什么是 provideLayout()？.let { setContentView(it) }？
What is provideLayout()?.let { setContentView(it) }?
所以我想了解更多关于 cameraX 的信息，它是 Android Jetpack 库中相对较新的成员。我一直在查看 GitHub 上的一个现有项目，以帮助我了解如何进行设置。但是问题是我是 kotlin 的新手（项目在 kotlin 中）。
项目中：Project
存在一个叫做"it"
的东西
可以在这里看到：Example
更具体地在以下行中：
provideLayout()?.let { setContentView(it) }
"it" 在整个项目中也重复了多次。我只想知道这是什么。 "it" 是变量吗？ "it" 是 Kotlin 特定代码吗？
此外，什么是：
provideLayout()?.let
这是如何工作的？它有什么作用？
更新
特定于 CommonsWare 解决方案（错误）：
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.memory.pod.debug, PID: 31521
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.memory.pod.debug/com.memory.pod.camerax.ui.home.HomeActivity}: android.view.InflateException: Binary XML file line #8 in com.memory.pod.debug:layout/activity_home: Binary XML file line #8 in com.memory.pod.debug:layout/activity_home: Error inflating class androidx.fragment.app.FragmentContainerView
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
Caused by: android.view.InflateException: Binary XML file line #8 in com.memory.pod.debug:layout/activity_home: Binary XML file line #8 in com.memory.pod.debug:layout/activity_home: Error inflating class androidx.fragment.app.FragmentContainerView
Caused by: android.view.InflateException: Binary XML file line #8 in com.memory.pod.debug:layout/activity_home: Error inflating class androidx.fragment.app.FragmentContainerView
Caused by: androidx.fragment.app.Fragment$InstantiationException: Unable to instantiate fragment com.arindam.camerax.ui.home.permission.PermissionsFragment: make sure class name exists
at androidx.fragment.app.FragmentFactory.loadFragmentClass(FragmentFactory.java:91)
at androidx.fragment.app.Fragment.instantiate(Fragment.java:546)
at androidx.fragment.app.FragmentContainer.instantiate(FragmentContainer.java:57)
at androidx.fragment.app.FragmentManager.instantiate(FragmentManager.java:390)
at androidx.navigation.fragment.FragmentNavigator.instantiateFragment(FragmentNavigator.java:132)
at androidx.navigation.fragment.FragmentNavigator.navigate(FragmentNavigator.java:162)
at androidx.navigation.fragment.FragmentNavigator.navigate(FragmentNavigator.java:58)
at androidx.navigation.NavGraphNavigator.navigate(NavGraphNavigator.java:71)
at androidx.navigation.NavGraphNavigator.navigate(NavGraphNavigator.java:28)
at androidx.navigation.NavController.navigate(NavController.java:935)
at androidx.navigation.NavController.onGraphCreated(NavController.java:577)
at androidx.navigation.NavController.setGraph(NavController.java:534)
at androidx.navigation.NavController.setGraph(NavController.java:499)
at androidx.navigation.NavController.setGraph(NavController.java:481)
at androidx.navigation.fragment.NavHostFragment.onCreate(NavHostFragment.java:237)
at androidx.fragment.app.Fragment.performCreate(Fragment.java:2684)
at androidx.fragment.app.FragmentStateManager.create(FragmentStateManager.java:280)
at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1175)
at androidx.fragment.app.FragmentManager.addAddedFragments(FragmentManager.java:2224)
at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:1997)
at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1953)
at androidx.fragment.app.FragmentManager.execSingleAction(FragmentManager.java:1818)
at androidx.fragment.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:303)
at androidx.fragment.app.FragmentContainerView.<init>(FragmentContainerView.java:166)
at androidx.fragment.app.FragmentLayoutInflaterFactory.onCreateView(FragmentLayoutInflaterFactory.java:51)
E/AndroidRuntime: at androidx.fragment.app.FragmentController.onCreateView(FragmentController.java:135)
at androidx.fragment.app.FragmentActivity.dispatchFragmentsOnCreateView(FragmentActivity.java:356)
at androidx.fragment.app.FragmentActivity.onCreateView(FragmentActivity.java:335)
at android.view.LayoutInflater.tryCreateView(LayoutInflater.java:1069)
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:997)
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:961)
at android.view.LayoutInflater.rInflate(LayoutInflater.java:1123)
at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:1084)
at android.view.LayoutInflater.inflate(LayoutInflater.java:682)
at android.view.LayoutInflater.inflate(LayoutInflater.java:534)
at android.view.LayoutInflater.inflate(LayoutInflater.java:481)
at androidx.appcompat.app.AppCompatDelegateImpl.setContentView(AppCompatDelegateImpl.java:555)
at androidx.appcompat.app.AppCompatActivity.setContentView(AppCompatActivity.java:161)
at com.memory.pod.camerax.ui.base.BaseActivity.onCreate(BaseActivity.kt:11)
at android.app.Activity.performCreate(Activity.java:7825)
at android.app.Activity.performCreate(Activity.java:7814)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1306)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
Caused by: java.lang.ClassNotFoundException: com.arindam.camerax.ui.home.permission.PermissionsFragment
at java.lang.Class.classForName(Native Method)
at java.lang.Class.forName(Class.java:454)
at androidx.fragment.app.FragmentFactory.loadClass(FragmentFactory.java:47)
at androidx.fragment.app.FragmentFactory.loadFragmentClass(FragmentFactory.java:88)
... 53 more
Caused by: java.lang.ClassNotFoundException: Didn't find class "com.arindam.camerax.ui.home.permission.PermissionsFragment" on path: DexPathList[[zip file "/data/app/com.memory.pod.debug-koXsvzgNs8unWof_uGUUgw==/base.apk"],nativeLibraryDirectories=[/data/app/com.memory.pod.debug-koXsvzgNs8unWof_uGUUgw==/lib/arm64, /system/lib64, /system/product/lib64]]
at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:196)
at java.lang.ClassLoader.loadClass(ClassLoader.java:379)
at java.lang.ClassLoader.loadClass(ClassLoader.java:312)
不是超链接突出显示部分的一侧是
com.memory.pod.camerax.ui.base.BaseActivity.onCreate(BaseActivity.kt:11)
Is "it" a variable?
it是传入lambda表达式的参数。
一段一段的...
provideLayout()是一个函数，returning一个对象。
?.是一个"safe call"，意味着如果对象是null，我们跳过调用
?.let() 是安全调用的特殊用法，表示 "execute this lambda expression if the object is not null, and pass that object in as a parameter; if the object is null, though, skip it and let's just go have lunch"
{ setContentView(it) } 是一个 lambda 表达式，如果由 provideLayout() 编辑的对象 return 不是 null，则由 let() 调用，并且该对象在 lambda 表达式中称为 it（其中 it 是 lambda 参数的默认名称，因为我们没有提供其他名称）
如果我们知道provideLayout()永远不会returnnull，或者如果setContentView()可以支持null，这可以简化为：
setContentView(provideLayout())
所以我想了解更多关于 cameraX 的信息，它是 Android Jetpack 库中相对较新的成员。我一直在查看 GitHub 上的一个现有项目，以帮助我了解如何进行设置。但是问题是我是 kotlin 的新手（项目在 kotlin 中）。
项目中：Project
存在一个叫做"it"
的东西可以在这里看到：Example
更具体地在以下行中：
provideLayout()?.let { setContentView(it) }
"it" 在整个项目中也重复了多次。我只想知道这是什么。 "it" 是变量吗？ "it" 是 Kotlin 特定代码吗？
此外，什么是：
provideLayout()?.let
这是如何工作的？它有什么作用？
更新
特定于 CommonsWare 解决方案（错误）：
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.memory.pod.debug, PID: 31521
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.memory.pod.debug/com.memory.pod.camerax.ui.home.HomeActivity}: android.view.InflateException: Binary XML file line #8 in com.memory.pod.debug:layout/activity_home: Binary XML file line #8 in com.memory.pod.debug:layout/activity_home: Error inflating class androidx.fragment.app.FragmentContainerView
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
Caused by: android.view.InflateException: Binary XML file line #8 in com.memory.pod.debug:layout/activity_home: Binary XML file line #8 in com.memory.pod.debug:layout/activity_home: Error inflating class androidx.fragment.app.FragmentContainerView
Caused by: android.view.InflateException: Binary XML file line #8 in com.memory.pod.debug:layout/activity_home: Error inflating class androidx.fragment.app.FragmentContainerView
Caused by: androidx.fragment.app.Fragment$InstantiationException: Unable to instantiate fragment com.arindam.camerax.ui.home.permission.PermissionsFragment: make sure class name exists
at androidx.fragment.app.FragmentFactory.loadFragmentClass(FragmentFactory.java:91)
at androidx.fragment.app.Fragment.instantiate(Fragment.java:546)
at androidx.fragment.app.FragmentContainer.instantiate(FragmentContainer.java:57)
at androidx.fragment.app.FragmentManager.instantiate(FragmentManager.java:390)
at androidx.navigation.fragment.FragmentNavigator.instantiateFragment(FragmentNavigator.java:132)
at androidx.navigation.fragment.FragmentNavigator.navigate(FragmentNavigator.java:162)
at androidx.navigation.fragment.FragmentNavigator.navigate(FragmentNavigator.java:58)
at androidx.navigation.NavGraphNavigator.navigate(NavGraphNavigator.java:71)
at androidx.navigation.NavGraphNavigator.navigate(NavGraphNavigator.java:28)
at androidx.navigation.NavController.navigate(NavController.java:935)
at androidx.navigation.NavController.onGraphCreated(NavController.java:577)
at androidx.navigation.NavController.setGraph(NavController.java:534)
at androidx.navigation.NavController.setGraph(NavController.java:499)
at androidx.navigation.NavController.setGraph(NavController.java:481)
at androidx.navigation.fragment.NavHostFragment.onCreate(NavHostFragment.java:237)
at androidx.fragment.app.Fragment.performCreate(Fragment.java:2684)
at androidx.fragment.app.FragmentStateManager.create(FragmentStateManager.java:280)
at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1175)
at androidx.fragment.app.FragmentManager.addAddedFragments(FragmentManager.java:2224)
at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:1997)
at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1953)
at androidx.fragment.app.FragmentManager.execSingleAction(FragmentManager.java:1818)
at androidx.fragment.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:303)
at androidx.fragment.app.FragmentContainerView.<init>(FragmentContainerView.java:166)
at androidx.fragment.app.FragmentLayoutInflaterFactory.onCreateView(FragmentLayoutInflaterFactory.java:51)
E/AndroidRuntime: at androidx.fragment.app.FragmentController.onCreateView(FragmentController.java:135)
at androidx.fragment.app.FragmentActivity.dispatchFragmentsOnCreateView(FragmentActivity.java:356)
at androidx.fragment.app.FragmentActivity.onCreateView(FragmentActivity.java:335)
at android.view.LayoutInflater.tryCreateView(LayoutInflater.java:1069)
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:997)
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:961)
at android.view.LayoutInflater.rInflate(LayoutInflater.java:1123)
at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:1084)
at android.view.LayoutInflater.inflate(LayoutInflater.java:682)
at android.view.LayoutInflater.inflate(LayoutInflater.java:534)
at android.view.LayoutInflater.inflate(LayoutInflater.java:481)
at androidx.appcompat.app.AppCompatDelegateImpl.setContentView(AppCompatDelegateImpl.java:555)
at androidx.appcompat.app.AppCompatActivity.setContentView(AppCompatActivity.java:161)
at com.memory.pod.camerax.ui.base.BaseActivity.onCreate(BaseActivity.kt:11)
at android.app.Activity.performCreate(Activity.java:7825)
at android.app.Activity.performCreate(Activity.java:7814)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1306)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
Caused by: java.lang.ClassNotFoundException: com.arindam.camerax.ui.home.permission.PermissionsFragment
at java.lang.Class.classForName(Native Method)
at java.lang.Class.forName(Class.java:454)
at androidx.fragment.app.FragmentFactory.loadClass(FragmentFactory.java:47)
at androidx.fragment.app.FragmentFactory.loadFragmentClass(FragmentFactory.java:88)
... 53 more
Caused by: java.lang.ClassNotFoundException: Didn't find class "com.arindam.camerax.ui.home.permission.PermissionsFragment" on path: DexPathList[[zip file "/data/app/com.memory.pod.debug-koXsvzgNs8unWof_uGUUgw==/base.apk"],nativeLibraryDirectories=[/data/app/com.memory.pod.debug-koXsvzgNs8unWof_uGUUgw==/lib/arm64, /system/lib64, /system/product/lib64]]
at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:196)
at java.lang.ClassLoader.loadClass(ClassLoader.java:379)
at java.lang.ClassLoader.loadClass(ClassLoader.java:312)
不是超链接突出显示部分的一侧是
com.memory.pod.camerax.ui.base.BaseActivity.onCreate(BaseActivity.kt:11)
Is "it" a variable?
it是传入lambda表达式的参数。
一段一段的...
provideLayout()是一个函数，returning一个对象。?.是一个"safe call"，意味着如果对象是null，我们跳过调用?.let()是安全调用的特殊用法，表示 "execute this lambda expression if the object is notnull, and pass that object in as a parameter; if the object isnull, though, skip it and let's just go have lunch"{ setContentView(it) }是一个 lambda 表达式，如果由provideLayout()编辑的对象 return 不是null，则由let()调用，并且该对象在 lambda 表达式中称为it（其中it是 lambda 参数的默认名称，因为我们没有提供其他名称）
如果我们知道provideLayout()永远不会returnnull，或者如果setContentView()可以支持null，这可以简化为：
setContentView(provideLayout())