_plot = $APlot;    }    public function Use()    {        if (!$this->_plot instanceof APlot){            echo '未定策略，或无效策略'.PHP_EOL;            return;        }        $this->_plot->Use();    }}$context = new ContextPlot();$context->SetPlot(new PlotA());$context->Use(); // 触发 PlotA 策略$context->SetPlot(new PlotB());$context->Use(); // 触发 PlotB 策略$context_two = new ContextPlot();$context_two->Use(); // 未定策略，或无效策略