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关键词layoutSubviews, layoutIfNeeded, setNeedsLayout
layoutSubviews官方描述
The default implementation of this method does nothing on iOS 5.1 and earlier. Otherwise, the default implementation uses any constraints you have set to determine the size and pos
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经典进程的同步问题
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本文摘自汤小丹主编《计算机操作系统》（第三版）2.4 经典进程的同步问
在多道程序环境下,进程同步问题十分重要,也是相当有趣的问题,因而吸引了不少 学者对它进行研究,由此而产生了一系列经典的进程同步问题,其中较有代表性的是“生 产者—消费者”问题、“读者—写者问题”、“哲学家进餐问题”等等。通过对这些问题的研 究和学习,可以帮助我们更好地理解进程同步的概念及实现方法。
生产者—消费者问题
前面我们已经对生产者—消费者问题(The proceducer-consumer problem)做了
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本文摘自汤小丹主编《计算机操作系统》（第三版）2.3 进程同步
在 OS 中引入进程后,虽然提高了资源的利用率和系统的吞吐量,但由于进程的异步性, 也会给系统造成混乱,尤其是在他们争用临界资源时。例如,当多个进程去争用一台打印 机时,有可能使多个进程的输出结果交织在一起,难于区分;而当多个进程去争用共享变 量、表格、链表时,有可能致使数据处理出错。进程同步的主要任务是对多个相关进程在 执行次序上进行协调,以使并发执行的诸进程之间能有效地共享资源和相互合作,从而使 程序的执行具有可再现性。
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关键词三级模式，两级映象
What模式
引入在数据模型中有型(Type)和值(Value)的概念。型就是某一类数据结构和属性的说明，值就是具体的赋值。
模式模式(Schema)是数据库中全体数据的逻辑结构和特征描述，是数据库的型。
实例模式的一个具体值称为模式的一个实例(Instance)，同一个模式可以有多个实例。
模式与实例的关系模式是相对稳定的，而实例是不断变化的；模式反映的是数据的结构及其联系，而实例反映的是数据库某一刻的状态。
三级模式结构
数据库系统由外模式、模式、
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0. 关键词数据模型，概念模型，逻辑模型，实体，属性，E-R图
1. What 数据模型是什么
数据模型是对现实世界数据特征的抽象，是对现实世界的模拟。
数据模型要满足三方面的要求：
能够真实的模拟现实世界
要便于人的理解
要便于在计算机上实现
1.2 数据模型的分类
概念模型
逻辑模型
物理模型（不讨论）
1.2.1 概念模型
概念模型是站在用户的角度对数据进行建模，主要用于数据库设计。
1) 实体（Entity）客观存在并相互区别的事物称为实体，实体可以是具体的事物也可以是抽象的概
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