当找不到PODS的头文件时，检查import是否用的是
#import <>
引入的，用户本地文件夹内的头文件引入，通常用双引号
#import ""
路径为
target - building settings - User Header Search Paths - $ (SRCROOT)
但是如果引入的是本地的framework，需要在
target - build settings - framework search path
里面设置framework的路径，或者直接用$(srcroot) RECURSIVE 让framework在本地文件夹里面找
linux 命令中，- (single dash)是必选参数 --（double dash）是可选参数，命令分为系统自带命令（函数）和 安装的三方app(gem git ..)
- 什么是RPC
remote procedure call, call the remote service,is a protocol that one program can use to request a service from a program located in another computer on a network without having to understand the network's details.
= one program can use RPC to request a remote service without having to understand the network's details,RPC is a protocol.
application.delegate.window 和 application.keyWindow有什么区别
application.delegate.window -> used to present the app's visual content on the device's main screen
application.keyWindow -> the window be most recently send makeKeyAndVisiable message
总结：最好把子视图加到delegate.window上，因为如果APP后台前台切换的时候，keywindow会更改对象，就不会再是我们通常所认为的那个看得见的window了
什么是affine transformation
affine transformation is a function between affine spaces which preserves points,straight lines and planes.Examples include translation,scaling,reflection,rotation and compositions of them in any combination and sequence
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