In this section, the target

What does air safety mean
How to migrate code
How do I disable air security
Code specification examples

video

www.bilibili.com/video/bv1g5…

code

Github.com/ducafecat/g…

reference

dart.cn/null-safety
The dart. Cn/null – safety…
The dart. Cn/null – safety…
The dart. Cn/null – safety…
The dart. Cn/null – safety…

The body of the

What does air safety mean

Default non-null

String title = 'ducafecat';
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type?The operator

String? title = null;
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value!The operator

String? title = 'ducafecat';
StringnewTitle = title! ;Copy the code

value?The operator

String? title = 'ducafecat';
boolisEmpty = title? .isEmpty();Copy the code

value??The operator

String? title = 'ducafecat';
String newTitle = title ?? 'cat';
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lateThis is checked at run time. So use it only if you are sure it will be initialized before it is used

late String? title;
title = 'ducafecat';
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The List, the generic

type	Whether the set is nullable	Whether the data item is nullable
List	no	no
List?	yes	no
List<String? >	no	yes
List<String? >?	yes	yes

type	Whether the set is nullable	Whether the data item is nullable
Map<String, int>	no	no*
Map<String, int>?	yes	no*
Map<String, int? >	no	yes
Map<String, int? >?	yes	yes

* May return null

// It is possible to return null
int value = <String.int> {'one': 1} ['one']; // ERROR

// Need to add type?
int? value = <String.int> {'one': 1} ['one']; // OK

/ / or the value!
int value = <String.int> {'one': 1} ['one']! ;// OK

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Benefits brought about

Healthier code
Good user experience
Run faster
Compiled files are smaller

Enable and migrate

pubspec.yaml

Environment: the SDK: "> = 2.12.0 < 3.0.0"Copy the code

Transfer order

We strongly recommend that you migrate your code sequentially, starting with the dependencies at the bottom of the dependency relationship. For example, if C depends on B and B depends on A, the migration should be in the order of A -> B -> C.

Checking dependencies

Dart --version > Dart --version > Dart pub outdated --mode=null-safetyCopy the code

Upgrade depend on

Dart pub upgrade --null-safety dart pub upgradeCopy the code

The migration tool

> dart migrate
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Analysis of the

> dart analyze
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Disable air safety

Cli command

> dart --no-sound-null-safety run
> flutter run --no-sound-null-safety
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.vscode/launch.json

{
  // Use IntelliSense to learn about related attributes.
  // Hover to view descriptions of existing properties.
  / / for more information, please visit: https://go.microsoft.com/fwlink/?linkid=830387
  "version": "0.2.0"."configurations": [{"name": "getx_quick_start"."request": "launch"."type": "dart"."program": "lib/main.dart"."args": ["--no-sound-null-safety"]]}}Copy the code

Examples, specifications

The dart. Cn/null – safety…

Explicitly handle empty state

makeCoffee(String coffee, [String? dairy]) {
  if(dairy ! =null) {
    print('$coffee with $dairy');
  } else {
    print('Black $coffee'); }}Copy the code

** Top-level variables and static fields must contain an initialization method. ** Since they can be accessed from anywhere in the program, the compiler cannot guarantee that they are assigned values before they are used. The only safe option is to require that the initialization expression itself be included to ensure that a value of the matching type is produced.

int topLevel = 0;

class SomeClass {
  static int staticField = 0;
}
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The fields of an instance must also include an initialization method when declared, either in the usual initialization form or in the constructor of the instance.

class SomeClass {
  int atDeclaration = 0;
  int initializingFormal;
  int initializationList;

  SomeClass(this.initializingFormal)
      : initializationList = 0;
}
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Local variables are the most flexible. A non-empty variable does not necessarily require an initialization method.

int tracingFibonacci(int n) {
  int result;
  if (n < 2) {
    result = n;
  } else {
    result = tracingFibonacci(n - 2) + tracingFibonacci(n - 1);
  }

  print(result);
  return result;
}
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Process analysis, where Dart willobjectThe type declared from itObjecttoList. The following program cannot run until air safety is introduced.

// With (or without) null safety:
bool isEmptyList(Object object) {
  if (object is List) {
    return object.isEmpty; // <-- OK!
  } else {
    return false;
  }
}

->

// Without null safety:
bool isEmptyList(Object object) {
  if (object is! List) return false;
  return object.isEmpty;
}
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Absolute assignment analysis

int tracingFibonacci(int n) {
  final int result;
  if (n < 2) {
    result = n;
  } else {
    result = tracingFibonacci(n - 2) + tracingFibonacci(n - 1);
  }

  print(result);
  return result;
}
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Warning of useless code

String checkList(List list) {
  if(list? .isEmpty) {return 'Got nothing';
  }
  return 'Got something';
}
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Lazy-loaded variables,lateThe modifier is “constrain variables at run time, not compile time.” This makeslateThis term is approximately equal towhenEnforce constraints on variables.

// Using null safety:
class Coffee {
  String? _temperature;

  void heat() { _temperature = 'hot'; }
  void chill() { _temperature = 'iced'; }

  String serve() => _temperature! + ' coffee'; } - >// Using null safety:
class Coffee {
  late String _temperature;

  void heat() { _temperature = 'hot'; }
  void chill() { _temperature = 'iced'; }

  String serve() => _temperature + ' coffee';
}

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late 与 finalUsed in combination with ordinaryfinalUnlike fields, you do not need to initialize them at declaration or construction time. You can load it later somewhere in the run. But you can only do itAt a timeAssigns a value, and it validates at run time.

// Using null safety:
class Coffee {
  late final String _temperature;

  void heat() { _temperature = 'hot'; }
  void chill() { _temperature = 'iced'; }

  String serve() => _temperature + ' coffee';
}
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All parameters here must be passed by name. parametera 和 cIs optional and can be omitted. parameterb 和 dIs required and must be passed when called. Note here that being required is independent of being nullable.

// Using null safety:
function({int? a, required int? b, int? c, required int? d}) {}
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Abstract field

abstract class Cup {
  Beverage get contents;
  setcontents(Beverage); } - >abstract class Cup {
  abstract Beverage contents;
}

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Some assignment calculations can be moved to static initializations.

// Initalized without values
ListQueue _context;
Float32List _buffer;
dynamic _readObject;

Vec2D(Map<String.dynamic> object) {
  _buffer = Float32List.fromList([0.0.0.0]);
  _readObject = object['container'];
  _context = ListQueue<dynamic> (); } - >// Initalized with values
final ListQueue _context = ListQueue<dynamic> ();final Float32List _buffer = Float32List.fromList([0.0.0.0]);
final dynamic _readObject;

Vec2D(Map<String.dynamic> object) : _readObject = object['container'];


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May returnnullFactory method of

factory StreamReader(dynamic data) {
  StreamReader reader;
  if (data is ByteData) {
    reader = BlockReader(data);
  } else if (data is Map) {
    reader = JSONBlockReader(data);
  }
  returnreader; } - >factory StreamReader(dynamic data) {
  if (data is ByteData) {
    // Move the readIndex forward for the binary reader.
    return BlockReader(data);
  } else if (data is Map) {
    return JSONBlockReader(data);
  } else {
    throw ArgumentError('Unexpected type for data'); }}Copy the code

This video document

Ducafecat. Tech / 2021/04/09 /…

GetX Quick Start code

Github.com/ducafecat/g…

News client code

Github.com/ducafecat/f…

Strapi manual translation

getstrapi.cn