Flutter进阶(2)：控件Widget的自定义与封装

前言

如果你不是没有任何开发经验，那么你一定知道任何app里都有可能有重用性比较高的控件。所以对于那么重用性比较高的，或者需要你自定义的控件的，我们需要将它们给封装起来，以便下次或者其他app中继续使用。这也正式本节想要说的内容Flutter中如何封装Widget。

下面我从自己实现一个满意的封装，分别介绍你可能用到的三种封装方式

• 1、函数式封装
• 2、以继承 StatefulWidget 的方式封装
• 3、继承父类式封装（推荐）

下面我们以登录页的文本框的自定义来谈封装。

用户名登录的UI图.png

一、函数式封装

/// 蓝色背景按钮(常用于：登录按钮)
/// 方法1:以函数的方法实现
FlatButton blueButton(String text, bool enable, VoidCallback enableOnPressed) {
  return FlatButton(
    child: Text(text),
    splashColor: Colors.transparent,
    color: Color(0xff01adfe),
    textColor: Colors.white,
    highlightColor: Color(0xff1393d7),
    disabledColor: Color(0xffd3d3d5),
    disabledTextColor: Colors.white,
    shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.circular(5.0)
    ),
    //onPressed: enable ? enableOnPressed : null,
    onPressed: enable ?  () {
      enableOnPressed();
    } : null,
  );
}

乍看没什么问题，好像很简洁。但当你也用这种方式来实现文本框的时候，其代码如下：

/// 文本框(常用于：登录用户名、密码文本框)
/// 方法1:以函数的方法实现
TextField loginTextField(String placeholder, String prefixIconImageName, ValueChanged<String> onSubmitted) {
  return TextField(
    //autofocus: shouldAutofocusUserNameTextField,
    style: TextStyle(color: Colors.black, fontSize: 17.0),
    decoration: InputDecoration(
      contentPadding: EdgeInsets.all(0.0),
      //labelText: "用户名",
      hintText: placeholder,
      //prefixIcon: Icon(Icons.person),
      prefixIcon: new Image.asset(
        prefixIconImageName,
        width: 14.0,
        height: 15.0,
      ),
      enabledBorder: loginTextFieldDecorationBorder(),
      focusedBorder: loginTextFieldDecorationBorder(),
      ),
      // keyboardType: TextInputType.text,
      // controller: _usernameController,
      // textInputAction: TextInputAction.next,
      // focusNode: usernameFocusNode,
      // onSubmitted: (text) {
      //   print("current userName:" + text);
      //   if (null == currentFocusNode) {
      //     currentFocusNode = FocusScope.of(context);
      //   }
      //   currentFocusNode.requestFocus(passwordFocusNode);
      // }
      onSubmitted: onSubmitted,
  );
}

// 文本框border
InputBorder loginTextFieldDecorationBorder() {
  return new OutlineInputBorder(
      borderSide: new BorderSide(color: Color(0xffd2d2d2), width: 0.6),
      borderRadius: new BorderRadius.circular(6.0)
  );
}

可见这种函数的方式，没办法处理过多属性的自定义。因为它并不像我们iOS中的UIView，可以对得到的控件在后续再定制。所以，在Flutter中这种函数式的封装不适合，因为它无法满足使用。

附：以下是iOS中的操作：

- (CJTextField *)userNameTextField {
    if (_userNameTextField == nil) {
        UIImage *normalImage = [UIImage imageNamed:@"login_username_gray"];
        UIImage *selectedImage = [UIImage imageNamed:@"login_username_blue"];
        _userNameTextField = [CJDemoTextFieldFactory textFieldWithNormalImage:normalImage selectedImage:selectedImage];
        _userNameTextField.placeholder = NSLocalizedString(@"用户名", nil);
        _userNameTextField.returnKeyType = UIReturnKeyNext;
        _userNameTextField.clearButtonMode = UITextFieldViewModeWhileEditing;
        _userNameTextField.delegate = self;
    }
    return _userNameTextField;
}

- (UIButton *)loginButton {
    if (_loginButton == nil) {
        _loginButton = [CJDemoButtonFactory blueButton];
        [_loginButton setTitle:NSLocalizedString(@"登录", nil) forState:UIControlStateNormal];
        _loginButton.enabled = NO;
        [_loginButton addTarget:self action:@selector(loginButtonAction) forControlEvents:UIControlEventTouchUpInside];
    }
    return _loginButton;
}

二、以继承 StatefulWidget 的方式封装

/// 文本框(常用于：登录用户名、密码文本框)
/// 方法2:以继承 StatefulWidget 的方式实现
class LoginTextField extends StatefulWidget {
  final String placeholder;
  final String prefixIconImageName;
  final bool autofocus;
  final TextEditingController controller;
  final TextInputAction textInputAction;
  final FocusNode focusNode;
  final ValueChanged<String> onSubmitted;
  final TextInputType keyboardType;

  LoginTextField({
    Key key,
    this.placeholder,
    this.prefixIconImageName,
    this.autofocus,
    this.keyboardType,
    this.controller,
    this.textInputAction,
    this.focusNode,
    this.onSubmitted

})  :  super(key: key);

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

class _LoginTextFieldState extends State<LoginTextField> {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: TextField(
        autofocus: widget.autofocus,
        style: TextStyle(color: Colors.black, fontSize: 17.0),
        decoration: InputDecoration(
          contentPadding: EdgeInsets.all(0.0),
          //labelText: "用户名",
          hintText: widget.placeholder,
          //prefixIcon: Icon(Icons.person),
          prefixIcon: new Image.asset(
            widget.prefixIconImageName,
            width: 14.0,
            height: 15.0,
          ),
          enabledBorder: loginTextFieldDecorationBorder(),
          focusedBorder: loginTextFieldDecorationBorder(),
        ),
        keyboardType: widget.keyboardType,
        controller: widget.controller,
        textInputAction: widget.textInputAction,
        focusNode: widget.focusNode,
        onSubmitted: widget.onSubmitted
        ),
    );
  }
}

// 文本框border
InputBorder loginTextFieldDecorationBorder() {
  return new OutlineInputBorder(
      borderSide: new BorderSide(color: Color(0xffd2d2d2), width: 0.6),
      borderRadius: new BorderRadius.circular(6.0)
  );
}

使用的时候：

TextField userNameTextField() {
    return  TextField(
        autofocus: shouldAutofocusUserNameTextField,
        style: TextStyle(color: Colors.black, fontSize: 17.0),
        decoration: InputDecoration(
          contentPadding: EdgeInsets.all(0.0),
          //labelText: "用户名",
          hintText: "用户名",
          //prefixIcon: Icon(Icons.person),
          prefixIcon: new Image.asset(
            userNameValid ? 'lib/Resources/login/login_username_blue.png' : 'lib/Resources/login/login_username_gray.png',
            width: 14.0,
            height: 15.0,
          ),
          enabledBorder: loginTextFieldDecorationBorder(),
          focusedBorder: loginTextFieldDecorationBorder(),
        ),
        keyboardType: TextInputType.text,
        controller: _usernameController,
        textInputAction: TextInputAction.next,
        focusNode: usernameFocusNode, //usernameFocusNode
        onSubmitted: (text) {
          print("current userName:" + text);
          if (null == currentFocusNode) {
            currentFocusNode = FocusScope.of(context);
          }
          currentFocusNode.requestFocus(passwordFocusNode);
        }
        );
  }

虽然使用上看似没什么问题，但是整个TextField的继承代码难道你不觉得有更简洁的写法吗？

所以下面将讲解直接继承TextFiled的方法。

三、继承父类的方式

/// 文本框(常用于：登录用户名、密码文本框)
/// 方法3:以继承 TextField 的方式实现
class LoginTextField extends TextField {
  LoginTextField({
    Key key,
    String text,

    String placeholder, 

    /// prefix icon 
    bool prefixIconSelected,
    String prefixIconNormalImageName,
    String prefixIconSelectedImageName,

    bool autofocus = false,
    bool obscureText = false,
    TextInputType keyboardType,
    TextEditingController controller,
    bool showClear = false,
    TextInputAction textInputAction,
    FocusNode focusNode,
    ValueChanged<String> onSubmitted,
  }) : super(
    key: key,
    autofocus: autofocus,
    obscureText: obscureText,
    style: TextStyle(color: Colors.black, fontSize: 17.0),
    decoration: InputDecoration(
      contentPadding: EdgeInsets.all(0.0),
      //labelText: "用户名",
      hintText: placeholder,
      //prefixIcon: Icon(Icons.person),
      prefixIcon: new Image.asset(
        !prefixIconSelected ? prefixIconNormalImageName :prefixIconSelectedImageName,
        width: 14.0,
        height: 15.0,
      ),
      suffixIcon: !showClear ? null : clearButtonWithOnPressed(controller.clear),
      enabledBorder: loginTextFieldDecorationBorder(),
      focusedBorder: loginTextFieldDecorationBorder(),
    ),
    keyboardType: keyboardType,
    controller: controller,
    textInputAction: textInputAction,
    focusNode: focusNode,
    onSubmitted: onSubmitted
    );
}

/// selected Image
class SelectedImage extends Image {
  SelectedImage({
    Key key,
    bool selected,
    String normalImageName,
    String selectedImageName,
  }) :super (
      key: key,
      image: AssetImage(!selected ? normalImageName :selectedImageName)
  );
}


/// 文本框border
InputBorder loginTextFieldDecorationBorder() {
  return new OutlineInputBorder(
      borderSide: new BorderSide(color: Color(0xffd2d2d2), width: 0.6),
      borderRadius: new BorderRadius.circular(6.0)
  );
}

使用时候

  // 用户名文本框
  LoginTextField userNameTextField() {
    return LoginTextField(
        placeholder: "用户名",
        prefixIconSelected: userNameValid,
        prefixIconNormalImageName: 'assets/images/login/login_username_gray.png',
        prefixIconSelectedImageName: 'assets/images/login/login_username_blue.png',
        autofocus: shouldAutofocusUserNameTextField,
        keyboardType: TextInputType.text,
        controller: _usernameController,
        textInputAction: TextInputAction.next,
        focusNode: usernameFocusNode,
        onSubmitted: (text) {
          print("current userName:" + text);
          if (null == currentFocusNode) {
            currentFocusNode = FocusScope.of(context);
          }
          currentFocusNode.requestFocus(passwordFocusNode);
        });
  }

可见这种方式，不管在封装时候，还是在使用时候，写的代码都是最简洁的。

所以，综上在Flutter中对于一个Widget的封装，我们采用直接继承其父类的方式来处理，且其具体的写法如上。