flutter Future 使用

下面情况是我使用过程中对Future的误解及处理过程，希望对大家理解有帮助

我写一个Future的函数，希望它被异步调用。代码可能如下：

@override
  Widget build(BuildContext context) {

      //code begin
      debugPrint(DateTime.now().toString() + ":build.1");
      myFutureFunc().then( (int){
          debugPrint(DateTime.now().toString() + ":myFutureFunc.then.ok");
      } );
      debugPrint(DateTime.now().toString() + ":build.2");
      //code end

    return new Scaffold(
        body: Container(
            child: Column(...)//这里忽略代码
            )
        )
    );
  }

  Future<int> myFutureFunc() async{
      debugPrint(DateTime.now().toString() + ":enter");

      return 0;
  }

返回如下：

I/flutter (25508): 2019-10-14 15:31:09.082448:build.1
I/flutter (25508): 2019-10-14 15:31:09.084144:enter
I/flutter (25508): 2019-10-14 15:31:09.084344:build.2
I/flutter (25508): 2019-10-14 15:31:09.120824:myFutureFunc.then.ok

感觉与普通函数没有任何区别。只是 then 函数调用慢了一点点

在函数中增加更多的处理。如下：

  Future<int> myFutureFunc() async{
      debugPrint(DateTime.now().toString() + ":enter1");
      int i=0;
      for(int i=0;i<1000;++i)
          for(int j=0;j<10000;++j)
              for(int k=0;k<10000;++k)
                  ++i;
      debugPrint(DateTime.now().toString() + ":enter2");

      return 0;
  }

结果如下：

I/flutter (25508): 2019-10-14 15:36:25.282415:build.1
I/flutter (25508): 2019-10-14 15:36:25.283749:enter1
I/flutter (25508): 2019-10-14 15:36:25.416389:enter2
I/flutter (25508): 2019-10-14 15:36:25.416638:build.2
I/flutter (25508): 2019-10-14 15:36:25.449129:myFutureFunc.then.ok

里面执行时间长短不影响，与上一次执行相同结果。

替换成 await 对应函数

  Future<int> myFutureFunc() async{
      debugPrint(DateTime.now().toString() + ":enter1");
      await Future.delayed( Duration(seconds: 5));
      debugPrint(DateTime.now().toString() + ":enter2");

      return 0;
  }

结果如下：

I/flutter (25508): 2019-10-14 15:38:22.445591:build.1
I/flutter (25508): 2019-10-14 15:38:22.447336:enter1
I/flutter (25508): 2019-10-14 15:38:22.447710:build.2
I/flutter (25508): 2019-10-14 15:38:27.452447:enter2
I/flutter (25508): 2019-10-14 15:38:27.453455:myFutureFunc.then.ok

可见，碰到了 await ，函数就返回了。再试试验证一下

Future<int> myFutureFunc() async{
      await Future.delayed(Duration(seconds: 0));
      debugPrint(DateTime.now().toString() + ":enter0");

      debugPrint(DateTime.now().toString() + ":enter1");
      await Future.delayed( Duration(seconds: 5));
      debugPrint(DateTime.now().toString() + ":enter2");

      return 0;
  }

I/flutter (25508): 2019-10-14 15:40:26.937157:build.1
I/flutter (25508): 2019-10-14 15:40:26.938874:build.2
I/flutter (25508): 2019-10-14 15:40:26.975199:enter0
I/flutter (25508): 2019-10-14 15:40:26.975438:enter1
I/flutter (25508): 2019-10-14 15:40:31.981472:enter2
I/flutter (25508): 2019-10-14 15:40:31.982531:myFutureFunc.then.ok

的确如此！

因此，函数运行时，只是碰到 await 才会返回 一个 Future 。 我们写异步时用 Future的关键就在这，所以上面的函数，正确的写法应该是：

@override
  Widget build(BuildContext context) {

      //code begin
      debugPrint(DateTime.now().toString() + ":build.1");

      Future((){ this.myFutureFunc();}).then((val){
          debugPrint(DateTime.now().toString() + ":myFutureFunc Done.");
      });

      debugPrint(DateTime.now().toString() + ":build.2");
      //code end

    return new Scaffold(
        body: Container(
            child: Column()// 忽略代码
        )
    );
  }

  int myFutureFunc(){
      debugPrint(DateTime.now().toString() + ":enter1");
      

      return 0;
  }

结果如下：

I/flutter (25508): 2019-10-14 16:00:58.093965:build.1
I/flutter (25508): 2019-10-14 16:00:58.094854:build.2
I/flutter (25508): 2019-10-14 16:00:58.130326:enter1
I/flutter (25508): 2019-10-14 16:00:58.130598:myFutureFunc Done.

它与js中的promise 很像！

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