Memento

Michelspacher s title-page is not, in fact, a monument to empirical learning, but takes the form of a memento mori. Far from applauding the new Renaissance humanism, on the contrary, he employs the science of anatomy as a metaphor to demonstrate the futility of rational knowledge in the face of death and decay.The prospect of immortality is available only to those who seek an intuitive Gnosis, a divine epiphany concerning God s work in Nature. 

The above design using The State Pattern addresses a simple one-way traffic scenario of the flow—the process can only go forward, not backward. In reality, the chemist may want to undo and roll back a step either because of human mistakes or the system did something that is against the chemist s intention. Undo is a common functionality in many productivity software systems, such as Microsoft Word. Fortunately, there is another GoF design pattern called Memento, which provides an elegant solution. 

To perform undo, the system has to remember the states of the objects along the way in the process. Memento is the object that stores previous states on behalf of the subject object and allows the subject object to recover its previous states when necessary. 

The Memento Pattern Without violating encapsulation, capture and externalize an object s state so that the object can be restored to this state later. 

Suppose the compound state transition only involves structure changes and undo only needs to recover the structure and the compound state. Figure 12.6 is the class diagram of the Memento Pattern for the Compound object. 

Figure 12.6 The class diagram of the Memento Pattern for the Compound object.

Sometimes it is necessary to perform undo multiple steps. In this case, create a Caretaker class that holds a stack of Memento objects. If redo is desired, then have two stacks in the Caretaker, one for undo and one for redo, and the Memento object that is popped from the undo stack gets pushed into the redo stack and vice versa. The code example is as follows ... 

Figure 12.7 is a sequence diagram that shows undo and redo using Memento Pattern. 

Cleaning out your work area can be a melancholy experience. You can reduce the impact by starting a few weeks before your last day. Begin by removing a few nonessentials such as books, pictures, and personal mementos. As time permits, clean out your files. This helps you disengage from your work and prevents the air of finality that comes with a one-day massive move out. 

Dusha s flame Who would not scour the centuries for some memento of her light Any man who had been her lover, he speculated, might well be driven mad with the desire for just one more caress of her hand. 

He promises to give away his expensive elixir to anyone who will give him a memento, a handkerchief. 

A few seconds later, Miss Muxdroozol opens her door. Bob, hi, what s up She has a cup of coffee in one hand and a blow dryer in the other. Currently, her hair is almost shoulder length, fashionably cut, with wonderful crimson streaks in it. A few strands of hair wriggle as if they have a life of their own, more like liquid solder than anything biological. Her cheekbones are high and pronounced, her mouth tenderly curved, her eyes skillfully shadowed. The only imperfection she has (though Bob thinks it only adds to her allure) is a F-shaped scar near her left eyebrow—a memento of a childhood accident. She seems self-conscious about it. 

The reclamation of the industrial area will prepare the Bagnoli area for the building of an urban park (included in the urban development plan for the city s eastern sector), which will represent a tangible sign of the environmental recovery of the area. The park will also preserve some structures as a memento of the industrial history of the area. 

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