[fluent] dart technique (independent main function entry | nullable type determination | default value setting)

One 、 Independent main function entry

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stay dart Declaration in the document main() function , You can leave Flutter Environmental Science , Execute this independently main function ;

///  Detachable  Flutter  Functions executed independently by the environment void main(){ 
      print("main");}

main There are two triangular icons on the left side of the function , Click the button , The... Can be executed main function ;

Two 、 Nullable type determination

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Nullable type determination : Not sure if the object is empty , adopt ?. Method call , ( Be similar to Kotlin The nullable type of ) , ? The function of is to determine whether it is empty , If it is empty , Terminate the call , No null pointer exception will be reported ;

///  Detachable  Flutter  Functions executed independently by the environment void main(){ 
      print("main");  Student student;  /// 1.  Security call  :  Not sure if the object is empty  ,  adopt  ?.  Method call   /// (  Be similar to  Kotlin  The nullable type of  )  /// ?  The function of is to determine whether it is empty  ,  If it is empty  ,  Terminate the call   print(" Print  student  name  : ${student?.name}");}class Student{ 
      String name;  int age;  Student(this.name, this.age);}

Execution results :

main Print  student  name  : null

3、 ... and 、 Default settings

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Use ?? You can set a default value for a null value , If a value is not obtained , Or get null , You can set a default value for this variable or expression ;

student?.name??"Tom"

The function of the above code is if student?.name Value is empty , Then return to “Tom” The default value is ;

Code example :

  /// 2.  Set the default value   ///  Use  ??  You can set a default value for a null value   ///  Here if  student?.name  It's empty  ,  The default value is  "Tom"  print(" Print  student  name  : ${student?.name??"Tom"}");

Execution results :

 Print  student  name  : Tom

Four 、 Complete code example

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Complete code example :

///  Detachable  Flutter  Functions executed independently by the environment void main(){ 
      print("main");  Student student;  /// 1.  Security call  :  Not sure if the object is empty  ,  adopt  ?.  Method call   /// (  Be similar to  Kotlin  The nullable type of  )  /// ?  The function of is to determine whether it is empty  ,  If it is empty  ,  Terminate the call   print(" Print  student  name  : ${student?.name}");  /// 2.  Set the default value   ///  Use  ??  You can set a default value for a null value   ///  Here if  student?.name  It's empty  ,  The default value is  "Tom"  print(" Print  student  name  : ${student?.name??"Tom"}");}class Student{ 
      String name;  int age;  Student(this.name, this.age);}

Code execution result :

main Print  student  name  : null Print  student  name  : Tom

5、 ... and 、 Related resources

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Reference material :

• Flutter Official website : https://flutter.dev/
• Flutter Developing documents : https://flutter.cn/docs ( Strongly recommend )
• official GitHub Address : https://github.com/flutter
• Flutter The Chinese community : https://flutter.cn/
• Flutter Practical tutorial : https://flutter.cn/docs/cookbook
• Flutter CodeLab : https://codelabs.flutter-io.cn/
• Dart Chinese document : https://dart.cn/
• Dart Developer website : https://api.dart.dev/
• Flutter Chinese net ( unofficial , The translation is very good ) : https://flutterchina.club/ , http://flutter.axuer.com/docs/
• Flutter Related issues : https://flutterchina.club/faq/ ( It is recommended to watch it at the introductory stage )

Blog source download :

• GitHub Address : https://github.com/han1202012/flutter_app_hello ( Keep updating with the progress of the blog , There may not be the source code of this blog )

• Blog source snapshot : https://download.csdn.net/download/han1202012/15463304( The source code snapshot of this blog , You can find the source code of this blog )