/** 获取屏幕宽 */
#define kScreenWidth \
([[UIScreen mainScreen] respondsToSelector:@selector(nativeBounds)] ? [UIScreen mainScreen].nativeBounds.size.width/[UIScreen mainScreen].nativeScale : [UIScreen mainScreen].bounds.size.width)
/** 获取屏幕高 */
#define kScreenHeight \
([[UIScreen mainScreen] respondsToSelector:@selector(nativeBounds)] ? [UIScreen mainScreen].nativeBounds.size.height/[UIScreen mainScreen].nativeScale : [UIScreen mainScreen].bounds.size.height)
/** iPhone5 坐标适配 */
#define ScreenZoomScaleFive kScreenWidth/320.0f // iPhone5
#define ScreenFive(value) value *ScreenZoomScaleFive
#define LColorFromRGB(rgbValue) [UIColor \
colorWithRed:((float)((rgbValue & 0xFF0000) >> 16))/255.0 \
green:((float)((rgbValue & 0xFF00) >> 8))/255.0 \
blue:((float)(rgbValue & 0xFF))/255.0 alpha:1.0]
6.缩放因子（scale factor between logic point and device pixel）
（1）Scale起源
早期的iPhone3GS的屏幕分辨率是320*480（PPI=163），iOS绘制图形（CGPoint/CGSize/CGRect）均以point为单位（measured in points）：
1 point = 1 pixel（Point Per Inch=Pixel Per Inch=PPI）
后来在iPhone4中，同样大小（3.5 inch）的屏幕采用了Retina显示技术，横、纵向方向像素密度都被放大到2倍，像素分辨率提高到(320x2)x(480x2)= 960x640（PPI=326）， 显像分辨率提升至iPhone3GS的4倍（1个Point被渲染成1个2x2的像素矩阵）。
但是对于开发者来说，iOS绘制图形的API依然沿袭point(pt，注意区分印刷行业的“磅”)为单位。在同样的逻辑坐标系下（320x480）：
1 point = scale*pixel（在iPhone4~6中，缩放因子scale=2；在iPhone6+中，缩放因子scale=3）。
可以理解为：
scale=绝对长度比（point/pixel）=单位长度内的数量比（pixel/point）
（2）UIScreen.scale
UIScreen.h中定义了该属性：
// The natural scale factor associated with the screen.(read-only)
@property(nonatomic,readonly) CGFloat scale NS_AVAILABLE_IOS(4_0);
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This value reflects the scale factor needed to convert from the default logical coordinate space into the device coordinate space of this screen.
The default logical coordinate space is measured using points. For standard-resolution displays, the scale factor is 1.0 and one point equals one pixel. For Retina displays, the scale factor is 2.0 and one point is represented by four pixels.
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为了自动适应分辨率，系统会根据设备实际分辨率，自动给UIScreen.scale赋值，该属性对开发者只读。
（3）UIScreen.nativeScale
iOS8新增了nativeScale属性：
// Native scale factor of the physical screen
@property(nonatomic,readonly) CGFloat nativeScale NS_AVAILABLE_IOS(8_0);
以下是iPhone6+下的输出，初步看来nativeScale与scale没有太大区别：
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(lldb)p (CGFloat)[[UIScreen mainScreen] scale]
(CGFloat) $1 = 3
(lldb) p(CGFloat)[[UIScreen mainScreen] nativeScale]
(CGFloat) $2 = 3
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（4）机型判别
在同样的逻辑分辨率下，可以通过scale参数识别是iPhone3GS还是iPhone4(s)。以下基于nativeScale参数，定义了探测机型是否为iPhone6+的宏：
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// not UIUserInterfaceIdiomPad
#define IS_IPHONE (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone)
// detect iPhone6 Plus based on its native scale
#define IS_IPHONE_6PLUS (IS_IPHONE && [[UIScreenmainScreen] nativeScale] == 3.0f)
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那么，同样的分辨率和scale，如何区分机型iPhone4与4s、iPhone5与5s呢？通过[[UIDevice currentDevice] model]只能判别iPhone、iPad、iPod大类，要判断iPhone具体机型型号，则需要通过sysctlbyname("hw.machine")获取详细的设备参数信息予以甄别。
