Question

上下文：这是javascript tutorial中的任务。该代码应生成2018-10-14 13:11:36.411 13542-13542/io.github.louistsaitszho.loft E/AndroidRuntime: FATAL EXCEPTION: main Process: io.github.louistsaitszho.loft, PID: 13542 java.lang.RuntimeException: StrictMode ThreadPolicy violation at android.os.StrictMode$AndroidBlockGuardPolicy.onThreadPolicyViolation(StrictMode.java:1705) at android.os.StrictMode$AndroidBlockGuardPolicy.lambda$handleViolationWithTimingAttempt$0(StrictMode.java:1619) at android.os.-$$Lambda$StrictMode$AndroidBlockGuardPolicy$9nBulCQKaMajrWr41SB7f7YRT1I.run(Unknown Source:6) at android.os.Handler.handleCallback(Handler.java:873) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:193) at android.app.ActivityThread.main(ActivityThread.java:6669) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) Caused by: android.os.strictmode.DiskReadViolation at android.os.StrictMode$AndroidBlockGuardPolicy.onReadFromDisk(StrictMode.java:1504) at java.io.UnixFileSystem.getBooleanAttributes(UnixFileSystem.java:241) at java.io.File.isDirectory(File.java:845) at dalvik.system.DexPathList$Element.maybeInit(DexPathList.java:696) at dalvik.system.DexPathList$Element.findResource(DexPathList.java:729) at dalvik.system.DexPathList.findResources(DexPathList.java:526) at dalvik.system.BaseDexClassLoader.findResources(BaseDexClassLoader.java:174) at java.lang.ClassLoader.getResources(ClassLoader.java:839) at java.util.ServiceLoader$LazyIterator.hasNextService(ServiceLoader.java:349) at java.util.ServiceLoader$LazyIterator.hasNext(ServiceLoader.java:402) at java.util.ServiceLoader$1.hasNext(ServiceLoader.java:488) at kotlin.collections.CollectionsKt___CollectionsKt.toCollection(_Collections.kt:1062) at kotlin.collections.CollectionsKt___CollectionsKt.toMutableList(_Collections.kt:1095) at kotlin.collections.CollectionsKt___CollectionsKt.toList(_Collections.kt:1086) at kotlinx.coroutines.experimental.MainDispatcherLoader.<clinit>(Dispatchers.kt:97) at kotlinx.coroutines.experimental.Dispatchers.getMain(Dispatchers.kt:54) at io.github.louistsaitszho.loft.ScopedFragment.getCoroutineContext(ScopedFragment.kt:20) at kotlinx.coroutines.experimental.CoroutineContextKt.newCoroutineContext(CoroutineContext.kt:81) at kotlinx.coroutines.experimental.BuildersKt__Builders_commonKt.launch(Builders.common.kt:49) at kotlinx.coroutines.experimental.BuildersKt.launch(Unknown Source:1) at kotlinx.coroutines.experimental.BuildersKt__Builders_commonKt.launch$default(Builders.common.kt:46) at kotlinx.coroutines.experimental.BuildersKt.launch$default(Unknown Source:1) at io.github.louistsaitszho.loft.splash.SplashFragment.onStart(SplashFragment.kt:28) at android.support.v4.app.Fragment.performStart(Fragment.java:2372) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1467) at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1759) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1827) at android.support.v4.app.FragmentManagerImpl.dispatchStateChange(FragmentManager.java:3244) at android.support.v4.app.FragmentManagerImpl.dispatchStart(FragmentManager.java:3206) at android.support.v4.app.Fragment.performStart(Fragment.java:2378) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1467) at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1759) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1827) at android.support.v4.app.FragmentManagerImpl.dispatchStateChange(FragmentManager.java:3244) at android.support.v4.app.FragmentManagerImpl.dispatchStart(FragmentManager.java:3206) at android.support.v4.app.FragmentController.dispatchStart(FragmentController.java:206) at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:605) 2018-10-14 13:11:36.412 13542-13542/io.github.louistsaitszho.loft E/AndroidRuntime: at android.support.v7.app.AppCompatActivity.onStart(AppCompatActivity.java:177) at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1391) at android.app.Activity.performStart(Activity.java:7157) at android.app.ActivityThread.handleStartActivity(ActivityThread.java:2937) at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:180) at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:165) at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:142) at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) at android.os.Handler.dispatchMessage(Handler.java:106)的子类ExtendedClock，该子类允许控制台以自定义的时间间隔显示当前时间。

Clock

这是本教程中给出的解决方案中的怪异行：

Class Clock {
  constructor({ template }) {
    this._template = template;
  }

  _render() {
    let date = new Date();

    let hours = date.getHours();
    if (hours < 10) hours = '0' + hours;

    let mins = date.getMinutes();
    if (mins < 10) min = '0' + mins;

    let secs = date.getSeconds();
    if (secs < 10) secs = '0' + secs;

    let output = this._template
      .replace('h', hours)
      .replace('m', mins)
      .replace('s', secs);

    console.log(output);
  }

  stop() {
    clearInterval(this._timer);
  }

  start() {
    this._render();
    this._timer = setInterval(() => this._render(), 1000);
  }
}

} }; 另一个问题：我的代码如下。为什么这行不通？

class ExtendedClock extends Clock {
  constructor(options) {
    super(options);
    let { precision=1000 } = options;  // what is this?
    this._precision = precision;
  }

  start() {
    this._render();
    this._timer = setInterval(() => this._render(), this._precision);

Answer 1

这是具有默认值的对象分解语法。如果您要销毁的对象包含一个precision键，则将使用该值，否则，将使用1000。

如果键存在，将使用其值：

const options = { precision: 800 };
const { precision = 1000 } = options;
console.log(precision); // logs 800

但如果密钥不存在，将使用默认值：

const options = {};
const { precision = 1000 } = options;
console.log(precision); // logs 1000

您的代码可能无法正常工作，因为当您调用super.start()时，超类会使用

开始循环

setInterval(() => this._render(), 1000);

您的代码在调用此函数后会启动一个循环，但是现在这两个循环都在运行，导致渲染函数每隔1000ms被超类的setInterval调用一次，然后又每隔precision ms调用一次您子类的循环。

而不是在循环开始时调用super.start()，而是尝试仅调用this._render()。

Answer 2

如果有属性，此奇怪的代码只是覆盖默认值的便捷方法。

let options = { precision: 999, template: 'h:m:s' };
let { precision = 1000 } = options; // precision === 999

let options = { template: 'h:m:s' };
let { precision = 1000 } = options; // precision === 1000

对于第二个问题，我需要更多输入。您会遇到什么错误？

有人可以解释这个看似奇怪的赋值{{key = value} = arguments`

2 个答案: