本节换一种方式解读，把我消化过的东西反刍出来可能这样大家容易理解些，knockout.js大量使用闭包，非常难读。
我们从viewModel看起：
function MyViewModel() {
this.firstName = $.observable('Planet');
this.lastName = $.observable('Earth');
this.fullName = $.computed({
getter: function () {
return this.firstName() + " " + this.lastName();
},
setter: function (value) {
var lastSpacePos = value.lastIndexOf(" ");
if (lastSpacePos > 0) { // Ignore values with no space character
this.firstName(value.substring(0, lastSpacePos)); // Update "firstName"
this.lastName(value.substring(lastSpacePos + 1)); // Update "lastName"
}
},
scope: this
});
}
var a = new MyViewModel();
a.fullName("xxx yyy")
这里包含两种observable，没有依赖的与有依赖的，有依赖的通过没有依赖的计算出来，因此叫做computed！
但不管怎么样，它们都是返回一个函数，我们通过如下代码就可以模拟它们了：
//注：这里会用到mass Framework的种子模块的API https://github.com/RubyLouvre/mass-Framework/blob/master/src/mass.js
//observable的传参必须是基本类型
var validValueType = $.oneObject("Null,NaN,Undefined,Boolean,Number,String");
$.observable = function(value){
var v = value;//将上一次的传参保存到v中,ret与它构成闭包
function ret(neo){
if(arguments.length){ //setter
if(!validValueType[$.type(neo)]){
$.error("arguments must be primitive type!")
return ret
}
if(v !== neo ){
v = neo;
}
return ret;
}else{ //getter
return v;
}
}
value = validValueType[$.type(value)] ? value : void 0;
ret(arguments[0]);//必须先执行一次
return ret
}
$.computed = function(obj, scope){//为一个惰性函数，会重写自身
//computed是由多个$.observable组成
var getter, setter
if(typeof obj == "function"){
getter = obj
}else if(obj && typeof obj == "object"){
getter = obj.getter;
setter = obj.setter;
scope = obj.scope;
}
var v
var ret = function(neo){
if(arguments.length ){
if(typeof setter == "function"){//setter不一定存在的
if(!validValueType[$.type(neo)]){
$.error("arguments must be primitive type!")
return ret
}
if(v !== neo ){
setter.call(scope, neo);
v = neo;
}
}
return ret;
}else{
v = getter.call(scope);
return v;
}
}
ret(); //必须先执行一次
return ret;
}
因此当我们执行new MyViewModel(),就会依次执行$.observable, $.observable, $.computed, $.computed中的参数的getter, getter再调用两个observable。
问题来了，当我们调用computed时，总会通知其依赖（即firstName ，lastName）进行更新，但firstName 发生改变时没有手段通知fullName 进行更新。ko把这逻辑写在dependencyDetection模块中。我简化如下：
$.dependencyDetection = (function () {
var _frames = [];
return {
begin: function (ret) {
_frames.push(ret);
},
end: function () {
_frames.pop();
},
collect: function (self) {
if (_frames.length > 0) {
if(!self.list)
self.list = [];
var fn = _frames[_frames.length - 1];
if ( self.list.indexOf( fn ) >= 0)
return;
self.list.push(fn);
}
}
};
})();
我们把它加入到 $.computed 与 $.observable中，再添加一个发布更新函数valueWillMutate
var validValueType = $.oneObject("Null,NaN,Undefined,Boolean,Number,String")
$.dependencyDetection = (function () {
var _frames = [];
return {
begin: function (ret) {
_frames.push(ret);
},
end: function () {
_frames.pop();
},
collect: function (self) {
if (_frames.length > 0) {
if(!self.list)
self.list = [];
var fn = _frames[_frames.length - 1];
if ( self.list.indexOf( fn ) >= 0)
return;
self.list.push(fn);
}
}
};
})();
$.valueWillMutate = function(observable){
var list = observable.list
if($.type(list,"Array")){
for(var i = 0, el; el = list[i++];){
el();
}
}
}
$.observable = function(value){
var v = value;//将上一次的传参保存到v中,ret与它构成闭包
function ret(neo){
if(arguments.length){ //setter
if(!validValueType[$.type(neo)]){
$.error("arguments must be primitive type!")
return ret
}
if(v !== neo ){
v = neo;
$.valueWillMutate(ret);//向依赖者发送通知
}
return ret;
}else{ //getter
$.dependencyDetection.collect(ret);//收集被依赖者
return v;
}
}
value = validValueType[$.type(value)] ? value : void 0;
ret(arguments[0]);//必须先执行一次
return ret
}
$.computed = function(obj, scope){//为一个惰性函数，会重写自身
//computed是由多个$.observable组成
var getter, setter
if(typeof obj == "function"){
getter = obj
}else if(obj && typeof obj == "object"){
getter = obj.getter;
setter = obj.setter;
scope = obj.scope;
}
var v
var ret = function(neo){
if(arguments.length ){
if(typeof setter == "function"){//setter不一定存在的
if(!validValueType[$.type(neo)]){
$.error("arguments must be primitive type!")
return ret
}
if(v !== neo ){
setter.call(scope, neo);
v = neo;
}
}
return ret;
}else{
$.dependencyDetection.begin(ret);//让其依赖知道自己的存在
v = getter.call(scope);
$.dependencyDetection.end();
return v;
}
}
ret(); //必须先执行一次
return ret;
}
function MyViewModel() {
this.firstName = $.observable('Planet');
this.lastName = $.observable('Earth');
this.fullName = $.computed({
getter: function () {
return this.firstName() + " " + this.lastName();
},
setter: function (value) {
var lastSpacePos = value.lastIndexOf(" ");
if (lastSpacePos > 0) { // Ignore values with no space character
this.firstName(value.substring(0, lastSpacePos)); // Update "firstName"
this.lastName(value.substring(lastSpacePos + 1)); // Update "lastName"
}
},
scope: this
});
this.card = $.computed(function(){
return this.fullName() +" 屌丝"
},this)
}
var a = new MyViewModel();
//============测试代码============
$.log(a.firstName())//Planet
$.log(a.lastName())//Earth
$.log(a.fullName())//Planet Earth 通过上面两个计算出来
a.fullName("xxx yyy");//更新fullName会自动更新firstName与lastName
$.log(a.firstName())//xxx
$.log(a.lastName())//yyy
a.firstName("ooo");//更新firstName会自动更新fullName
$.log(a.fullName())//ooo yyy
$.log(a.card())//ooo yyy 屌丝
到这里viewModel中的每个域（firstName, lastName, fullName）只要存在依赖关系都能相互通知了。