton; public AddressBar() { super(JPanel.class); // 1.Create widget newButton = new QPushButton("New"); addressEdit = new QLineEdit(); goButton = new QPushButton("Go"); // 2.Set property addressEdit.getSwingWidget().setColumns(10); // 3.Connect signal-slot connect(newButton, QPushButton.CLICKED, this, NEW_BUTTON_CLICKED); connect(addressEdit, QLineEdit.RETURN_PRESSED, this, HANDLE_GO_TO_ADDRESS); connect(goButton, QPushButton.CLICKED, this, HANDLE_GO_TO_ADDRESS); // 4.Add to layout getSwingWidget().add(newButton.getSwingWidget()); getSwingWidget().add(addressEdit.getSwingWidget()); getSwingWidget().add(goButton.getSwingWidget()); } public void handleGoToAddress() { emit(GO_TO_ADDRESS, addressEdit.getSwingWidget().getText(), "test string"); } }

TabBar.java

public class TabBar extends JTabbedPane { /** * SLOT */ public static final String HANDLE_NEW_TAB = "handleNewTab()"; public static final String HANDLE_GO_TO_SITE = "goToSite(String,String)"; public TabBar() { handleNewTab(); } public void handleNewTab() { WebView tab = new WebView(); add("blank", tab); } public void goToSite(String url, String testStr) { System.out.println("Receive url: " + url + ", " + testStr); WebView tab = (WebView) getSelectedComponent(); tab.load(url); } }

MainWindow.java

public class MainWindow extends JFrame { public static void main(String[] args) { JFrame window = new MainWindow(); window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); window.setSize(320, 340); window.setVisible(true); } public MainWindow() { // 1.Create widget AddressBar addressBar = new AddressBar(); TabBar tabBar = new TabBar(); // 2.Set property // 3.Connect signal-slot QObject.connect(addressBar, AddressBar.NEW_BUTTON_CLICKED, tabBar, TabBar.HANDLE_NEW_TAB); QObject.connect(addressBar, AddressBar.GO_TO_ADDRESS, tabBar, TabBar.HANDLE_GO_TO_SITE); // 4.Add to layout GridBagLayout layout = new GridBagLayout(); setLayout(layout); GridBagConstraints grid = new GridBagConstraints(); grid.fill = GridBagConstraints.BOTH; grid.gridx = grid.gridy = 0; grid.weightx = 1.0; grid.weighty = 0.1; add(addressBar.getSwingWidget(), grid); grid.fill = GridBagConstraints.BOTH; grid.gridx = 0; grid.gridy = 1; grid.weightx = 1.0; grid.weighty = 0.9; add(tabBar, grid); } } @SuppressWarnings("serial") class WebView extends JEditorPane { public WebView() { setEditable(false); } public void load(final String url) { SwingUtilities.invokeLater(new Runnable() { @Override public void run() { try { WebView.this.setPage(url); } catch (IOException e) { e.printStackTrace(); } } }); } }

测试一下吧，运行起来的效果就是这样。

新建Tab页和前往该地址事件都可以成功地从AddressBar传递到TabBar。怎么样，这种Java版的

SIGNAL-SLOT是不是很方便。多开拓自己的视野，借鉴优秀的思想，我们才能做出更好的设计！

希望你喜欢本文。