Question

到目前为止，每当我需要在Widget中使用条件语句时，我已经完成了以下操作（使用Center和Containers作为简化的虚拟示例）：

new Center(
  child: condition == true ? new Container() : new Container()
)

虽然当我尝试使用if / else语句时，会导致死代码警告：

new Center(
  child: 
    if(condition == true){
      new Container();
    }else{
      new Container();
    }
)

有趣的是，我尝试使用switch case语句，它给了我相同的警告，因此我无法运行代码。我做错了什么，或者是否因为没有颤抖的人认为有死代码而无法使用if / else或switch语句？

Answer 1

实际上，您可以使用if/else和switch以及飞镖/颤振内联的任何其他语句。

使用立即匿名功能

class StatmentExample extends StatelessWidget {
  Widget build(BuildContext context) {
    return Text((() {
      if(true){
        return "tis true";}

      return "anything but true";
    })());
  }
}

即将您的语句包装在函数中

(() {
  // your code here
}())

我强烈建议不要在您的UI“标记”中直接放置太多逻辑，但是我发现Dart中的类型推断需要一点工作，因此有时在类似情况下会很有用。

使用三元运算符

condition? Text("True"): null,

使用If或For语句或集合中的散布运算符

children: [
  ...manyItems,
  oneItem,
  if(canIKickIt)
    ...kickTheCan
  for (item in items)
    Text(item)

使用方法

child: getWidget()

Widget getWidget() {
  if (x > 5) ...
  //more logic here and return a Widget

重新定义switch语句

作为三元运算符的另一种选择，您可以创建switch语句的函数版本，例如下面的帖子https://stackoverflow.com/a/57390589/1058292。

  child: case2(myInput,
  {
    1: Text("Its one"),
    2: Text("Its two"),
  }, Text("Default"));

Answer 2

其他答案没有提及，您也可以：

使用构建器小部件

Builder widget用于允许在需要子窗口小部件时使用闭包：

一个柏拉图式小部件，它调用一个闭包来获取其子小部件。

在需要逻辑来构建窗口小部件的任何时候都很方便，它避免了创建专用功能的需要。而且我发现与使用立即匿名功能相比，它使所做的工作更加清晰。

您将Builder小部件用作子级，并在其builder方法中提供逻辑。以您的示例为例：

Center(
  child: Builder(
    builder: (context) {
      if (condition) {
        return Container();
      } else {
        return Center();
      }
    }
  )
)

在这样一个简单的示例中，三元运算符就足够了（child: condition ? Container() : Center()），但是如果需要更多的逻辑，Builder提供了一个方便的位置。

我也同意其他张贴者的意见，认为应该从UI代码中提取逻辑，但是当仅使用switch或if / else语句来确定要构建的小部件时，我宁愿将其保留在此处（前提是条件很简单，例如在ViewModel中测试一个值）。生成器小部件可以帮助您解决问题。

Answer 3

在Dart中，if/else和switch是语句而不是表达式。它们不会返回值，因此您无法将它们传递给构造函数参数。如果构建方法中有很多条件逻辑，那么尝试简化它是一个好习惯。例如，您可以将自包含逻辑移动到方法，并使用if/else语句初始化您可以在以后使用的局部变量。

使用方法和if / else

Widget _buildChild() {
  if (condition) {
    return ...
  }
  return ...
}

Widget build(BuildContext context) {
  return new Container(child: _buildChild());
}

使用`if/else`

Widget build(BuildContext context) {
  Widget child;
  if (condition) {
    child = ...
  } else {
    child = ...
  }
  return new Container(child: child);
}

Answer 4

我个人将if / else语句用于带有此类block语句的儿童。它仅在上述Dart 2.3.0版中支持。

如果/否则

Column(
    children: [
        if (_selectedIndex == 0) ...[
          DayScreen(),
        ] else ...[
          StatsScreen(),
        ],
    ],
 ),

如果/否则，如果

Column(
    children: [
        if (_selectedIndex == 0) ...[
          DayScreen(),
        ] else if(_selectedIndex == 1)...[
          StatsScreen(),
        ],
    ],
 ),

Answer 5

为便于记录，Dart 2.3增加了在Collection文字中使用if / else语句的功能。现在可以通过以下方式完成此操作：

return Column(children: <Widget>[
  Text("hello"),
  if (condition)
     Text("should not render if false"),
  Text("world")
],);

Flutter Issue #28181 - Inline conditional rendering in list

Answer 6

这是解决方案。我已经解决了。这是代码

child: _status(data[index]["status"]),

Widget _status(status) {
  if (status == "3") {
    return Text('Process');
  } else if(status == "1") {
    return Text('Order');
  } else {
    return Text("Waiting");
  }
}

Answer 7

除了三元运算符，如果您需要在条件语句之前执行操作，您还可以使用 Builder 小部件。

Container(
  child: Builder(builder: (context) {
     /// some operation here ...
     if(someCondition) {
       return Text('A');
     }
     else return Text('B');
    })
 )

Answer 8

我发现，使用条件逻辑构建Flutter UI的一种简单方法是将逻辑保留在UI之外。这是一个返回两种不同颜色的函数：

Color getColor(int selector) {
  if (selector % 2 == 0) {
    return Colors.blue;
  } else {
    return Colors.blueGrey;
  }
}

下面该函数用于设置CircleAvatar的背景。

new ListView.builder(
  itemCount: users.length,
  itemBuilder: (BuildContext context, int index) {
    return new Column(
      children: <Widget>[
        new ListTile(
          leading: new CircleAvatar(
            backgroundColor: getColor(index),
            child: new Text(users[index].name[0])
          ),
          title: new Text(users[index].login),
          subtitle: new Text(users[index].name),
        ),
        new Divider(height: 2.0),
      ],
    );
  },
);

非常整洁，因为您可以在多个小部件中重复使用颜色选择器功能。

Answer 9

使用几个月后大声笑？：我发现我可以使用它：

Column(
     children: [
       if (true) Text('true') else Text('false'),
     ],
   )

Answer 10

您可以简单地使用条件语句a==b?c:d

例如：

Container(
  color: Colors.white,
  child: ('condition')?
   Widget1(...)
  :Widget2(...)
)

我希望你有主意。

Answer 11

这是一篇很棒的文章和对话。我尝试使用所述的三元运算符。但是代码无法正常工作，导致出现错误。

Column(children: [ condition? Text("True"): null,],);

上面的三元示例是小姐率先。 Dart将返回一个错误，返回一个空值而不是小部件。您不能返回null。正确的方法是返回小部件：

Column(children: [ condition? Text("True"): Text("false"),],);

为了使三元组工作，您需要返回一个Widget。如果您不想返回任何内容，则可以返回一个空容器。

Column(children: [ condition? Text("True"): Container(),],);

祝你好运。

Answer 12

带有按钮

bool _paused = false;

CupertinoButton(
  child: _paused ? Text('Play') : Text('Pause'),
  color: Colors.blue,
  onPressed: () {
    setState(() {
      _paused = !_paused;
    });
  },
),

Answer 13

如果要进行条件渲染，请执行以下操作：

Column(
   children: <Widget>[
     if (isCondition == true)
        Text('The condition is true'),
   ],
 );

但是，如果您想使用第三级（如果不是）条件怎么办？当子窗口小部件是多层时。

您可以将其用于解决方案flutter_conditional_rendering，这是一个flutter软件包，可增强条件渲染，支持if-else和switch条件。

其他条件：

Column(
      children: <Widget>[
        Conditional.single(
          context: context,
          conditionBuilder: (BuildContext context) => someCondition == true,
          widgetBuilder: (BuildContext context) => Text('The condition is true!'),
          fallbackBuilder: (BuildContext context) => Text('The condition is false!'),
        ),
      ],
    );

切换条件：

Column(
      children: <Widget>[
        ConditionalSwitch.single<String>(
          context: context,
          valueBuilder: (BuildContext context) => 'A',
          caseBuilders: {
            'A': (BuildContext context) => Text('The value is A!'),
            'B': (BuildContext context) => Text('The value is B!'),
          },
          fallbackBuilder: (BuildContext context) => Text('None of the cases matched!'),
        ),
      ],
    );

如果要有条件地呈现窗口小部件(List<Widget>)列表，而不是单个窗口。使用Conditional.list()和ConditionalSwitch.list()!

Answer 14

在我的应用中，我创建了一个WidgetChooser小部件，因此可以在无条件逻辑的小部件之间进行选择：

WidgetChooser(
      condition: true,
      trueChild: Text('This widget appears if the condition is true.'),
      falseChild: Text('This widget appears if the condition is false.'),
    );

这是WidgetChooser小部件的来源：

import 'package:flutter/widgets.dart';

class WidgetChooser extends StatelessWidget {
  final bool condition;
  final Widget trueChild;
  final Widget falseChild;

  WidgetChooser({@required this.condition, @required this.trueChild, @required this.falseChild});

  @override
  Widget build(BuildContext context) {
    if (condition) {
      return trueChild;
    } else {
      return falseChild;
    }
  }
}

Answer 15

您可以在dart中将三元运算符用于条件语句，它的使用很简单

(condition) ? statement1 : statement2

如果condition为true，则将执行statement1，否则为statement2。

举一个实际的例子

Center(child: condition ? Widget1() : Widget2())

请记住，如果您打算将null用作Widget2，则最好使用SizedBox.shrink()，因为某些父窗口小部件在获得null子代之后会引发异常。 / p>

Answer 16

如果您使用小部件列表，则可以使用以下方法：

class HomePage extends StatelessWidget {
  bool notNull(Object o) => o != null;
  @override
  Widget build(BuildContext context) {
    var condition = true;
    return Scaffold(
      appBar: AppBar(
        title: Text("Provider Demo"),
      ),
      body: Center(
          child: Column(
        children: <Widget>[
          condition? Text("True"): null,
          Container(
            height: 300,
            width: MediaQuery.of(context).size.width,
            child: Text("Test")
          )
        ].where(notNull).toList(),
      )),
    );
  }
}

Answer 17

Flutter 中的条件渲染可以通过 proviso 包轻松完成。它有一套全面的条件小部件和构建器，可以使条件语句代码更具可读性和更简单。

API 和帮助程序包括但不限于：

条件小部件和构建器：

ConditionWidget(
  condition: starred,
  widget: Icon(
    Icons.favorite
  ),
  fallback: fallbackWidget
)

ConditionBuilder(
  condition: (_) => someCondition,
  trueBuilder: (_) => trueWidget,
  fallbackBuilder: (_) => fallbackWidget
)

切换大小写条件：

SwitchCaseBuilder.widget<String>(
  context: context,
  condition: (_) => '1',
  caseBuilders: {'1': (_) => someWidget(), '2': (_) => someWidget()},
  fallbackBuilder: (_) => fallbackWidget,
);

甚至是有条件的父小部件

ConditionalWrap(
  shouldWrap: shouldWrapChildInParent,
  child: Container(),
  parentBuilder: (child) => Container(
    child: child,
  ),
)

API 支持单个或多个小部件呈现。欢迎您试一试。

Answer 18

使用conditional_builder 1.0.2

请参阅以下链接

Click here

Answer 19

****您还可以通过此方法使用条件** **

 int _moneyCounter = 0;
  void _rainMoney(){
    setState(() {
      _moneyCounter +=  100;
    });
  }

new Expanded(
          child: new Center(
            child: new Text('\$$_moneyCounter', 

            style:new TextStyle(
              color: _moneyCounter > 1000 ? Colors.blue : Colors.amberAccent,
              fontSize: 47,
              fontWeight: FontWeight.w800
            )

            ),
          ) 
        ),

Answer 20

另一种选择：对于带有很多条件的类似“ javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.778 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.779 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.779 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.780 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_RSA_WITH_AES_256_GCM_SHA384 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.780 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.780 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.781 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.781 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.781 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.782 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_RSA_WITH_AES_128_GCM_SHA256 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.782 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.782 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.782 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.783 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.783 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.783 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.783 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_RSA_WITH_AES_256_CBC_SHA256 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.783 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.784 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.784 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: T LS_DHE_RSA_WITH_AES_256_CBC_SHA256 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.784 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_DHE_DSS_WITH_AES_256_CBC_SHA256 for TLS11 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.784 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.784 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.784 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.784 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_RSA_WITH_AES_256_GCM_SHA384 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.785 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.785 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.785 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.785 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.785 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.785 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_RSA_WITH_AES_128_GCM_SHA256 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.785 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.785 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.786 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.786 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.786 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.786 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.786 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_RSA_WITH_AES_256_CBC_SHA256 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.786 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.786 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.786 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_DHE_RSA_WITH_AES_256_CBC_SHA256 for TLS10 javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.787 MYT|HandshakeContext.java:294|Ignore unsupported cipher suite: TLS_DHE_DSS_WITH_AES_256_CBC_SHA256 for TLS10 javax.net.ssl|WARNING|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.818 MYT|SignatureScheme.java:282|Signature algorithm, ed25519, is not supported by the underlying providers javax.net.ssl|WARNING|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.818 MYT|SignatureScheme.java:282|Signature algorithm, ed448, is not supported by the underlying providers javax.net.ssl|ALL|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.822 MYT|SignatureScheme.java:358|Ignore disabled signature sheme: rsa_md5 javax.net.ssl|INFO|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.822 MYT|AlpnExtension.java:161|No available application protocols javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.823 MYT|SSLExtensions.java:256|Ignore, context unavailable extension: application_layer_protocol_negotiation javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.823 MYT|SSLExtensions.java:256|Ignore, context unavailable extension: renegotiation_info javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.825 MYT|ClientHello.java:651|Produced ClientHello handshake message ( "ClientHello": { "client version" : "TLSv1.2", "random" : "FB BC CD 7C 17 65 86 49 3E 1C 15 37 24 94 7D E7 60 44 1B B8 F4 18 21 D0 E1 B1 31 0D E1 80 D6 A7", "session id" : "", "cipher suites" : "[TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384(0xC02C), TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256(0xC02B), TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384(0xC030), TLS_RSA_WITH_AES_256_GCM_SHA384(0x009D), TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384(0xC02E), TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384(0xC032), TLS_DHE_RSA_WITH_AES_256_GCM_SHA384(0x009F), TLS_DHE_DSS_WITH_AES_256_GCM_SHA384(0x00A3), TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256(0xC02F), TLS_RSA_WITH_AES_128_GCM_SHA256(0x009C), TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256(0xC02D), TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256(0xC031), TLS_DHE_RSA_WITH_AES_128_GCM_SHA256(0x009E), TLS_DHE_DSS_WITH_AES_128_GCM_SHA256(0x00A2), TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384(0xC024), TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384(0xC028), TLS_RSA_WITH_AES_256_CBC_SHA256(0x003D), TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384(0xC026), TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384(0xC02A), TLS_DHE_RSA_WITH_AES_256_CBC_SHA256(0x006B), TLS_DHE_DSS_WITH_AES_256_CBC_SHA256(0x006A), TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA(0xC00A), TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA(0xC014), TLS_RSA_WITH_AES_256_CBC_SHA(0x0035), TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA(0xC005), TLS_ECDH_RSA_WITH_AES_256_CBC_SHA(0xC00F), TLS_DHE_RSA_WITH_AES_256_CBC_SHA(0x0039), TLS_DHE_DSS_WITH_AES_256_CBC_SHA(0x0038), TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256(0xC023), TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256(0xC027), TLS_RSA_WITH_AES_128_CBC_SHA256(0x003C), TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256(0xC025), TLS_ECDH_RSA_WITH_AES_128_CBC_SHA256(0xC029), TLS_DHE_RSA_WITH_AES_128_CBC_SHA256(0x0067), TLS_DHE_DSS_WITH_AES_128_CBC_SHA256(0x0040), TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA(0xC009), TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA(0xC013), TLS_RSA_WITH_AES_128_CBC_SHA(0x002F), TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA(0xC004), TLS_ECDH_RSA_WITH_AES_128_CBC_SHA(0xC00E), TLS_DHE_RSA_WITH_AES_128_CBC_SHA(0x0033), TLS_DHE_DSS_WITH_AES_128_CBC_SHA(0x0032), TLS_EMPTY_RENEGOTIATION_INFO_SCSV(0x00FF)]", "compression methods" : "00", "extensions" : [ "server_name (0)": { type=host_name (0), value=mq.tpc-ohcis.moh.gov.my }, "status_request (5)": { "certificate status type": ocsp "OCSP status request": { "responder_id": <empty> "request extensions": { <empty> } } }, "supported_groups (10)": { "versions": [secp256r1, secp384r1, secp521r1, sect283k1, sect283r1, sect409k1, sect409r1, sect571k1, sect571r1, secp256k1, ffdhe2048, ffdhe3072, ffdhe4096, ffdhe6144, ffdhe8192] }, "ec_point_formats (11)": { "formats": [uncompressed] }, "signature_algorithms (13)": { "signature schemes": [ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384, ecdsa_secp512r1_sha512, rsa_pss_rsae_sha256, rsa_pss_rsae_sha384, rsa_pss_rsae_sha512, rsa_pss_pss_sha256, rsa_pss_pss_sha384, rsa_pss_pss_sha512, rsa_pkcs1_sha256, rsa_pkcs1_sha384, rsa_pkcs1_sha512, dsa_sha256, ecdsa_sha224, rsa_sha224, dsa_sha224, ecdsa_sha1, rsa_pkcs1_sha1, dsa_sha1] }, "signature_algorithms_cert (50)": { "signature schemes": [ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384, ecdsa_secp512r1_sha512, rsa_pss_rsae_sha256, rsa_pss_rsae_sha384, rsa_pss_rsae_sha512, rsa_pss_pss_sha256, rsa_pss_pss_sha384, rsa_pss_pss_sha512, rsa_pkcs1_sha256, rsa_pkcs1_sha384, rsa_pkcs1_sha512, dsa_sha256, ecdsa_sha224, rsa_sha224, dsa_sha224, ecdsa_sha1, rsa_pkcs1_sha1, dsa_sha1] }, "status_request_v2 (17)": { "cert status request": { "certificate status type": ocsp_multi "OCSP status request": { "responder_id": <empty> "request extensions": { <empty> } } } }, "extended_master_secret (23)": { <empty> }, "supported_versions (43)": { "versions": [TLSv1.2, TLSv1.1, TLSv1] } ] } ) javax.net.ssl|DEBUG|43|SimpleAsyncTaskExecutor-1|2019-07-03 17:35:01.829 MYT|Alert.java:238|Received alert message ( "Alert": { "level" : "fatal", "description": "insufficient_security" } )”的语句，我喜欢使用地图：

switch's

在不触摸条件语句的情况下将元素添加/删除到条件列表更加容易。

另一个例子：

return Card(
        elevation: 0,
        margin: EdgeInsets.all(1),
        child: conditions(widget.coupon)[widget.coupon.status] ??
            (throw ArgumentError('invalid status')));


conditions(Coupon coupon) => {
      Status.added_new: CheckableCouponTile(coupon.code),
      Status.redeemed: SimpleCouponTile(coupon.code),
      Status.invalid: SimpleCouponTile(coupon.code),
      Status.valid_not_redeemed: SimpleCouponTile(coupon.code),
    };

Answer 21

child: Container(
   child: isFile == true ? 
            Image.network(pathfile, width: 300, height: 200, fit: BoxFit.cover) : 
            Text(message.subject.toString(), style: TextStyle(color: Colors.white),
      ),
),

Answer 22

我不知道这是否是一个好的做法，但我正在使用：

class StatmentExample extends StatelessWidget {
  Widget build(BuildContext context) {
    return pageValue==1 ? Page1():pageValue== 2? Page2():pageValue==3 ? Page3():Page4();
  }
}

Answer 23

这样做

Widget showIf(bool shouldShow, Widget widget) {
if (shouldShow) {
  return widget;
} else {
  return Container();
}}

所以当你想展示一些有条件的东西时，你喜欢说

Column(children: [showIf(myConditionIsTrue, myComplexWidget)])

Answer 24

您可以在以下人员配置中使用构建器：我考虑了一个条件，我们可以将图像 url 设为 null，因此如果为 null，我会显示一个缩小大小的框，因为它没有完全无效的小部件属性。

Builder(builder: (BuildContext context) {
  if (iconPath != null) {
    return ImageIcon(
      AssetImage(iconPath!),
      color: AppColors.kPrimaryColor,
    );
  } else {
    return SizedBox.shrink();
  }
})

Answer 25

仅在振动小部件时

if(bool = true) Container(

child: ....

),

OR

if(bool = true) Container(

child: ....

) else new Container(child: lalala),

如何在Flutter Widget（Center Widget）的子属性中使用条件语句

25 个答案:

使用立即匿名功能

使用三元运算符

使用If或For语句或集合中的散布运算符

使用方法

重新定义switch语句

使用构建器小部件

使用方法和if / else

使用`if/else`

如何在Flutter Widget（Center Widget）的子属性中使用条件语句

25 个答案:

使用立即匿名功能

使用三元运算符

使用If或For语句或集合中的散布运算符

使用方法

重新定义switch语句

使用构建器小部件

使用方法和if / else

使用if/else

使用`if/else`