AVFoundation相机预览层不工作

所以，我试图使用AVFoundation实现一个摄像头。我想我做的一切正确。这是我在做的

So, I am trying to implement a camera using AVFoundation. I think I do everything right. this is what i am doing

1. 创建会话


2. 获取视频类型的设备


3. 通过设备循环播放照相机


4. 使用＃3中提及的设备获取设备输入并将其添加到会话


5. 创建 AVCaptureStillImageOutput


6. 类型的输出设置输出设置并将其添加到会话


7. 从我的视图2获取CALayer（将在下面解释我的意思2）


8. 创建 AVCaptureVideoPreviewLayer


9. 将它添加到＃7中提到的图层


10. 开始运行会话

1. create session
2. get devices of video type
3. loop through devices to get the camera at the back
4. get a device input using the device mentioned in #3 and add it to the session
5. create an output of type AVCaptureStillImageOutput
6. set output settings and add it to the session
7. get a CALayer from my view 2(will explain below what I mean by view 2)
8. create an instance of AVCaptureVideoPreviewLayer
9. add it to the layer mentioned in #7
10. start running the session

所以我有2个视图一个在另一个。上面的是View 1，下面的是View 2. View 1应该提供自定义的摄像头控件。

So I have 2 views one over the other. The one on top is View 1 and the one below is view 2. View 1 is supposed to provide with custom camera controls.

这里是代码：

self.session = [[AVCaptureSession alloc]init];
[self.session setSessionPreset:AVCaptureSessionPresetHigh];
NSArray *devices = [[NSArray alloc]init];
devices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
for (AVCaptureDevice *device in devices){
    if([device position] == AVCaptureDevicePositionBack){
        self.device = device;
        break;
    }
}
NSError *error;
self.input = [[AVCaptureDeviceInput alloc]initWithDevice:self.device error:&error];
if([self.session canAddInput:self.input]){
    [self.session addInput:self.input];    
}


self.stillImageOutput = [[AVCaptureStillImageOutput alloc]init];
NSDictionary *outputSettings = @{AVVideoCodecKey : AVVideoCodecJPEG};
[self.stillImageOutput setOutputSettings:outputSettings];

[self.session addOutput:self.stillImageOutput];

CALayer *cameraLayer = self.cameraView.layer;
self.cameraView.backgroundColor = [UIColor clearColor];

AVCaptureVideoPreviewLayer *preview = [[AVCaptureVideoPreviewLayer alloc]initWithSession:self.session];
[cameraLayer addSublayer:preview];

[self.session startRunning];

我得到的是View 1（它有一个.png图像作为背景，洞，使其下的视图，视图2可以看到），视图2是可见的，但我看不到我应该做的。因为我改变了视图2的背景颜色以清除颜色，我看到所有的黑色。

What I get is View 1(it has a .png image as its background. the image has a hole so that the view under it, view 2 can be visible) and view 2 is visible but I dont see what I am supposed to. Because I changed the background color for view 2 to clear color I see all black. I am supposed to see what the camera sees.