简述vue的双向绑定原理

// 1.实现一个监听器Observer，用来劫持并监听所有属性，如果有变动的，就通知订阅者
function Observer(data) {
    this.data = data;
    this.walk(data);
}

Observer.prototype = {
    walk: function(data) {
        var self = this;
        Object.keys(data).forEach(function(key) {
            self.defineReactive(data, key, data[key]);
        });
    },
    defineReactive: function(data, key, val) {
        var dep = new Dep();
        var childObj = observe(val);
        Object.defineProperty(data, key, {
            enumerable: true,
            configurable: true,
            get: function getter () {
                if (Dep.target) {
                    dep.addSub(Dep.target);
                }
                return val;
            },
            set: function setter (newVal) {
                if (newVal === val) {
                    return;
                }
                val = newVal;
                dep.notify();
            }
        });
    }
};

function observe(value, vm) {
    if (!value || typeof value !== ‘object‘) {
        return;
    }
    return new Observer(value);
};

function Dep () {
    this.subs = [];
}
Dep.prototype = {
    addSub: function(sub) {
        this.subs.push(sub);
    },
    notify: function() {
        this.subs.forEach(function(sub) {
            sub.update();
        });
    }
};
Dep.target = null;

// 2.实现一个订阅者Watcher，可以收到属性的变化通知并执行相应的函数，从而更新视图。
function Watcher(vm, exp, cb) {
    this.cb = cb;
    this.vm = vm;
    this.exp = exp;
    this.value = this.get();  // 将自己添加到订阅器的操作
}

Watcher.prototype = {
    update: function() {
        this.run();
    },
    run: function() {
        var value = this.vm.data[this.exp];
        var oldVal = this.value;
        if (value !== oldVal) {
            this.value = value;
            this.cb.call(this.vm, value, oldVal);
        }
    },
    get: function() {
        Dep.target = this;  // 缓存自己
        var value = this.vm.data[this.exp]  // 强制执行监听器里的get函数
        Dep.target = null;  // 释放自己
        return value;
    }
};

// 3.实现一个解析器Compile，可以扫描和解析每个节点的相关指令，并根据初始化模板数据以及初始化相应的订阅器。
function Compile(el, vm) {
    this.vm = vm;
    this.el = document.querySelector(el);
    this.fragment = null;
    this.init();
}

Compile.prototype = {
    init: function () {
        if (this.el) {
            this.fragment = this.nodeToFragment(this.el);
            this.compileElement(this.fragment);
            this.el.appendChild(this.fragment);
        } else {
            console.log(‘Dom元素不存在‘);
        }
    },
    nodeToFragment: function (el) {
        var fragment = document.createDocumentFragment();
        var child = el.firstChild;
        while (child) {
            // 将Dom元素移入fragment中
            fragment.appendChild(child);
            child = el.firstChild
        }
        return fragment;
    },
    compileElement: function (el) {
        var childNodes = el.childNodes;
        var self = this;
        [].slice.call(childNodes).forEach(function(node) {
            var reg = /\{\{(.*)\}\}/;
            var text = node.textContent;

            if (self.isElementNode(node)) {  
                self.compile(node);
            } else if (self.isTextNode(node) && reg.test(text)) {
                self.compileText(node, reg.exec(text)[1]);
            }

            if (node.childNodes && node.childNodes.length) {
                self.compileElement(node);
            }
        });
    },
    compile: function(node) {
        var nodeAttrs = node.attributes;
        var self = this;
        Array.prototype.forEach.call(nodeAttrs, function(attr) {
            var attrName = attr.name;
            if (self.isDirective(attrName)) {
                var exp = attr.value;
                var dir = attrName.substring(2);
                if (self.isEventDirective(dir)) {  // 事件指令
                    self.compileEvent(node, self.vm, exp, dir);
                } else {  // v-model 指令
                    self.compileModel(node, self.vm, exp, dir);
                }
                node.removeAttribute(attrName);
            }
        });
    },
    compileText: function(node, exp) {
        var self = this;
        var initText = this.vm[exp];
        this.updateText(node, initText);
        new Watcher(this.vm, exp, function (value) {
            self.updateText(node, value);
        });
    },
    compileEvent: function (node, vm, exp, dir) {
        var eventType = dir.split(‘:‘)[1];
        var cb = vm.methods && vm.methods[exp];

        if (eventType && cb) {
            node.addEventListener(eventType, cb.bind(vm), false);
        }
    },
    compileModel: function (node, vm, exp, dir) {
        var self = this;
        var val = this.vm[exp];
        this.modelUpdater(node, val);
        new Watcher(this.vm, exp, function (value) {
            self.modelUpdater(node, value);
        });

        node.addEventListener(‘input‘, function(e) {
            var newValue = e.target.value;
            if (val === newValue) {
                return;
            }
            self.vm[exp] = newValue;
            val = newValue;
        });
    },
    updateText: function (node, value) {
        node.textContent = typeof value == ‘undefined‘ ? ‘‘ : value;
    },
    modelUpdater: function(node, value, oldValue) {
        node.value = typeof value == ‘undefined‘ ? ‘‘ : value;
    },
    isDirective: function(attr) {
        return attr.indexOf(‘v-‘) == 0;
    },
    isEventDirective: function(dir) {
        return dir.indexOf(‘on:‘) === 0;
    },
    isElementNode: function (node) {
        return node.nodeType == 1;
    },
    isTextNode: function(node) {
        return node.nodeType == 3;
    }
}