主要通过controller 实现，具体代码如下

TextField(　　　　　　　　　　//输入键盘类型
              keyboardType: TextInputType.text,
              autofocus: true,
              decoration: InputDecoration(
                border: OutlineInputBorder(
                    borderRadius: BorderRadius.circular(30),
                    borderSide: BorderSide.none),
              ),
              onChanged: (value) {
                this._keyword = value;
              },
              controller: TextEditingController.fromValue(TextEditingValue(
                  text: '${this._keyword == null ? "" : this._keyword}',  //判断keyword是否为空
                  // 保持光标在最后

                  selection: TextSelection.fromPosition(TextPosition(
                      affinity: TextAffinity.downstream,
                      offset: '${this._keyword}'.length)))),
            ),