错误代码：Snapshotting a view that has not been rendered results in an empty snapshot. Ensure your view has been rendered at least once before snapshotting or snapshot after screen updates.

问题分析：iOS8在调用系统相机拍照时，会有一两秒的停顿，然后再弹出UIImagePickConroller，IOS7是没有这个问题的，在找了无数遍都没能解决这个问题，有说要将imagePickController设置为全局变量，有说要延时0.5秒再presentViewController的，各显神通，但很遗憾的都没能解决这个问题，今天特意单独写个Demo来研究此问题，终于取得了突破性的进展！

其实根本原因不在于系统拍照控制器上面，而是执行presentViewController这个动作本身！我们可以查看下UIViewController这个类，他有一个属性

@property(nonatomic,assign) UIModalPresentationStyle modalPresentationStyle NS_AVAILABLE_IOS(3_2);

这是一个枚举值，在iOS7的SDK中，定义如下：

typedef 

NS_ENUM

(

NSInteger

, UIModalPresentationStyle) {

    

UIModalPresentationFullScreen = 0,

#if __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_3_2

    

UIModalPresentationPageSheet,

    

UIModalPresentationFormSheet,

    

UIModalPresentationCurrentContext,

#endif

#if __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_7_0

    

UIModalPresentationCustom,

    

UIModalPresentationNone = -1,       

#endif       

};

 

 

在iOS8的SDK中定义如下：

typedef 

NS_ENUM

(

NSInteger

, UIModalPresentationStyle) {

        

UIModalPresentationFullScreen = 0,

        

UIModalPresentationPageSheet 

NS_ENUM_AVAILABLE_IOS

(3_2),

        

UIModalPresentationFormSheet 

NS_ENUM_AVAILABLE_IOS

(3_2),

        

UIModalPresentationCurrentContext 

NS_ENUM_AVAILABLE_IOS

(3_2),

        

UIModalPresentationCustom 

NS_ENUM_AVAILABLE_IOS

(7_0),

        

UIModalPresentationOverFullScreen 

NS_ENUM_AVAILABLE_IOS

(8_0),

        

UIModalPresentationOverCurrentContext 

NS_ENUM_AVAILABLE_IOS

(8_0),

        

UIModalPresentationPopover 

NS_ENUM_AVAILABLE_IOS

(8_0),

        

UIModalPresentationNone 

NS_ENUM_AVAILABLE_IOS

(7_0) = -1, 

解决问题的关键部分来了，IOS8多了一个样式UIModalPresentationOverCurrentContext，IOS8中presentViewController时请将控制器的modalPresentationStyle设置为UIModalPresentationOverCurrentContext，问题解决！！

if 

([[[UIDevice currentDevice] systemVersion] floatValue]>=8.0) {

    

self

.modalPresentationStyle=UIModalPresentationOverCurrentContext;

}