C++ 命令模式讲解和代码示例
命令是一种行为设计模式， 它可将请求或简单操作转换为一个对象。
此类转换让你能够延迟进行或远程执行请求， 还可将其放入队列中。
复杂度：
流行度：
使用示例： 命令模式在 C++ 代码中很常见。 大部分情况下， 它被用于代替包含行为的参数化 UI 元素的回调函数， 此外还被用于对任务进行排序和记录操作历史记录等。
识别方法： 命令模式可以通过抽象或接口类型 （发送者） 中的行为方法来识别， 该类型调用另一个不同的抽象或接口类型 （接收者） 实现中的方法， 该实现则是在创建时由命令模式的实现封装。 命令类通常仅限于一些特殊行为。
概念示例
本例说明了命令设计模式的结构并重点回答了下面的问题：
- 它由哪些类组成？
- 这些类扮演了哪些角色？
- 模式中的各个元素会以何种方式相互关联？
main.cc: 概念示例
/**
* The Command interface declares a method for executing a command.
*/
class Command {
public:
virtual ~Command() {
}
virtual void Execute() const = 0;
};
/**
* Some commands can implement simple operations on their own.
*/
class SimpleCommand : public Command {
private:
std::string pay_load_;
public:
explicit SimpleCommand(std::string pay_load) : pay_load_(pay_load) {
}
void Execute() const override {
std::cout << "SimpleCommand: See, I can do simple things like printing (" << this->pay_load_ << ")\n";
}
};
/**
* The Receiver classes contain some important business logic. They know how to
* perform all kinds of operations, associated with carrying out a request. In
* fact, any class may serve as a Receiver.
*/
class Receiver {
public:
void DoSomething(const std::string &a) {
std::cout << "Receiver: Working on (" << a << ".)\n";
}
void DoSomethingElse(const std::string &b) {
std::cout << "Receiver: Also working on (" << b << ".)\n";
}
};
/**
* However, some commands can delegate more complex operations to other objects,
* called "receivers."
*/
class ComplexCommand : public Command {
/**
* @var Receiver
*/
private:
Receiver *receiver_;
/**
* Context data, required for launching the receiver's methods.
*/
std::string a_;
std::string b_;
/**
* Complex commands can accept one or several receiver objects along with any
* context data via the constructor.
*/
public:
ComplexCommand(Receiver *receiver, std::string a, std::string b) : receiver_(receiver), a_(a), b_(b) {
}
/**
* Commands can delegate to any methods of a receiver.
*/
void Execute() const override {
std::cout << "ComplexCommand: Complex stuff should be done by a receiver object.\n";
this->receiver_->DoSomething(this->a_);
this->receiver_->DoSomethingElse(this->b_);
}
};
/**
* The Invoker is associated with one or several commands. It sends a request to
* the command.
*/
class Invoker {
/**
* @var Command
*/
private:
Command *on_start_;
/**
* @var Command
*/
Command *on_finish_;
/**
* Initialize commands.
*/
public:
~Invoker() {
delete on_start_;
delete on_finish_;
}
void SetOnStart(Command *command) {
this->on_start_ = command;
}
void SetOnFinish(Command *command) {
this->on_finish_ = command;
}
/**
* The Invoker does not depend on concrete command or receiver classes. The
* Invoker passes a request to a receiver indirectly, by executing a command.
*/
void DoSomethingImportant() {
std::cout << "Invoker: Does anybody want something done before I begin?\n";
if (this->on_start_) {
this->on_start_->Execute();
}
std::cout << "Invoker: ...doing something really important...\n";
std::cout << "Invoker: Does anybody want something done after I finish?\n";
if (this->on_finish_) {
this->on_finish_->Execute();
}
}
};
/**
* The client code can parameterize an invoker with any commands.
*/
int main() {
Invoker *invoker = new Invoker;
invoker->SetOnStart(new SimpleCommand("Say Hi!"));
Receiver *receiver = new Receiver;
invoker->SetOnFinish(new ComplexCommand(receiver, "Send email", "Save report"));
invoker->DoSomethingImportant();
delete invoker;
delete receiver;
return 0;
}
Output.txt: 执行结果
Invoker: Does anybody want something done before I begin?
SimpleCommand: See, I can do simple things like printing (Say Hi!)
Invoker: ...doing something really important...
Invoker: Does anybody want something done after I finish?
ComplexCommand: Complex stuff should be done by a receiver object.
Receiver: Working on (Send email.)
Receiver: Also working on (Save report.)
