dart 变量不等于它们在 object 的构造函数中设置的值

Question

I have this class called branch:

class Branch {
  String id;
  String businessName;
  String businessAddress;
  String businessPhone;
  int status;

  Branch({
    businessName = "unknown",
    businessAddress = "unknown",
    businessPhone = "unknown",
    id = "unknown",
    status = -1,
  });

Then I have this simple stateful widget:

class PageMain extends StatefulWidget {

  PageMain({Key key}) : super(key: key);

  @override
  _PageMainState createState() => _PageMainState();
}

class _PageMainState extends State<PageMain> {
  bool _isInitalized = false;    

  void _initializeOnFirstBuild(){
    if(!_isInitialized) {
      Branch testBranch = new Branch(businessName: "test");
      print(testBranch.businessName);
      _isInitialized = true;
    }
  }

  void didChangeDependencies() {
    // TODO: implement didChangeDependencies
    super.didChangeDependencies();
    _initializeOnFirstBuild();
  }
}

when trying to print the name of the business or any other variable that is stored and set in the constructer it returns null. (returns returns "null" instead of "test")

Answer 1

Initialisation in constructor is wrong. You need to use this keyword.

Branch({
    this.businessName = "unknown",
    this.businessAddress = "unknown",
    this.businessPhone = "unknown",
    this.id = "unknown",
    this.status = -1,
});

Answer 2

Please don't use didChangeDependencies .

According to the documentation:

Called when a dependency of this State object changes.

And :

This method is also called immediately after initState. It is safe to call BuildContext.dependOnInheritedWidgetOfExactType from this method.

https://api.flutter.dev/flutter/widgets/SingleTickerProviderStateMixin/didChangeDependencies.html

However, I would not right your business logic in this method, because this method is not guaranteed to always get called, unless any dependencies within the widget change. If nothing changes, then this method does not get called.

There are first off, two problems. First you are not overriding the method in the _PageMainState widget. If you are not overriding the method, then how is _initializeOnFirstBuild supposed to get called if only the base logic runs? So, add an @override attribute on top of the didChangeDependencies method.

@override 
void didChangeDependencies() {
    // TODO: implement didChangeDependencies
    super.didChangeDependencies();
    _initializeOnFirstBuild();
  }

Then it should work.

Second, please instead write your initialisation of the Branch class directly in the initState method, like this:

 class PageMain extends StatefulWidget {
    
      PageMain({Key key}) : super(key: key);
    
      @override
      _PageMainState createState() => _PageMainState();
    }
    
    class _PageMainState extends State<PageMain> {
      bool _isInitalized = false;    
    
      void _initializeOnFirstBuild(){
        if(!_isInitialized) {
          Branch testBranch = new Branch(businessName: "test");
          print(testBranch.businessName);
          _isInitialized = true;
        }
      }
    
      @override
      void initState() {
        super.initState();
        _initializeOnFirstBuild();
      }
    }