AVCaptureConnection.previewLayer 在委托 AVCaptureMetadataOutputObjectsDelegate 中为 nil
AVCaptureConnection.previewLayer is nil in delegate AVCaptureMetadataOutputObjectsDelegate
下面的代码可以 运行 在 iPhone 上，当针对任何 QR 码时，应该打印内容。下面二维码指向example.com.
问题是代理应该提供 connection: AVCaptureConnection 而它确实提供了，但是它的 previewLayer 属性 是 nil.
可以将以下代码粘贴到一个新的空 Xcode 项目中。如果您禁用（注释掉）第 57 行，并启用第 56 行，它工作正常。但我想将委托放在 CaptureView class 之外。我如何设置捕获使得 AVCaptureMetadataOutputObjectsDelegate 它的 previewLayer 属性 不是 nil?
import UIKit
import AVFoundation
func printMetadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], previewLayer: AVCaptureVideoPreviewLayer) {
for object in metadataObjects {
let visualCodeObject = previewLayer.transformedMetadataObject(for: object)
guard let object = visualCodeObject, let barcode = object as? AVMetadataMachineReadableCodeObject else {
NSLog("Ignoring object that is not AVMetadataMachineReadableCodeObject")
continue
}
guard let barcodeString = barcode.stringValue else {
NSLog("Captured something that's not a string")
continue
}
NSLog("Captured string %@", barcodeString)
}
}
class CaptureView: UIView, AVCaptureMetadataOutputObjectsDelegate {
private let previewLayer = AVCaptureVideoPreviewLayer()
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
printMetadataOutput(output, didOutput: metadataObjects, previewLayer: self.previewLayer)
}
override func layoutSubviews() {
super.layoutSubviews()
self.previewLayer.frame = self.frame
}
init(frame: CGRect, delegate: AVCaptureMetadataOutputObjectsDelegate) {
guard let captureDevice = AVCaptureDevice.default(for: .video) else {
fatalError("Couldn't find default capture device")
}
guard let captureDeviceInput = try? AVCaptureDeviceInput(device: captureDevice) else {
super.init(frame: frame)
return
}
let captureSession = AVCaptureSession()
captureSession.addInput(captureDeviceInput)
self.previewLayer.session = captureSession
self.previewLayer.videoGravity = .resizeAspectFill
super.init(frame: frame)
self.layer.addSublayer(self.previewLayer)
self.previewLayer.frame = self.frame
captureSession.startRunning()
let metadataOutput = AVCaptureMetadataOutput()
// metadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
metadataOutput.setMetadataObjectsDelegate(delegate, queue: DispatchQueue.main)
metadataOutput.rectOfInterest = CGRect(x: 0, y: 0, width: 1, height: 1)
if captureSession.canAddOutput(metadataOutput) {
captureSession.addOutput(metadataOutput)
} else {
fatalError("Can't add metadata output to capture session")
}
metadataOutput.metadataObjectTypes = [.qr]
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
class MetadataDelegate: NSObject, AVCaptureMetadataOutputObjectsDelegate {
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
guard let previewLayer = connection.videoPreviewLayer else {
print("previewLayer was nil")
return
}
printMetadataOutput(output, didOutput: metadataObjects, previewLayer: previewLayer)
}
}
class ViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate {
private let metadataDelegate = MetadataDelegate()
override func viewDidLoad() {
let captureView = CaptureView(frame: CGRect(), delegate: self.metadataDelegate)
captureView.frame = self.view.frame
captureView.autoresizingMask = [.flexibleHeight, .flexibleWidth]
self.view.addSubview(captureView)
}
}
我发现了这个错误。
事实上，即使当你启用这条线时：
metadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
在CaptureView的对应metadataOutput(_, didOutput:, from:,)中，connection.videoPreviewLayer仍然是nil。正如 开发人员文档 所说：
This property is the set if you initialized the connection using init(inputPort:videoPreviewLayer:) or connectionWithInputPort:videoPreviewLayer:.
所以，在这两种情况下，connection.videoPreviewLayer 都将为零。
我已经稍微更新了您的代码以使其按您想要的方式工作。
import UIKit
import AVFoundation
func printMetadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], previewLayer: AVCaptureVideoPreviewLayer) {
for object in metadataObjects {
let visualCodeObject = previewLayer.transformedMetadataObject(for: object)
guard let object = visualCodeObject, let barcode = object as? AVMetadataMachineReadableCodeObject else {
NSLog("Ignoring object that is not AVMetadataMachineReadableCodeObject")
continue
}
guard let barcodeString = barcode.stringValue else {
NSLog("Captured something that's not a string")
continue
}
NSLog("Captured string %@", barcodeString)
}
}
class CaptureView: UIView, AVCaptureMetadataOutputObjectsDelegate {
let previewLayer = AVCaptureVideoPreviewLayer()
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
if connection.videoPreviewLayer == nil {
print("connection.videoPreviewLayer was nil")
}
printMetadataOutput(output, didOutput: metadataObjects, previewLayer: self.previewLayer)
}
override func layoutSubviews() {
super.layoutSubviews()
self.previewLayer.frame = self.frame
}
init(frame: CGRect, delegate: AVCaptureMetadataOutputObjectsDelegate) {
guard let captureDevice = AVCaptureDevice.default(for: .video) else {
fatalError("Couldn't find default capture device")
}
guard let captureDeviceInput = try? AVCaptureDeviceInput(device: captureDevice) else {
super.init(frame: frame)
return
}
let captureSession = AVCaptureSession()
captureSession.addInput(captureDeviceInput)
self.previewLayer.session = captureSession
self.previewLayer.videoGravity = .resizeAspectFill
super.init(frame: frame)
self.layer.addSublayer(self.previewLayer)
self.previewLayer.frame = self.frame
captureSession.startRunning()
let metadataOutput = AVCaptureMetadataOutput()
// metadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
metadataOutput.setMetadataObjectsDelegate(delegate, queue: DispatchQueue.main)
metadataOutput.rectOfInterest = CGRect(x: 0, y: 0, width: 1, height: 1)
if captureSession.canAddOutput(metadataOutput) {
captureSession.addOutput(metadataOutput)
} else {
fatalError("Can't add metadata output to capture session")
}
metadataOutput.metadataObjectTypes = [.qr]
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
class MetadataDelegate: NSObject, AVCaptureMetadataOutputObjectsDelegate {
var previewLayer: AVCaptureVideoPreviewLayer?
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
guard let previewLayer = previewLayer else {
print("previewLayer was nil")
return
}
printMetadataOutput(output, didOutput: metadataObjects, previewLayer: previewLayer)
}
}
class ViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate {
private let metadataDelegate = MetadataDelegate()
override func viewDidLoad() {
let captureView = CaptureView(frame: CGRect(), delegate: self.metadataDelegate)
metadataDelegate.previewLayer = captureView.previewLayer
captureView.frame = self.view.frame
captureView.autoresizingMask = [.flexibleHeight, .flexibleWidth]
self.view.addSubview(captureView)
}
}
正如 videoPreviewLayer 文档所述：
This property is the set if you initialized the connection using
init(inputPort:videoPreviewLayer:) or
connectionWithInputPort:videoPreviewLayer:.
因此，为了在 videoPreviewLayer 属性 中获取值，您必须手动设置 AVCaptureConnection 对象。
相反，我建议将 AVCaptureMetadataOutputObjectsDelegate 隐藏在您可以声明的自定义协议后面：
protocol CaptureViewMetadataOutputObjectsDelegate {
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection, previewLayer: AVCaptureVideoPreviewLayer)
}
然后，在您的 CaptureView 中实施 AVCaptureMetadataOutputObjectsDelegate 协议，并调用您的协议的函数传递所需的 AVCaptureVideoPreviewLayer。您的代码将是这样的：
class CaptureView: UIView, AVCaptureMetadataOutputObjectsDelegate {
private let previewLayer = AVCaptureVideoPreviewLayer()
private let delegate: CaptureViewMetadataOutputObjectsDelegate
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
delegate.metadataOutput(output, didOutput: metadataObjects, from: connection, previewLayer: previewLayer)
// printMetadataOutput(output, didOutput: metadataObjects, previewLayer: self.previewLayer)
}
override func layoutSubviews() {
super.layoutSubviews()
self.previewLayer.frame = self.frame
}
init(frame: CGRect, delegate: CaptureViewMetadataOutputObjectsDelegate) {
self.delegate = delegate
guard let captureDevice = AVCaptureDevice.default(for: .video) else {
fatalError("Couldn't find default capture device")
}
guard let captureDeviceInput = try? AVCaptureDeviceInput(device: captureDevice) else {
super.init(frame: frame)
return
}
let captureSession = AVCaptureSession()
captureSession.addInput(captureDeviceInput)
self.previewLayer.session = captureSession
self.previewLayer.videoGravity = .resizeAspectFill
super.init(frame: frame)
self.layer.addSublayer(self.previewLayer)
self.previewLayer.frame = self.frame
captureSession.startRunning()
let metadataOutput = AVCaptureMetadataOutput()
metadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
// metadataOutput.setMetadataObjectsDelegate(delegate, queue: DispatchQueue.main)
metadataOutput.rectOfInterest = CGRect(x: 0, y: 0, width: 1, height: 1)
if captureSession.canAddOutput(metadataOutput) {
captureSession.addOutput(metadataOutput)
} else {
fatalError("Can't add metadata output to capture session")
}
metadataOutput.metadataObjectTypes = [.qr]
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
class MetadataDelegate: CaptureViewMetadataOutputObjectsDelegate {
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection, previewLayer: AVCaptureVideoPreviewLayer) {
printMetadataOutput(output, didOutput: metadataObjects, previewLayer: previewLayer)
}
}
class ViewController: UIViewController {
private let metadataDelegate = MetadataDelegate()
override func viewDidLoad() {
let captureView = CaptureView(frame: CGRect(), delegate: self.metadataDelegate)
captureView.frame = self.view.frame
captureView.autoresizingMask = [.flexibleHeight, .flexibleWidth]
self.view.addSubview(captureView)
}
}
更新：经过一番研究，我确实在init(inputPort:videoPreviewLayer:)函数文档中找到了这条语句：
When using addInput(_:): or addOutput(_:), connections are
automatically formed between all compatible inputs and outputs. You do
not need to manually create and add connections to the session unless
you use the primitive addInputWithNoConnections(_:) and
addOutputWithNoConnections(_:) methods.
Tha 意味着当您将设备相机添加为输入并将 AVCaptureMetadataOutput 添加为输出时，所有兼容的 AVCaptureConnection 都会自动创建。
我尝试使用以下代码创建另一个 AVCaptureConnection：
if let port = captureDeviceInput.ports.first(where: { [=12=].mediaType == .video }) {
let con = AVCaptureConnection(inputPort: port, videoPreviewLayer: self.previewLayer)
if captureSession.canAddConnection(con) {
captureSession.addConnection(con)
}
}
但是 canAddConnection(_:) 函数总是 returns false.
之后我打印了 AVCaptureSession 的 connections 数组，我看到了以下内容：
(lldb) po captureSession.connections
[<AVCaptureConnection: 0x280d67980 (AVCaptureDeviceInput: 0x280d119a0 Back Camera) -> (AVCaptureVideoPreviewLayer: 0x280d6ba40) [type:vide][enabled:1][active:1]>, <AVCaptureConnection: 0x280d7bee0 (AVCaptureDeviceInput: 0x280d119a0 Back Camera) -> (AVCaptureMetadataOutput: 0x280d700e0) [type:mobj][enabled:1][active:1]>]
因此，已创建一个 AVCaptureConnection 并使用后置摄像头作为输入和一个 AVCaptureVideoPreviewLayer 实例（可能是您创建的 previewLayer 属性）另一个以后置摄像头作为输入，你传递给 AVCaptureSession.
的 AVCaptureMetadataOutput
显然，第一个确实对 videoPreviewLayer 属性 有一定的价值：
(lldb) po captureSession.connections[0].videoPreviewLayer
▿ Optional<AVCaptureVideoPreviewLayer>
- some : <AVCaptureVideoPreviewLayer:0x280d6ba40; position = CGPoint (0 0); bounds = CGRect (0 0; 0 0); sublayers = (<CALayer: 0x280d6bc20>); masksToBounds = YES; allowsGroupOpacity = YES; inheritsTiming = NO; >
显然，您在 metadataOutput(_:didOutput:from:) 函数中获得的 AVCaptureConnection 实例将始终是第二个实例。将后置摄像头与 AVCaptureMetadataOutput.
相关联的那个
下面的代码可以 运行 在 iPhone 上，当针对任何 QR 码时，应该打印内容。下面二维码指向example.com.
问题是代理应该提供 connection: AVCaptureConnection 而它确实提供了，但是它的 previewLayer 属性 是 nil.
可以将以下代码粘贴到一个新的空 Xcode 项目中。如果您禁用（注释掉）第 57 行，并启用第 56 行，它工作正常。但我想将委托放在 CaptureView class 之外。我如何设置捕获使得 AVCaptureMetadataOutputObjectsDelegate 它的 previewLayer 属性 不是 nil?
import UIKit
import AVFoundation
func printMetadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], previewLayer: AVCaptureVideoPreviewLayer) {
for object in metadataObjects {
let visualCodeObject = previewLayer.transformedMetadataObject(for: object)
guard let object = visualCodeObject, let barcode = object as? AVMetadataMachineReadableCodeObject else {
NSLog("Ignoring object that is not AVMetadataMachineReadableCodeObject")
continue
}
guard let barcodeString = barcode.stringValue else {
NSLog("Captured something that's not a string")
continue
}
NSLog("Captured string %@", barcodeString)
}
}
class CaptureView: UIView, AVCaptureMetadataOutputObjectsDelegate {
private let previewLayer = AVCaptureVideoPreviewLayer()
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
printMetadataOutput(output, didOutput: metadataObjects, previewLayer: self.previewLayer)
}
override func layoutSubviews() {
super.layoutSubviews()
self.previewLayer.frame = self.frame
}
init(frame: CGRect, delegate: AVCaptureMetadataOutputObjectsDelegate) {
guard let captureDevice = AVCaptureDevice.default(for: .video) else {
fatalError("Couldn't find default capture device")
}
guard let captureDeviceInput = try? AVCaptureDeviceInput(device: captureDevice) else {
super.init(frame: frame)
return
}
let captureSession = AVCaptureSession()
captureSession.addInput(captureDeviceInput)
self.previewLayer.session = captureSession
self.previewLayer.videoGravity = .resizeAspectFill
super.init(frame: frame)
self.layer.addSublayer(self.previewLayer)
self.previewLayer.frame = self.frame
captureSession.startRunning()
let metadataOutput = AVCaptureMetadataOutput()
// metadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
metadataOutput.setMetadataObjectsDelegate(delegate, queue: DispatchQueue.main)
metadataOutput.rectOfInterest = CGRect(x: 0, y: 0, width: 1, height: 1)
if captureSession.canAddOutput(metadataOutput) {
captureSession.addOutput(metadataOutput)
} else {
fatalError("Can't add metadata output to capture session")
}
metadataOutput.metadataObjectTypes = [.qr]
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
class MetadataDelegate: NSObject, AVCaptureMetadataOutputObjectsDelegate {
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
guard let previewLayer = connection.videoPreviewLayer else {
print("previewLayer was nil")
return
}
printMetadataOutput(output, didOutput: metadataObjects, previewLayer: previewLayer)
}
}
class ViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate {
private let metadataDelegate = MetadataDelegate()
override func viewDidLoad() {
let captureView = CaptureView(frame: CGRect(), delegate: self.metadataDelegate)
captureView.frame = self.view.frame
captureView.autoresizingMask = [.flexibleHeight, .flexibleWidth]
self.view.addSubview(captureView)
}
}
我发现了这个错误。
事实上，即使当你启用这条线时：
metadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
在CaptureView的对应metadataOutput(_, didOutput:, from:,)中，connection.videoPreviewLayer仍然是nil。正如 开发人员文档 所说：
This property is the set if you initialized the connection using init(inputPort:videoPreviewLayer:) or connectionWithInputPort:videoPreviewLayer:.
所以，在这两种情况下，connection.videoPreviewLayer 都将为零。
我已经稍微更新了您的代码以使其按您想要的方式工作。
import UIKit
import AVFoundation
func printMetadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], previewLayer: AVCaptureVideoPreviewLayer) {
for object in metadataObjects {
let visualCodeObject = previewLayer.transformedMetadataObject(for: object)
guard let object = visualCodeObject, let barcode = object as? AVMetadataMachineReadableCodeObject else {
NSLog("Ignoring object that is not AVMetadataMachineReadableCodeObject")
continue
}
guard let barcodeString = barcode.stringValue else {
NSLog("Captured something that's not a string")
continue
}
NSLog("Captured string %@", barcodeString)
}
}
class CaptureView: UIView, AVCaptureMetadataOutputObjectsDelegate {
let previewLayer = AVCaptureVideoPreviewLayer()
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
if connection.videoPreviewLayer == nil {
print("connection.videoPreviewLayer was nil")
}
printMetadataOutput(output, didOutput: metadataObjects, previewLayer: self.previewLayer)
}
override func layoutSubviews() {
super.layoutSubviews()
self.previewLayer.frame = self.frame
}
init(frame: CGRect, delegate: AVCaptureMetadataOutputObjectsDelegate) {
guard let captureDevice = AVCaptureDevice.default(for: .video) else {
fatalError("Couldn't find default capture device")
}
guard let captureDeviceInput = try? AVCaptureDeviceInput(device: captureDevice) else {
super.init(frame: frame)
return
}
let captureSession = AVCaptureSession()
captureSession.addInput(captureDeviceInput)
self.previewLayer.session = captureSession
self.previewLayer.videoGravity = .resizeAspectFill
super.init(frame: frame)
self.layer.addSublayer(self.previewLayer)
self.previewLayer.frame = self.frame
captureSession.startRunning()
let metadataOutput = AVCaptureMetadataOutput()
// metadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
metadataOutput.setMetadataObjectsDelegate(delegate, queue: DispatchQueue.main)
metadataOutput.rectOfInterest = CGRect(x: 0, y: 0, width: 1, height: 1)
if captureSession.canAddOutput(metadataOutput) {
captureSession.addOutput(metadataOutput)
} else {
fatalError("Can't add metadata output to capture session")
}
metadataOutput.metadataObjectTypes = [.qr]
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
class MetadataDelegate: NSObject, AVCaptureMetadataOutputObjectsDelegate {
var previewLayer: AVCaptureVideoPreviewLayer?
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
guard let previewLayer = previewLayer else {
print("previewLayer was nil")
return
}
printMetadataOutput(output, didOutput: metadataObjects, previewLayer: previewLayer)
}
}
class ViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate {
private let metadataDelegate = MetadataDelegate()
override func viewDidLoad() {
let captureView = CaptureView(frame: CGRect(), delegate: self.metadataDelegate)
metadataDelegate.previewLayer = captureView.previewLayer
captureView.frame = self.view.frame
captureView.autoresizingMask = [.flexibleHeight, .flexibleWidth]
self.view.addSubview(captureView)
}
}
正如 videoPreviewLayer 文档所述：
This property is the set if you initialized the connection using
init(inputPort:videoPreviewLayer:)orconnectionWithInputPort:videoPreviewLayer:.
因此，为了在 videoPreviewLayer 属性 中获取值，您必须手动设置 AVCaptureConnection 对象。
相反，我建议将 AVCaptureMetadataOutputObjectsDelegate 隐藏在您可以声明的自定义协议后面：
protocol CaptureViewMetadataOutputObjectsDelegate {
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection, previewLayer: AVCaptureVideoPreviewLayer)
}
然后，在您的 CaptureView 中实施 AVCaptureMetadataOutputObjectsDelegate 协议，并调用您的协议的函数传递所需的 AVCaptureVideoPreviewLayer。您的代码将是这样的：
class CaptureView: UIView, AVCaptureMetadataOutputObjectsDelegate {
private let previewLayer = AVCaptureVideoPreviewLayer()
private let delegate: CaptureViewMetadataOutputObjectsDelegate
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
delegate.metadataOutput(output, didOutput: metadataObjects, from: connection, previewLayer: previewLayer)
// printMetadataOutput(output, didOutput: metadataObjects, previewLayer: self.previewLayer)
}
override func layoutSubviews() {
super.layoutSubviews()
self.previewLayer.frame = self.frame
}
init(frame: CGRect, delegate: CaptureViewMetadataOutputObjectsDelegate) {
self.delegate = delegate
guard let captureDevice = AVCaptureDevice.default(for: .video) else {
fatalError("Couldn't find default capture device")
}
guard let captureDeviceInput = try? AVCaptureDeviceInput(device: captureDevice) else {
super.init(frame: frame)
return
}
let captureSession = AVCaptureSession()
captureSession.addInput(captureDeviceInput)
self.previewLayer.session = captureSession
self.previewLayer.videoGravity = .resizeAspectFill
super.init(frame: frame)
self.layer.addSublayer(self.previewLayer)
self.previewLayer.frame = self.frame
captureSession.startRunning()
let metadataOutput = AVCaptureMetadataOutput()
metadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
// metadataOutput.setMetadataObjectsDelegate(delegate, queue: DispatchQueue.main)
metadataOutput.rectOfInterest = CGRect(x: 0, y: 0, width: 1, height: 1)
if captureSession.canAddOutput(metadataOutput) {
captureSession.addOutput(metadataOutput)
} else {
fatalError("Can't add metadata output to capture session")
}
metadataOutput.metadataObjectTypes = [.qr]
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
class MetadataDelegate: CaptureViewMetadataOutputObjectsDelegate {
func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection, previewLayer: AVCaptureVideoPreviewLayer) {
printMetadataOutput(output, didOutput: metadataObjects, previewLayer: previewLayer)
}
}
class ViewController: UIViewController {
private let metadataDelegate = MetadataDelegate()
override func viewDidLoad() {
let captureView = CaptureView(frame: CGRect(), delegate: self.metadataDelegate)
captureView.frame = self.view.frame
captureView.autoresizingMask = [.flexibleHeight, .flexibleWidth]
self.view.addSubview(captureView)
}
}
更新：经过一番研究，我确实在init(inputPort:videoPreviewLayer:)函数文档中找到了这条语句：
When using
addInput(_:):oraddOutput(_:), connections are automatically formed between all compatible inputs and outputs. You do not need to manually create and add connections to the session unless you use the primitiveaddInputWithNoConnections(_:)andaddOutputWithNoConnections(_:)methods.
Tha 意味着当您将设备相机添加为输入并将 AVCaptureMetadataOutput 添加为输出时，所有兼容的 AVCaptureConnection 都会自动创建。
我尝试使用以下代码创建另一个 AVCaptureConnection：
if let port = captureDeviceInput.ports.first(where: { [=12=].mediaType == .video }) {
let con = AVCaptureConnection(inputPort: port, videoPreviewLayer: self.previewLayer)
if captureSession.canAddConnection(con) {
captureSession.addConnection(con)
}
}
但是 canAddConnection(_:) 函数总是 returns false.
之后我打印了 AVCaptureSession 的 connections 数组，我看到了以下内容：
(lldb) po captureSession.connections
[<AVCaptureConnection: 0x280d67980 (AVCaptureDeviceInput: 0x280d119a0 Back Camera) -> (AVCaptureVideoPreviewLayer: 0x280d6ba40) [type:vide][enabled:1][active:1]>, <AVCaptureConnection: 0x280d7bee0 (AVCaptureDeviceInput: 0x280d119a0 Back Camera) -> (AVCaptureMetadataOutput: 0x280d700e0) [type:mobj][enabled:1][active:1]>]
因此，已创建一个 AVCaptureConnection 并使用后置摄像头作为输入和一个 AVCaptureVideoPreviewLayer 实例（可能是您创建的 previewLayer 属性）另一个以后置摄像头作为输入，你传递给 AVCaptureSession.
AVCaptureMetadataOutput
显然，第一个确实对 videoPreviewLayer 属性 有一定的价值：
(lldb) po captureSession.connections[0].videoPreviewLayer
▿ Optional<AVCaptureVideoPreviewLayer>
- some : <AVCaptureVideoPreviewLayer:0x280d6ba40; position = CGPoint (0 0); bounds = CGRect (0 0; 0 0); sublayers = (<CALayer: 0x280d6bc20>); masksToBounds = YES; allowsGroupOpacity = YES; inheritsTiming = NO; >
显然，您在 metadataOutput(_:didOutput:from:) 函数中获得的 AVCaptureConnection 实例将始终是第二个实例。将后置摄像头与 AVCaptureMetadataOutput.