Command mode belongs to the behavior mode of an object. Command mode encapsulates a request or operation into an object, and separates the responsibility of issuing the command from the responsibility of executing the command through abstraction of the command. The implementation of command mode can provide the functions of revocation and recovery of commands. The specific structure diagram is shown below.
2000年夏季的某个清晨，一个年轻的单身女人赤裸地暴死在自己的床上。是情杀？奸杀？还是仇杀？ 西峰市刑警大队的干警们就此展开了一系列的调查。可是，当刑警们展开侦破时却发现整个案情扑朔迷离，死者王玲生前生活在纸醉金迷之中，并拥有众多的情人；除了死者的通讯录、手机等物品蹊跷的丢失外，现场并未发现凶手的蛛丝马迹。案件的调查漫无头绪，真凶是谁？是鬼鬼祟祟的男邻居，是经常偷窥她的强奸前科犯；还是她的情…
《邪恶力量》（Supernatural）由美国WB电视台制作，讲述灵异超自然现象大多来自于美国的都市传说和民间传说。
朱珠和王旺一家住在西北地区的沙漠边上，为了治沙，朱珠和王旺一家人开始植树。他们冒风沙、抗烈日，肩扛人拉运树苗，一桶一桶挑水灌溉，每天往返数十里。持之以恒数十年，他们的辛劳初见成效。然而王旺不幸患了骨癌，不久就去世了。朱珠面对如此打击并没有屈服，她继续带领一家老小植树治沙，凭着不屈不挠的精神，将万亩不毛之地变成了绿色的海洋。
虽说深秋季节，道路两旁的树叶凋零、衰草枯黄，但那怒放的金黄野菊，却给萧瑟深秋染上了一抹明艳。
Since the single mode is instantiated only once, the first call returns an instance object of a. When we continue to call, the instance of b is the instance of a, so aa is printed below.
Load Module by Path
秦国刺客来袭，已经被侍卫击杀，惊扰各位休息了。
李敬武忙道：两边都传。
于是，张槐只在家住了一晚，又匆匆地赶去湖州府城了。
One of them taught me to be a gentle woman.
自从被称作《灼热的万圣节》的那一天开始， 世界因新的战略级魔法师登场而震撼。
只有局中人，才知道这究竟有多难。
保护伞公司在海底建造了逼真的模拟城市，并使用克隆人作为T病毒的实验对象，为了抵达出口，爱丽丝一行人必须穿越东京、纽约、华盛顿和莫斯科四座城市，而每座城市中，都有属于它的丧尸们在蠢蠢欲动。
十年前的一场残酷的细菌侵袭灭绝了斯堪的纳维亚的绝大多数人口，一对年轻兄妹开始了一段寻觅安全处所的旅途。兄妹二人唯一的向导是父亲的日记本，里面记录了细菌与新世界的危险。他们加入其他年轻幸存者的行列，开始了险象环生的跨境之行。面对致命灾难，这帮人必须迅速意识到，最大的危险并非来自外界，而是来自内部。
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Observer mode: pay attention to encapsulating object notification, support the change of communication object, realize the change of object state, notify and update the objects that depend on it.
Know the principle man: Very good, your experience value has improved a lot. However, we can't start fighting monsters yet. After all, no monster can be directly sprayed to death. You lack tools. Then at this stage, we need to strengthen our programming ability. First find a framework to come down, read the source code, what? You said you can't read the source code, it doesn't matter, there are a lot of experience reading the source code online. Of course, the basis of these experiences is without exception: read more and write more. On this basis, find trick again. After the source code of the framework is set down, change some codes to run, debug them, and then constantly find out the reason to see how each API is written, and try to write it yourself. Write more and practice more. After 30 years of hard coding, you will definitely get something.
