OC基础(12)

2022-09-24 20:07:10

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new方法实现原理

本小节知识:

  1. 【掌握】new方法实现原理

1.new方法实现原理

  • 完整的创建一个可用的对象:Person *p=[Person new];
  • new方法的内部会分别调用两个方法来完成3件事情:
    • (1)使用alloc方法来分配存储空间(返回分配的对象);
    • (2)使用init方法来对对象进行初始化。
    • (3)返回对象的首地址
This method is a combination of alloc and init. Like alloc, it initializes the isa instance variable of the new object so it points to the class data structure. It then invokes the init method to complete the initialization process.
  • 可以把new方法拆开如下:
    • (1)调用类方法+alloc分配存储空间,返回未经初始化的对象Person *p1=[person alloc];
    • (2)调用对象方法-init进行初始化,返回对象本身 Person *p2=[p1 init]; + (3)以上两个过程整合为一句:Person *p=[[Person alloc] init];

  • 说明：

    • alloc 与 init合起来称为构造方法，表示构造一个对象
    • alloc 方法为对象分配存储空间，并将所分配这一块区域全部清0. objc The isa instance variable of the new instance is initialized to a data structure that describes the class; memory for all other instance variables is set to 0.
    • init方法是初始化方法（构造方法），用来对象成员变量进行初始化，默认实现是一个空方法。 An object isn’t ready to be used until it has been initialized. The init method defined in the NSObject class does no initialization; it simply returns self.

  • 所以下面两句的作用是等价的 objc Person *p1 = [Person new]; Person *p = [[Person alloc] init];

  • iOS 程序通常使用[[类名 alloc] init] 的方式创建对象，因为这个可以与其他initWithXX:...的初始化方法，统一来。代码更加统一

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padding-left: 30px;
margin-left: -30px;
cursor: pointer;
position: absolute;
top: 0;
left: 0;
bottom: 0; }

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padding: 0;
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font-size: 14px; }

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ul, ol {
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dl {
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dl dt > :last-child {
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dl dd {
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dl dd > :last-child {
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blockquote {
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color: #777777; }
blockquote > :first-child {
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blockquote > :last-child {
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table {
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table tr {
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background-color: white;
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table tr:nth-child(2n) {
background-color: #f8f8f8; }
table tr th {
font-weight: bold;
border: 1px solid #cccccc;
margin: 0;
padding: 6px 13px; }
table tr td {
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margin-top: 0; }
table tr th :last-child, table tr td :last-child {
margin-bottom: 0; }

img {
max-width: 100%; }

span.frame {
display: block;
overflow: hidden; }
span.frame > span {
border: 1px solid #dddddd;
display: block;
float: left;
overflow: hidden;
margin: 13px 0 0;
padding: 7px;
width: auto; }
span.frame span img {
display: block;
float: left; }
span.frame span span {
clear: both;
color: #333333;
display: block;
padding: 5px 0 0; }
span.align-center {
display: block;
overflow: hidden;
clear: both; }
span.align-center > span {
display: block;
overflow: hidden;
margin: 13px auto 0;
text-align: center; }
span.align-center span img {
margin: 0 auto;
text-align: center; }
span.align-right {
display: block;
overflow: hidden;
clear: both; }
span.align-right > span {
display: block;
overflow: hidden;
margin: 13px 0 0;
text-align: right; }
span.align-right span img {
margin: 0;
text-align: right; }
span.float-left {
display: block;
margin-right: 13px;
overflow: hidden;
float: left; }
span.float-left span {
margin: 13px 0 0; }
span.float-right {
display: block;
margin-left: 13px;
overflow: hidden;
float: right; }
span.float-right > span {
display: block;
overflow: hidden;
margin: 13px auto 0;
text-align: right; }

code, tt {
margin: 0 2px;
padding: 0 5px;
white-space: nowrap;
border: 1px solid #eaeaea;
background-color: #f8f8f8;
border-radius: 3px; }

pre code {
margin: 0;
padding: 0;
white-space: pre;
border: none;
background: transparent; }

.highlight pre {
background-color: #f8f8f8;
border: 1px solid #cccccc;
font-size: 13px;
line-height: 19px;
overflow: auto;
padding: 6px 10px;
border-radius: 3px; }

pre {
background-color: #f8f8f8;
border: 1px solid #cccccc;
font-size: 13px;
line-height: 19px;
overflow: auto;
padding: 6px 10px;
border-radius: 3px; }
pre code, pre tt {
background-color: transparent;
border: none; }

sup {
font-size: 0.83em;
vertical-align: super;
line-height: 0;
}
* {
-webkit-print-color-adjust: exact;
}
@media screen and (min-width: 914px) {
body {
width: 854px;
margin:0 auto;
}
}
@media print {
table, pre {
page-break-inside: avoid;
}
pre {
word-wrap: break-word;
}
}
-->

类的本质

本小节知识:

  1. 【了解】类的本质
  2. 【掌握】如何获取类对象
  3. 【理解】类对象的用法
  4. 【理解】类对象的存储
  5. 【了解】OC实例对象类对象元数据之间关系

1.类的本质

  • 类的本质其实也是一个对象(类对象)
  • 程序中第一次使用该类的时候被创建，在整个程序中只有一份。
  • 此后每次使用都是这个类对象，它在程序运行时一直存在。
  • 类对象是一种数据结构,存储类的基本信息:类大小,类名称,类的版本，继承层次，以及消息与函数的映射表等
  • 类对象代表类,Class类型,对象方法属于类对象
  • 如果消息的接收者是类名,则类名代表类对象
  • 所有类的实例都由类对象生成,类对象会把实例的isa的值修改成自己的地址,每个实例的isa都指向该实例的类对象

2.如何获取类对象

  • 通过实例对象
格式：[实例对象   class ];

如：   [dog class];
  • 通过类名获取(类名其实就是类对象)
格式：[类名 class];

如：[Dog class]

3.类对象的用法

  • 用来调用类方法
[Dog test];

Class c = [Dog class];

[c test];
  • 用来创建实例对象
Dog *g = [Dog new];

Class c = [Dog class];

Dog *g1 = [c new];

4.类对象的存储

OC基础(12)

5.OC实例对象 类对象 元对象之间关系

  • Objective-C是一门面向对象的编程语言。

    • 每一个对象 都是一个类的实例。
    • 每一个对象 都有一个名为isa的指针,指向该对象的类。
    • 每一个类
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