Dart series: look at me for security. The security feature in dart is null safety

brief introduction

stay Dart 2.12 Introduced in null safety New features , in other words dart The default types in the program are non empty , Unless you tell the compiler , This type can be empty .

It looks like a small change , But this small change has led to a lot Dart Large version upgrade of package , This leads to the use of Dart2.12 Previous versions follow dart2.12 The later version is completely two different worlds .

Is it really so wonderful ？ Let's see Dart 2.12 null safety Characteristics of .

Non-nullable type

In feeling Non-nullable Before the type , Let's take a look at a piece of code ：

void main(){
  String name;
  print('name is $name.');
}

The code is simple , We define a String Type of name character string , And then in print Print it in the statement .

If you are dart 2.12 Before the release , Then there is no problem .

But here we are 2.12, May be an error ：

The non-nullable local variable 'name' must be assigned before it can be used.

Try giving it an initializer expression, or ensure that it's assigned on every execution path.

That is to say ,name It's not empty , You have to give him a value .

By forcing not to be empty , Ensure the security of the code , Very easy to use .

So if name If it can be empty, how to deal with it ？

Don't worry. , We can add... After the type that can be empty ? that will do ：

void main(){
  String? name;
  print('name is $name.');
}

Nullable List Of Strings and List Of Nullable Strings

If we're going to create a List,list It contains String, You can create ：

List<String> aListOfStrings = ['one', 'two', 'three'];

stay dart 2.12 in , Non null checking is also used in generics , therefore , By default List Medium String It can't be empty , If it has to be empty , It needs to be written like this ：

List<String?> aListOfNullableStrings = ['one', null, 'three'];

about list itself , It can't be empty , If empty , It needs to be written like this ：

List<String>? aNullableListOfStrings;

! The operator

If you think an object is not when it is used null, Then you can add... After the expression ！, As shown below ：

  String? word;
  word = aListOfNullableStrings.first;
  print(word!.length);

late keyword

occasionally , We know that an object must not be empty , But for now , It cannot be assigned immediately , You need to use late keyword .

Here's an example that uses late Example ：

class Team {
  late final Coach coach;
}

class Coach {
  late final Team team;
}

void main() {
  final myTeam = Team();
  final myCoach = Coach();
  myTeam.coach = myCoach;
  myCoach.team = myTeam;

  print('All done!');
}

In the above code , We have two classes that refer to each other , But both classes are not empty , If not used late, Will fail to compile .

Use late You can initialize the properties in the class at an appropriate time later , So as to ensure the operation of the code .

summary

That's all dart 2.12 The new null safety Use .

This article has been included in http://www.flydean.com/24-dart-null-safety/