NosuchMethodError: The getter 'pasterButtonLabel' was called on null.
Receiver: null
Tried calling: pasteButtonLabel

在最近 app store 提交审核时被拒了,然后得到了一个这样的截图

图片

在 flutter 中可能会会出现各种问题,因为之前遇到过这个问题,但是那是我另一个应用,这个忘了设置了

我快速搞了一下,重新提交了审核

解决方式

import 'package:flutter/cupertino.dart';
import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';

class ChineseCupertinoLocalizations implements CupertinoLocalizations {
  final materialDelegate = GlobalMaterialLocalizations.delegate;
  final widgetsDelegate = GlobalWidgetsLocalizations.delegate;
  final local = const Locale('zh');

  MaterialLocalizations ml;

  Future init() async {
    ml = await materialDelegate.load(local);
    print(ml.pasteButtonLabel);
  }

  @override
  String get alertDialogLabel => ml.alertDialogLabel;

  @override
  String get anteMeridiemAbbreviation => ml.anteMeridiemAbbreviation;

  @override
  String get copyButtonLabel => ml.copyButtonLabel;

  @override
  String get cutButtonLabel => ml.cutButtonLabel;

  @override
  DatePickerDateOrder get datePickerDateOrder => DatePickerDateOrder.mdy;

  @override
  DatePickerDateTimeOrder get datePickerDateTimeOrder =>
      DatePickerDateTimeOrder.date_time_dayPeriod;

  @override
  String datePickerDayOfMonth(int dayIndex) {
    return dayIndex.toString();
  }

  @override
  String datePickerHour(int hour) {
    return hour.toString().padLeft(2, "0");
  }

  @override
  String datePickerHourSemanticsLabel(int hour) {
    return "$hour" + "时";
  }

  @override
  String datePickerMediumDate(DateTime date) {
    return ml.formatMediumDate(date);
  }

  @override
  String datePickerMinute(int minute) {
    return minute.toString().padLeft(2, '0');
  }

  @override
  String datePickerMinuteSemanticsLabel(int minute) {
    return "$minute" + "分";
  }

  @override
  String datePickerMonth(int monthIndex) {
    return "$monthIndex";
  }

  @override
  String datePickerYear(int yearIndex) {
    return yearIndex.toString();
  }

  @override
  String get pasteButtonLabel => ml.pasteButtonLabel;

  @override
  String get postMeridiemAbbreviation => ml.postMeridiemAbbreviation;

  @override
  String get selectAllButtonLabel => ml.selectAllButtonLabel;

  @override
  String timerPickerHour(int hour) {
    return hour.toString().padLeft(2, "0");
  }

  @override
  String timerPickerHourLabel(int hour) {
    return "$hour".toString().padLeft(2, "0") + "时";
  }

  @override
  String timerPickerMinute(int minute) {
    return minute.toString().padLeft(2, "0");
  }

  @override
  String timerPickerMinuteLabel(int minute) {
    return minute.toString().padLeft(2, "0") + "分";
  }

  @override
  String timerPickerSecond(int second) {
    return second.toString().padLeft(2, "0");
  }

  @override
  String timerPickerSecondLabel(int second) {
    return second.toString().padLeft(2, "0") + "秒";
  }

  static const LocalizationsDelegate<CupertinoLocalizations> delegate =
      _ChineseDelegate();

  static Future<CupertinoLocalizations> load(Locale locale) async {
    var localizaltions = ChineseCupertinoLocalizations();
    await localizaltions.init();
    return SynchronousFuture<CupertinoLocalizations>(localizaltions);
  }
}

class _ChineseDelegate extends LocalizationsDelegate<CupertinoLocalizations> {
  const _ChineseDelegate();

  @override
  bool isSupported(Locale locale) {
    return locale.languageCode == 'zh';
  }

  @override
  Future<CupertinoLocalizations> load(Locale locale) {
    return ChineseCupertinoLocalizations.load(locale);
  }

  @override
  bool shouldReload(LocalizationsDelegate<CupertinoLocalizations> old) {
    return false;
  }
}

这个东西弄到项目里

然后在 Application 里配置一下

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
      localizationsDelegates: <LocalizationsDelegate<dynamic>>[
        ChineseCupertinoLocalizations.delegate, // 这里加上这个,是自定义的delegate

        DefaultCupertinoLocalizations.delegate, // 这个截止目前只包含英文

        // 下面两个是Material widgets的delegate, 包含中文
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
      ],
      supportedLocales: [
        const Locale('en', 'US'), // English
        const Locale('zh', 'Hans'), // China
        const Locale('zh', ''), // China
        // ... other locales the app supports
      ],
    );
  }
}

这样就能完成默认的中文的本地化,当然之前那个 Cupertino 的 delegate 实际上也是借助了 Material 中的 delegate 本地化

这里还要注意,一定要保证你的设备是本地语言是中文(因为很多朋友是使用模拟器开发的,默认是英文)

原因分析

这里要解析一波源码了,为什么会出现这种情况呢

主要原因就是在某个版本,加入了一整套的 Cupertino 相关的支持,但是又因为某些原因遗忘了非英文版本,造成了默认的情况下,不包含其他语言

而在未设置对应语言的 delegate 时,又没有一个默认的 delegate,从而造成空指针异常,进而抛出错误

源码解析

20190109135322.png

这个是 Cupertino 中,弹出剪切/复制/粘贴/全选那个按钮那个 toolbar 的配置界面,这里会使用到这个类,然而又因为得到的是空的,所以会报空指针异常

而获取的方式是使用CupertinoLocalizations.of(context);的方式

查看这个方法的定义处又可以跟踪到另一个地方

20190109135531.png

20190109135615.png

然后来到这段代码

Future<Map<Type, dynamic>> _loadAll(Locale locale, Iterable<LocalizationsDelegate<dynamic>> allDelegates) {
  final Map<Type, dynamic> output = <Type, dynamic>{};
  List<_Pending> pendingList;

  // Only load the first delegate for each delegate type that supports
  // locale.languageCode.
  final Set<Type> types = Set<Type>();
  final List<LocalizationsDelegate<dynamic>> delegates = <LocalizationsDelegate<dynamic>>[];
  for (LocalizationsDelegate<dynamic> delegate in allDelegates) {
    if (!types.contains(delegate.type) && delegate.isSupported(locale)) {
      types.add(delegate.type);
      delegates.add(delegate);
    }
  }

  for (LocalizationsDelegate<dynamic> delegate in delegates) {
    final Future<dynamic> inputValue = delegate.load(locale);
    dynamic completedValue;
    final Future<dynamic> futureValue = inputValue.then<dynamic>((dynamic value) {
      return completedValue = value;
    });
    if (completedValue != null) { // inputValue was a SynchronousFuture
      final Type type = delegate.type;
      assert(!output.containsKey(type));
      output[type] = completedValue;
    } else {
      pendingList ??= <_Pending>[];
      pendingList.add(_Pending(delegate, futureValue));
    }
  }

  // All of the delegate.load() values were synchronous futures, we're done.
  if (pendingList == null)
    return SynchronousFuture<Map<Type, dynamic>>(output);

  // Some of delegate.load() values were asynchronous futures. Wait for them.
  return Future.wait<dynamic>(pendingList.map<Future<dynamic>>((_Pending p) => p.futureValue))
    .then<Map<Type, dynamic>>((List<dynamic> values) {
      assert(values.length == pendingList.length);
      for (int i = 0; i < values.length; i += 1) {
        final Type type = pendingList[i].delegate.type;
        assert(!output.containsKey(type));
        output[type] = values[i];
      }
      return output;
    });
}

简单来说,这个方法是根据本地的语言读取所有支持这个语言的 delegate

然后问题来了, 没读取到怎么办呢

有几处代码需要关注一下

  1. 20190109143609.png

  2. 20190109143806.png

  3. 20190109144442.png

这里就看出来了,为什么会有空指针异常的原因 3 => 2 => 1 但是 1 也没读取到 ,自然就空指针了

后记

本篇介绍了解决方案 pastelabel copylabel 各种 label 空指针的原因和解决方案