Java中的观察者设计模式是个比较实用的设计模式，可以用于数据自动更新，但是C++中并没提供现成的类实现，于是我参照Java的 java.util.Observable，将java代码翻译为C++代码。自己实现了一个observable::observable，用法与Java的Observable类一样。
《JAVA设计模式之观察者模式》
https://www.cnblogs.com/porotin/p/7825656.html
实现代码
整个代码只有一个文件observable.hpp,java源码中的注释我也一并抄过来了。
/*
* observable.hpp
* learning from java.util.Observable,java.util.Observer
* Created on: 2021/09/15
* Author: guyadong
*/
#ifndef DTALK_OBSERVABLE_H_
#define DTALK_OBSERVABLE_H_
#include <memory>
#include <mutex>
#include <vector>
#include <algorithm>
namespace observable {
class observable;
/**
* A class can implement the <code>observer</code> interface when it
* wants to be informed of changes in observable objects.
* 对应 java.util.Observer 接口
*/
struct observer : public std::enable_shared_from_this<observer> {
/**
* This method is called whenever the observed object is changed. An
* application calls an <tt>Observable</tt> object's
* <code>notifyObservers</code> method to have all the object's
* observers notified of the change.
*
* @param o the observable object.
* @param arg an argument passed to the <code>notifyObservers</code>
* method.
*/
virtual void update(const observable &o, const std::shared_ptr<void>& arg) = 0;
};
using observer_ptr_type = std::shared_ptr<observer>;
/**
* This class represents an observable object, or "data"
* in the model-view paradigm. It can be subclassed to represent an
* object that the application wants to have observed.
* An observable object can have one or more observers. An observer
* may be any object that implements interface <tt>observer</tt>. After an
* observable instance changes, an application calling the
* <code>Observable</code>'s <code>notifyObservers</code> method
* causes all of its observers to be notified of the change by a call
* to their <code>update</code> method.
* <p>
* The order in which notifications will be delivered is unspecified.
* The default implementation provided in the Observable class will
* notify Observers in the order in which they registered interest, but
* subclasses may change this order, use no guaranteed order, deliver
* notifications on separate threads, or may guarantee that their
* subclass follows this order, as they choose.
* <p>
* Note that this notification mechanism is has nothing to do with threads
* and is completely separate from the <tt>wait</tt> and <tt>notify</tt>
* mechanism of class <tt>Object</tt>.
* <p>
* When an observable object is newly created, its set of observers is
* empty. Two observers are considered the same if and only if the
* <tt>equals</tt> method returns true for them.
*
*/
class observable : public std::enable_shared_from_this<observable> {
public:
observable() = default;
virtual ~observable() = default;
/** 复制构造函数 */
observable(const observable&o)
{
obs = o.obs;
changed = o.changed;
}
/** 赋值操作符 */
observable& operator=(const observable&o)
{
obs = o.obs;
changed = o.changed;
return *this;
}
/**
* Adds an observer to the set of observers for this object, provided
* that it is not the same as some observer already in the set.
* The order in which notifications will be delivered to multiple
* observers is not specified. See the class comment.
*
* @param o an observer to be added.
* @throws invalid_argument if the parameter o is null.
*/
void addObserver(const observer_ptr_type& o) {
std::lock_guard<std::mutex> guard(obs_mtx);
if(!o)
{
throw std::invalid_argument("o is null");
}
auto found = std::find_if(obs.begin(), obs.end(), [&](const observer_ptr_type&e) {
return o == e; });
if (found == obs.end())
{
obs.emplace_back(o);
}
}
/**
* Deletes an observer from the set of observers of this object.
* Passing <CODE>null</CODE> to this method will have no effect.
* @param o the observer to be deleted.
*/
void deleteObserver(const observer_ptr_type& o) {
if (o)
{
std::lock_guard<std::mutex> guard(obs_mtx);
auto found = std::find_if(obs.begin(), obs.end(), [&](const observer_ptr_type&e) {
return o == e; });
obs.erase(found);
}
}
/**
* If this object has changed, as indicated by the
* <code>hasChanged</code> method, then notify all of its observers
* and then call the <code>clearChanged</code> method to
* indicate that this object has no longer changed.
* <p>
* Each observer has its <code>update</code> method called with two
* arguments: this observable object and <code>null</code>. In other
* words, this method is equivalent to:
* <blockquote><tt>
* notifyObservers(null)</tt></blockquote>
*
*/
void notifyObservers() {
notifyObservers(nullptr);
}
/**
* If this object has changed, as indicated by the
* <code>hasChanged</code> method, then notify all of its observers
* and then call the <code>clearChanged</code> method to indicate
* that this object has no longer changed.
* <p>
* Each observer has its <code>update</code> method called with two
* arguments: this observable object and the <code>arg</code> argument.
*
* @param arg any object.
*/
void notifyObservers(const std::shared_ptr<void>& arg) {
/*
* a temporary array buffer, used as a snapshot of the state of
* current Observers.
*/
decltype(obs) arrLocal;
{
std::lock_guard<std::mutex> guard(obs_mtx);
if (!changed)
{
return;
}
arrLocal = obs;
clearChanged();
}
for (observer_ptr_type e : obs)
{
e->update(*this, arg);
}
}
/**
* Clears the observer list so that this object no longer has any observers.
*/
void deleteObservers() {
std::lock_guard<std::mutex> guard(obs_mtx);
obs.clear();
}
/**
* Tests if this object has changed.
*
* @return <code>true</code> if and only if the <code>setChanged</code>
* method has been called more recently than the
* <code>clearChanged</code> method on this object;
* <code>false</code> otherwise.
*/
bool hasChanged() const{
return changed;
}
/**
* Returns the number of observers of this <tt>Observable</tt> object.
*
* @return the number of observers of this object.
*/
int countObservers() const{
return (int)obs.size();
}
protected:
/**
* Marks this <tt>Observable</tt> object as having been changed; the
* <tt>hasChanged</tt> method will now return <tt>true</tt>.
*/
void setChanged() {
changed = true;
}
/**
* Indicates that this object has no longer changed, or that it has
* already notified all of its observers of its most recent change,
* so that the <tt>hasChanged</tt> method will now return <tt>false</tt>.
* This method is called automatically by the
* <code>notifyObservers</code> methods.
*
*/
void clearChanged() {
changed = false;
}
private:
std::vector<observer_ptr_type> obs;
/** std::mutex 作为 obs 字段的同步锁 */
std::mutex obs_mtx;
bool changed;
};
} /* namespace observable */
#endif /* DTALK_OBSERVABLE_H_ */
说明
因为 std::mutex删除了复制构造函数和赋值操作符,所以这里为了允许observable 可以使用复制构造和赋值操作，必须实现复制构造函数和赋值操作符。
C++中没有接口，observable::observer是个只定义纯虚函数的类，对应 java.util.Observer 接口。
调用示例
以下为代码片段，用于示例观察者模式如何使用
#include <observable.hpp>
/**
* OPTION 值改变观察者类(侦听器)
* 当值改变时通知侦听器
* 继承observable::observer实现update虚函数(接口方法)
*/
template<typename T>
struct ValueObserver : public observable::observer
{
using value_listener_type = std::function<void(const T&)>;
value_listener_type tmpl_listener;
ValueObserver(const value_listener_type& tmpl_listener) :tmpl_listener(tmpl_listener) {
}
virtual void update(const observable::observable &, const std::shared_ptr<void>& arg) final
{
if (arg)
{
auto opt = std::static_pointer_cast<BaseOption>(arg);
auto v = opt->getValue<T>();
if (v && tmpl_listener)
{
tmpl_listener(static_cast<const T&>(*v));
}
}
}
};
class BaseOption : public std::enable_shared_from_this<BaseOption >
{
//************************************
// 添加值改变侦听器
// @param const std::function<void(const T&)>& listener 侦听函数,为nullptr忽略
// @return dtalk::BaseOption& 当前对象
//************************************
template<typename T>
BaseOption& addListener(const std::function<void(const T&)>& listener)
{
if (listener) {
obs.addObserver(std::make_shared<ValueObserver<T>>(listener));
}
return *this;
}
dtalk::BaseOption& dtalk::BaseOption::setValue(const nlohmann::json & optionValue)
{
fields[__J_NAME(value)] = optionValue;
/** 当值被修改时调用observable::notifyObservers函数通知所有的侦听器 */
obs.notifyObservers(shared_from_this());
return *this;
}
private:
/** 保存所有需要序列化的字段 */
nlohmann::json fields;
/** observable 实例 */
observable::observable obs;
}