Vue.js-----轻量高效的MVVM框架（五、计算属性）

#基础例子

<div id="dr01">
    <h4>#基础例子</h4>
    <div>
        num01={{num01}}, num02={{num02}}
        <p>num01: vm定义的属性，num02：计算出来的属性</p>
    </div>
</div>

var dr01 = new Vue({
    el: "#dr01",
    data: {
        num01: 10,
    },
    computed: {
        num02: function() {
            return this.num01 * 5;
        }
    }
})

在js中定义组件dr01，初始化数据num01:10，computed中定义了num02（一个返回值为num01*5的方法）。

在页面中，{{num01}}为我们定义{{num02}}为计算出来的结果：

#计算属性vue.$watch

<div id="dr02">
    <h4>#计算属性vue.$watch</h4>
    <div>
        <span>FirstName01: </span><input type="text" v-model="firstName01" />
        <br />
        <span>LastName01: </span><input type="text" v-model="lastName01" />
        <br />
        <span>FullName01: </span><input type="text" readonly="readonly" v-model="fullName01" disabled="disabled" />
    </div>
    <br />
    <div>
        <span>FirstName02: </span><input type="text" v-model="firstName02" />
        <br />
        <span>LastName02: </span><input type="text" v-model="lastName02" />
        <br />
        <span>FullName02: </span><input type="text" readonly="readonly" v-model="fullName02" disabled="disabled" />
    </div>
</div>

var dr02 = new Vue({
    el: "#dr02",
    data: {
        firstName01: "Dark",
        lastName01: "Ranger",
        fullName01: "Dark Ranger",
        firstName02: "Slight",
        lastName02: "Wind"
    },
    computed: {
        fullName02: function() {
            return this.firstName02 + " " + this.lastName02;
        }
    }
});
dr02.$watch("firstName01", function(val) {
    this.fullName01 = val + " " + this.lastName01;
});
dr02.$watch("lastName01", function(val) {
    this.fullName01 = this.firstName01 + " " + val;
});

双向数据绑定的时候我们经常会遇到这样的情况：数据3=数据1+数据2，我们单独更新数据1或者数据2以后，数据3需要实时更新。这里我们有两种解决方案，但是肯定推荐第二种计算属性，为什么，简单！！！

1、在data中定义firstName01: "Dark",lastName01: "Ranger",为了初始化数据的时候fullName01也一样，同样定义fullName01: "Dark Ranger"

为了实时更新数据fullName01，我们需要对firstName01和lastName01进行观察，即加上dr02.$watch语句。

2、在data中定义firstName02: "Slight",lastName02: "Wind"，对fullName02我们采用计算属性的方式：把fullName02定义为一个返回值为firstName02+lastName02的方法，这样，当firstName02或者lastName02发生数据变更的时候，fullName02通过计算属性也会发生变化，进行实时更新。

　　初始化数据

　　firstname或者lastname数据更新后

#计算setter

上面的例子，我们考虑个问题，是的，当firstname或者lastname数据更新的时候，fullname数据也会更新；但是，当fullname数据发生变化的时候firstname和lastname并不会更新，因为在方式1中我们没有对fullname加入观察，如果加入观察，是可以更新，但是我们要更新10000条数据，也就要写对10000个数据进行观察的$watch方法，这里我们在方式2中对数据fullname加入get和set方法即可。

方式2的gettter和setter

<div id="dr03">
    <h4>#计算setter</h4>
    <span>FirstName: </span><input type="text" v-model="firstName" />
    <br />
    <span>LastName: </span><input type="text" v-model="lastName" />
    <br />
    <span>FullName: </span><input type="text" v-model="fullName" />
</div>

var dr03 = new Vue({
    el: "#dr03",
    data: {
        firstName: "Zhang",
        lastName: "Yao"
    },
    computed: {
        fullName: {
            get: function() {
                return this.firstName + " " + this.lastName;
            },
            set: function(newValue) {
                var names = newValue.split(" ");
                this.firstName = names[0];
                this.lastName = names[1];
            }
        }
    }
})

初始化数据：

当firstname、lastname、fullname中的一个发生变化的时候，其他数据也会实时更新

完整代码如下：

<!DOCTYPE html>
<html>

    <head>
        <meta charset="UTF-8">
        <script type="text/javascript" src="js/vue.js"></script>
        <title>计算属性</title>
    </head>

    <body>
        <div id="dr01">
            <h4>#基础例子</h4>
            <div>
                num01={{num01}}, num02={{num02}}
                <p>num01: vm定义的属性，num02：计算出来的属性</p>
            </div>
        </div>
        <div id="dr02">
            <h4>#计算属性vue.$watch</h4>
            <div>
                <span>FirstName01: </span><input type="text" v-model="firstName01" />
                <br />
                <span>LastName01: </span><input type="text" v-model="lastName01" />
                <br />
                <span>FullName01: </span><input type="text" readonly="readonly" v-model="fullName01" disabled="disabled" />
            </div>
            <br />
            <div>
                <span>FirstName02: </span><input type="text" v-model="firstName02" />
                <br />
                <span>LastName02: </span><input type="text" v-model="lastName02" />
                <br />
                <span>FullName02: </span><input type="text" readonly="readonly" v-model="fullName02" disabled="disabled" />
            </div>
        </div>
        <div id="dr03">
            <h4>#计算setter</h4>
            <span>FirstName: </span><input type="text" v-model="firstName" />
            <br />
            <span>LastName: </span><input type="text" v-model="lastName" />
            <br />
            <span>FullName: </span><input type="text" v-model="fullName" />
        </div>
        <script>
            var dr01 = new Vue({
                el: "#dr01",
                data: {
                    num01: 10,
                },
                computed: {
                    num02: function() {
                        return this.num01 * 5;
                    }
                }
            })
            var dr02 = new Vue({
                el: "#dr02",
                data: {
                    firstName01: "Dark",
                    lastName01: "Ranger",
                    fullName01: "Dark Ranger",
                    firstName02: "Slight",
                    lastName02: "Wind"
                },
                computed: {
                    fullName02: function() {
                        return this.firstName02 + " " + this.lastName02;
                    }
                }
            });
            dr02.$watch("firstName01", function(val) {
                this.fullName01 = val + " " + this.lastName01;
            });
            dr02.$watch("lastName01", function(val) {
                this.fullName01 = this.firstName01 + " " + val;
            });
            var dr03 = new Vue({
                el: "#dr03",
                data: {
                    firstName: "Zhang",
                    lastName: "Yao"
                },
                computed: {
                    fullName: {
                        get: function() {
                            return this.firstName + " " + this.lastName;
                        },
                        set: function(newValue) {
                            var names = newValue.split(" ");
                            this.firstName = names[0];
                            this.lastName = names[1];
                        }
                    }
                }
            })
        </script>
    </body>

</html>