TextFormField：在构建过程中调用的setState（）或markNeedsBuild（）

Question

我正在尝试将首次按钮设置为禁用，并且当用户输入金额时将其启用，按钮禁用效果很好，但是当我在TextFormField中输入金额时，它给出以下错误。

   I/flutter (29519): ??? EXCEPTION CAUGHT BY WIDGETS LIBRARY 
   ????????????????????????????????????????????????????????????
   I/flutter (29519): The following assertion was thrown building 
   TextFormField(dirty, state: _TextFormFieldState#b89da):
   I/flutter (29519): setState() or markNeedsBuild() called during build.
   I/flutter (29519): This HomePage widget cannot be marked as needing to 
   build because the framework is already in the
   I/flutter (29519): process of building widgets. A widget can be marked as 
   needing to be built during the build phase
   I/flutter (29519): only if one of its ancestors is currently building. 
   This exception is allowed because the framework
   I/flutter (29519): builds parent widgets before children, which means a 
   dirty descendant will always be built.
   I/flutter (29519): Otherwise, the framework might not visit this widget 
   during this build phase.
   I/flutter (29519): The widget on which setState() or markNeedsBuild() was 
   called was:

我的代码：

var _onPressed;
if (isButtonDisabled) {
  _onPressed = () {
    print("Hello");
  };
}

TextFormField代码：

child: TextFormField(
  decoration: InputDecoration(
    contentPadding: EdgeInsets.only(
      left: 12.0,
      right: 12.0,
      top: 12.0,
      bottom: 12.0
    ),
    labelText: 'Enter amount',
    hintText: 'Enter amount',
    hintStyle: TextStyle(
      color: Colors.red,
      fontSize: 14.0
    ),
    border: OutlineInputBorder(
      borderRadius: BorderRadius.circular(0.0)
    ),
  ),
  keyboardType: TextInputType.number,
  autovalidate: true,
  validator: (String txt) {
    if (txt.length == 2) {
      setState(() {
        isButtonDisabled = true;
      });
    } else {
      setState(() {
        isButtonDisabled = false;
      });
    }
  }
),       

按钮代码：

FlatButton(
  child: Text("Confirm"),
  onPressed: _onPressed,
)                                  

Answer 1

setState从中删除

   if (txt.length == 2){
     setState((){
     isButtonDisabled = true;
     });
   } else {
     setState((){
       isButtonDisabled = false;
      });
     }}),   

至

   if (txt.length == 2){
     isButtonDisabled = true;
   } else {
       isButtonDisabled = false;
   }}),   

仅当您传递如下函数时，才需要调用setState()直接在中执行的代码build()

onPressed: () {
  // code here is not executed in `build()`, 
  it's just passed to `onPressed` to be executed when the button is tapped
  // therefore here `setState()` is required for state changes 
}

validator: () {...} 看起来很相似，但是不会更新小部件的状态。