Snapshotting a view that has not been rendered results in an empty snapshot. …

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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，在百度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

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@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：

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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：

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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！！

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if ([[[UIDevice currentDevice] systemVersion] floatValue]>=8.0) {
    self.modalPresentationStyle=UIModalPresentationOverCurrentContext;
}

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