Compole.js

function Compile(el, vm) {
    this.$vm = vm;
    this.$el = this.isElementNode(el) ? el : document.querySelector(el);

    if (this.$el) {
        this.$fragment = this.node2Fragment(this.$el);
        this.init();
        this.$el.appendChild(this.$fragment);
    }
}

Compile.prototype = {
    constructor: Compile,
    node2Fragment: function (el) {
        var fragment = document.createDocumentFragment(),
            child;

        // 将原生节点拷贝到fragment
        while (child = el.firstChild) {
            fragment.appendChild(child);
        }

        return fragment;
    },

    init: function () {
        this.compileElement(this.$fragment);
    },

    compileElement: function (el) {
        var childNodes = el.childNodes,
            me = this;

        [].slice.call(childNodes).forEach(function (node) {
            var text = node.textContent;
            var reg = /\{\{(.*)\}\}/;

            if (me.isElementNode(node)) {
                me.compile(node);
            } else if (me.isTextNode(node) && reg.test(text)) {
                me.compileText(node, RegExp.$1.trim());
            }

            if (node.childNodes && node.childNodes.length) {
                me.compileElement(node);
            }
        });
    },

    compile: function (node) {
        var nodeAttrs = node.attributes,
            me = this;

        [].slice.call(nodeAttrs).forEach(function (attr) {
            var attrName = attr.name;
            if (me.isDirective(attrName)) {
                var exp = attr.value;
                var dir = attrName.substring(2);
                // 事件指令
                if (me.isEventDirective(dir)) {
                    compileUtil.eventHandler(node, me.$vm, exp, dir);
                    // 普通指令
                } else {
                    compileUtil[dir] && compileUtil[dir](node, me.$vm, exp);
                }

                node.removeAttribute(attrName);
            }
        });
    },

    compileText: function (node, exp) {
        compileUtil.text(node, this.$vm, exp);
    },

    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;
    }
};

// 指令处理集合
var compileUtil = {
    text: function (node, vm, exp) {
        this.bind(node, vm, exp, 'text');
    },

    html: function (node, vm, exp) {
        this.bind(node, vm, exp, 'html');
    },

    model: function (node, vm, exp) {
        this.bind(node, vm, exp, 'model');

        var me = this,
            val = this._getVMVal(vm, exp);
        node.addEventListener('input', function (e) {
            var newValue = e.target.value;
            if (val === newValue) {
                return;
            }

            me._setVMVal(vm, exp, newValue);
            val = newValue;
        });
    },

    class: function (node, vm, exp) {
        this.bind(node, vm, exp, 'class');
    },

    bind: function (node, vm, exp, dir) {
        var updaterFn = updater[dir + 'Updater'];

        updaterFn && updaterFn(node, this._getVMVal(vm, exp));

        new Watcher(vm, exp, function (value, oldValue) {
            updaterFn && updaterFn(node, value, oldValue);
        });
    },

    // 事件处理
    eventHandler: function (node, vm, exp, dir) {
        var eventType = dir.split(':')[1],
            fn = vm.$options.methods && vm.$options.methods[exp];

        if (eventType && fn) {
            node.addEventListener(eventType, fn.bind(vm), false);
        }
    },

    _getVMVal: function (vm, exp) {
        var val = vm;
        exp = exp.split('.');
        exp.forEach(function (k) {
            val = val[k];
        });
        return val;
    },

    _setVMVal: function (vm, exp, value) {
        var val = vm;
        exp = exp.split('.');
        exp.forEach(function (k, i) {
            // 非最后一个key,更新val的值
            if (i < exp.length - 1) {
                val = val[k];
            } else {
                val[k] = value;
            }
        });
    }
};

var updater = {
    textUpdater: function (node, value) {
        node.textContent = typeof value == 'undefined' ? '' : value;
    },

    htmlUpdater: function (node, value) {
        node.innerHTML = typeof value == 'undefined' ? '' : value;
    },

    classUpdater: function (node, value, oldValue) {
        var className = node.className;
        className = className.replace(oldValue, '').replace(/\s$/, '');

        var space = className && String(value) ? ' ' : '';

        node.className = className + space + value;
    },

    modelUpdater: function (node, value, oldValue) {
        node.value = typeof value == 'undefined' ? '' : value;
    }
};

Observer.js

function Observer(data) {
    this.data = data;
    this.walk(data);
}

Observer.prototype = {
    constructor: Observer,
    walk: function(data) {
        var me = this;
        Object.keys(data).forEach(function(key) {
            me.convert(key, data[key]);
        });
    },
    convert: function(key, val) {
        this.defineReactive(this.data, key, val);
    },

    defineReactive: function(data, key, val) {
        var dep = new Dep();
        var childObj = observe(val);

        Object.defineProperty(data, key, {
            enumerable: true, // 可枚举
            configurable: false, // 不能再define
            get: function() {
                if (Dep.target) {
                    dep.depend();
                }
                return val;
            },
            set: function(newVal) {
                if (newVal === val) {
                    return;
                }
                val = newVal;
                // 新的值是object的话,进行监听
                childObj = observe(newVal);
                // 通知订阅者
                dep.notify();
            }
        });
    }
};

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

    return new Observer(value);
};

var uid = 0;

function Dep() {
    this.id = uid++;
    this.subs = [];
}

Dep.prototype = {
    addSub: function(sub) {
        this.subs.push(sub);
    },

    depend: function() {
        Dep.target.addDep(this);
    },

    removeSub: function(sub) {
        var index = this.subs.indexOf(sub);
        if (index != -1) {
            this.subs.splice(index, 1);
        }
    },

    notify: function() {
        this.subs.forEach(function(sub) {
            sub.update();
        });
    }
};

Dep.target = null;

Watcher.js

function Watcher(vm, expOrFn, cb) {
    this.cb = cb;
    this.vm = vm;
    this.expOrFn = expOrFn;
    this.depIds = {};

    if (typeof expOrFn === 'function') {
        this.getter = expOrFn;
    } else {
        this.getter = this.parseGetter(expOrFn.trim());
    }

    this.value = this.get();
}

Watcher.prototype = {
    constructor: Watcher,
    update: function () {
        this.run();
    },
    run: function () {
        var value = this.get();
        var oldVal = this.value;
        if (value !== oldVal) {
            this.value = value;
            this.cb.call(this.vm, value, oldVal);
        }
    },
    addDep: function (dep) {
        // 1. 每次调用run()的时候会触发相应属性的getter
        // getter里面会触发dep.depend(),继而触发这里的addDep
        // 2. 假如相应属性的dep.id已经在当前watcher的depIds里,说明不是一个新的属性,仅仅是改变了其值而已
        // 则不需要将当前watcher添加到该属性的dep里
        // 3. 假如相应属性是新的属性,则将当前watcher添加到新属性的dep里
        // 如通过 vm.child = {name: 'a'} 改变了 child.name 的值,child.name 就是个新属性
        // 则需要将当前watcher(child.name)加入到新的 child.name 的dep里
        // 因为此时 child.name 是个新值,之前的 setter、dep 都已经失效,如果不把 watcher 加入到新的 child.name 的dep中
        // 通过 child.name = xxx 赋值的时候,对应的 watcher 就收不到通知,等于失效了
        // 4. 每个子属性的watcher在添加到子属性的dep的同时,也会添加到父属性的dep
        // 监听子属性的同时监听父属性的变更,这样,父属性改变时,子属性的watcher也能收到通知进行update
        // 这一步是在 this.get() --> this.getVMVal() 里面完成,forEach时会从父级开始取值,间接调用了它的getter
        // 触发了addDep(), 在整个forEach过程,当前wacher都会加入到每个父级过程属性的dep
        // 例如:当前watcher的是'child.child.name', 那么child, child.child, child.child.name这三个属性的dep都会加入当前watcher
        if (!this.depIds.hasOwnProperty(dep.id)) {
            dep.addSub(this);
            this.depIds[dep.id] = dep;
        }
    },
    get: function () {
        Dep.target = this;
        var value = this.getter.call(this.vm, this.vm);
        Dep.target = null;
        return value;
    },

    parseGetter: function (exp) {
        if (/[^\w.$]/.test(exp)) return;

        var exps = exp.split('.');

        return function (obj) {
            for (var i = 0, len = exps.length; i < len; i++) {
                if (!obj) return;
                obj = obj[exps[i]];
            }
            return obj;
        }
    }
};

Mvvm.js

function MVVM(options) {
    this.$options = options || {};
    var data = this._data = this.$options.data;
    var me = this;

    // 数据代理
    // 实现 vm.xxx -> vm._data.xxx
    Object.keys(data).forEach(function(key) {
        me._proxyData(key);
    });

    this._initComputed();

    observe(data, this);

    this.$compile = new Compile(options.el || document.body, this)
}

MVVM.prototype = {
    constructor: MVVM,
    $watch: function(key, cb, options) {
        new Watcher(this, key, cb);
    },

    _proxyData: function(key, setter, getter) {
        var me = this;
        setter = setter || 
        Object.defineProperty(me, key, {
            configurable: false,
            enumerable: true,
            get: function proxyGetter() {
                return me._data[key];
            },
            set: function proxySetter(newVal) {
                me._data[key] = newVal;
            }
        });
    },

    _initComputed: function() {
        var me = this;
        var computed = this.$options.computed;
        if (typeof computed === 'object') {
            Object.keys(computed).forEach(function(key) {
                Object.defineProperty(me, key, {
                    get: typeof computed[key] === 'function' 
                            ? computed[key] 
                            : computed[key].get,
                    set: function() {}
                });
            });
        }
    }
};

笨蛋、笨蛋!