Objective - c Chapter 1 -2 Hello world

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Objective - c   Chapter 1  Hello world    

1.1

1.2.On the Welcome screen, click "Create a new Xcode project" (see Figure 2-1), or just choose
File ->New ->New Project.

1.3.在main里写如下代码

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {

    @autoreleasepool {

        // insert code here...

        NSLog(@"Hello, World!");

    }

    return 0;

}

Build and run the program by clicking the Run button or pressing ?R.

Open the Xcode console window (by selecting View Debug Area Activate Console or pressing ??C), which displays your program‘s output,

Now Let‘s pull it apart and see how it works .

1.4.1

#import

#import <Foundation/Foundation.h>

#import guarantees that a header file will be included only once, no matter how many times the #import directive is actually seen for that file.

#import 保证头文件只能被包含一次，不论#import 引入多少次这个文件。

1.4.2 

Introducing Frameworks

A framework is a collection of parts—header files, libraries, images, sounds, and more—collected together into a single unit.

一个框架包含了头文件，库，图片，声音甚至更多到一个单元中。

The header files for the Foundation framework take up nearly a megabyte of disk storage and contain more than 14,000 lines of code, spread across over a hundred files. When you include the master header file with #import <Foundation/Foundation.h>, you get that whole vast collection.

Xcode is smart: it speeds up the task by using precompiled headers, a compressed and digested form of the header that‘s loaded quickly when you #import it.

Xcode 很聪明： 它使用预编译头文件，压缩

1.4.3 

NSLog and @"Strings " 

NSLog (@"Hello, Objective-C!");

The NS Prefix: A Prescription Against Name Collisions

NS前缀： 预防名字冲突。

Rather than break compatibility with code already written for NextSTEP, Apple just continued to use the "NS" prefix. It‘s a historical curiosity now, like your appendix.

历史原因，原来NestStep的缩写NS。所以一直这么简写了。

1.4.4

NSString: Where it‘s @

A string in double quotes preceded by an at sign means that the quoted string should be treated as a Cocoa NSString element.

一个在双引号前加at 符号@ 意思是说被引用的字符串应该视为cocoa NSSting 元素。

an NSString is a sequence of characters in Cocoa.

一个NSstring是一些列在cocoa中得字符。

NSString elements have a huge number of features packed into them and are used by Cocoa any time a string is needed. Here are just a few of the things an NSString can do:

   Tell you its length
   Compare itself to another string
   Convert itself to an integer or floating-point value

You can tell Xcode to always treat warnings as  errors . 

设置xcode使得对待警告如同错误。

1.5 Are you the  boolean type ?

Many languages have a Boolean type, which is, of course, a fancy term for variables that store true and false values. Objective-C is no exception.

oc 也有Boolean 类型。

类型是Bool 。

Objective - c Chapter 1 -2 Hello world

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原文地址：http://www.cnblogs.com/ljlkfx/p/4477711.html