Vue2实时监听表单变化的方法

<template>
    <section>
        <el-dialog :title="formTitle" :visible.sync="dialogFormVisible" :before-close="cancel">
            <el-form :model="form" :rules="rules" ref="form">
            </el-form>
            <div slot="footer" class="dialog-footer">
                <el-button size="medium" type="primary" @click="addSubmit" :loading="addLoading" :disabled="unChange">确 定</el-button>
            </div>
        </el-dialog>
    </section>
</template>

<script>    
    export default {
        props: ["dialogFormVisible","form","formTitle"],
        data() {
            return {
                unChange: true,
                preForm: JSON.parse(JSON.stringify(this.form)) //深拷贝对象
            };
        },
        watch: {
            form:{
                handler:function(nowVal,oldVal){
                    var $this = this;
                    for(let i in $this.form){
                        if(nowVal[i] != $this.preForm[i]) {
                            $this.unChange = false;
                            break;
                        }else {
                            $this.unChange = true;
                        }
                    }
                },
                deep:true
            }
        },
        methods: {
            addSubmit() {
                var $this = this;
            }
        },
        mounted() {
            var $this = this;
        }
    };
</script>