AngularJS 源码分析3

回顾

上次说到了rootScope里的$watch方法中的解析监控表达式，即而引出了对parse的分析,今天我们接着这里继续挖代码.

$watch续

先上一块$watch代码

$watch: function(watchExp, listener, objectEquality) {        var scope = this,            get = compileToFn(watchExp, 'watch'),            array = scope.$$watchers,            watcher = {              fn: listener,              last: initWatchVal,              get: get,              exp: watchExp,              eq: !!objectEquality            };        lastDirtyWatch = null;        // in the case user pass string, we need to compile it, do we really need this ?        if (!isFunction(listener)) {          var listenFn = compileToFn(listener || noop, 'listener');          watcher.fn = function(newVal, oldVal, scope) {listenFn(scope);};        }        if (typeof watchExp == 'string' && get.constant) {          var originalFn = watcher.fn;          watcher.fn = function(newVal, oldVal, scope) {            originalFn.call(this, newVal, oldVal, scope);            arrayRemove(array, watcher);          };        }        if (!array) {          array = scope.$$watchers = [];        }        // we use unshift since we use a while loop in $digest for speed.        // the while loop reads in reverse order.        array.unshift(watcher);        return function deregisterWatch() {          arrayRemove(array, watcher);          lastDirtyWatch = null;        };      }

这里的get = compileToFn(watchExp, 'watch'),上篇已经分析完了，这里返回的是一个执行表达式的函数,接着往下看，这里初始化了一个watcher对象,用来保存一些监听相关的信息,简单的说明一下

• fn, 代表监听函数,当监控表达式新旧不相等时会执行此函数
• last, 保存最后一次发生变化的监控表达式的值
• get, 保存一个监控表达式对应的函数,目的是用来获取表达式的值然后用来进行新旧对比的
• exp, 保存一个原始的监控表达式
• eq, 保存$watch函数的第三个参数，表示是否进行深度比较

然后会检查传递进来的监听参数是否为函数,如果是一个有效的字符串，则通过parse来解析生成一个函数,否则赋值为一个noop占位函数,最后生成一个包装函数，函数体的内容就是执行刚才生成的监听函数,默认传递当前作用域.

接着会检查监控表达式是否为字符串并且执行表达式的constant为true,代表这个字符串是一个常量,那么，系统在处理这种监听的时候，执行完一次监听函数之后就会删除这个$watch.最后往当前作用域里的$$watchers数组头中添加$watch信息,注意这里的返回值,利用JS的闭包保留了当前的watcher，然后返回一个函数，这个就是用来删除监听用的.

$eval

这个$eval也是挺方便的函数,假如你想直接在程序里执行一个字符串的话，那么可以这么用

$scope.name = '2';$scope.$eval('1+name'); // ==> 会输出12

大家来看看它的函数体

return $parse(expr)(this, locals);

其实就是通过parse来解析成一个执行表达式函数,然后传递当前作用域以及额外的参数,返回这个执行表达式函数的值

$evalAsync

evalAsync函数的作用就是延迟执行表达式,并且执行完不管是否异常,触发dirty check.

 if (!$rootScope.$$phase && !$rootScope.$$asyncQueue.length) {          $browser.defer(function() {            if ($rootScope.$$asyncQueue.length) {              $rootScope.$digest();            }          });        }this.$$asyncQueue.push({scope: this, expression: expr});

可以看到当前作用域内部有一个$$asyncQueue异步队列，保存着所有需要延迟执行的表达式,此处的表达式可以是字符串或者函数,因为这个表达式最终会调用$eval方法,注意这里调用了$browser服务的defer方法,从ng->browser.js源码里可以看到,其实这里就是调用setTimeout来实现的.

self.defer = function(fn, delay) {    var timeoutId;    outstandingRequestCount++;    timeoutId = setTimeout(function() {      delete pendingDeferIds[timeoutId];      completeOutstandingRequest(fn);    }, delay || 0);    pendingDeferIds[timeoutId] = true;    return timeoutId;  };

上面的代码主要是延迟执行函数，另外pendingDeferIds对象保存所有setTimeout返回的id,这个会在self.defer.cancel这里可以取消执行延迟执行.

说digest方法之前，还有一个方法要说说

$postDigest

这个方法跟evalAsync不同的时，它不会主动触发digest方法,只是往postDigestQueue队列中增加执行表达式，它会在digest体内最后执行，相当于在触发dirty check之后，可以执行别的一些逻辑.

this.$$postDigestQueue.push(fn);

下面我们来重点说说digest方法

$digest

digest方法是dirty check的核心,主要思路是先执行$$asyncQueue队列中的表达式,然后开启一个loop来的执行所有的watch里的监听函数,前提是前后两次的值是否不相等,假如ttl超过系统默认值，则dirth check结束,最后执行$$postDigestQueue队列里的表达式.

$digest: function() {        var watch, value, last,            watchers,            asyncQueue = this.$$asyncQueue,            postDigestQueue = this.$$postDigestQueue,            length,            dirty, ttl = TTL,            next, current, target = this,            watchLog = [],            logIdx, logMsg, asyncTask;        beginPhase('$digest');        lastDirtyWatch = null;        do { // "while dirty" loop          dirty = false;          current = target;          while(asyncQueue.length) {            try {              asyncTask = asyncQueue.shift();              asyncTask.scope.$eval(asyncTask.expression);            } catch (e) {              clearPhase();              $exceptionHandler(e);            }            lastDirtyWatch = null;          }          traverseScopesLoop:          do { // "traverse the scopes" loop            if ((watchers = current.$$watchers)) {              // process our watches              length = watchers.length;              while (length--) {                try {                  watch = watchers[length];                  // Most common watches are on primitives, in which case we can short                  // circuit it with === operator, only when === fails do we use .equals                  if (watch) {                    if ((value = watch.get(current)) !== (last = watch.last) &&                        !(watch.eq                            ? equals(value, last)                            : (typeof value == 'number' && typeof last == 'number'                               && isNaN(value) && isNaN(last)))) {                      dirty = true;                      lastDirtyWatch = watch;                      watch.last = watch.eq ? copy(value) : value;                      watch.fn(value, ((last === initWatchVal) ? value : last), current);                      if (ttl < 5) {                        logIdx = 4 - ttl;                        if (!watchLog[logIdx]) watchLog[logIdx] = [];                        logMsg = (isFunction(watch.exp))                            ? 'fn: ' + (watch.exp.name || watch.exp.toString())                            : watch.exp;                        logMsg += '; newVal: ' + toJson(value) + '; oldVal: ' + toJson(last);                        watchLog[logIdx].push(logMsg);                      }                    } else if (watch === lastDirtyWatch) {                      // If the most recently dirty watcher is now clean, short circuit since the remaining watchers                      // have already been tested.                      dirty = false;                      break traverseScopesLoop;                    }                  }                } catch (e) {                  clearPhase();                  $exceptionHandler(e);                }              }            }            // Insanity Warning: scope depth-first traversal            // yes, this code is a bit crazy, but it works and we have tests to prove it!            // this piece should be kept in sync with the traversal in $broadcast            if (!(next = (current.$$childHead ||                (current !== target && current.$$nextSibling)))) {              while(current !== target && !(next = current.$$nextSibling)) {                current = current.$parent;              }            }          } while ((current = next));          // `break traverseScopesLoop;` takes us to here          if((dirty || asyncQueue.length) && !(ttl--)) {            clearPhase();            throw $rootScopeMinErr('infdig',                '{0} $digest() iterations reached. Aborting!\n' +                'Watchers fired in the last 5 iterations: {1}',                TTL, toJson(watchLog));          }        } while (dirty || asyncQueue.length);        clearPhase();        while(postDigestQueue.length) {          try {            postDigestQueue.shift()();          } catch (e) {            $exceptionHandler(e);          }        }      }

通过上面的代码，可以看出,核心就是两个loop，外loop保证所有的model都能检测到,内loop则是真实的检测每个watch,watch.get就是计算监控表达式的值,这个用来跟旧值进行对比,假如不相等，则执行监听函数

注意这里的watch.eq这是是否深度检查的标识,equals方法是angular.js里的公共方法，用来深度对比两个对象,这里的不相等有一个例外，那就是NaN ===NaN,因为这个永远都是false,所以这里加了检查

!(watch.eq    ? equals(value, last)    : (typeof value == 'number' && typeof last == 'number'       && isNaN(value) && isNaN(last)))

比较完之后,把新值传给watch.last,然后执行watch.fn也就是监听函数,传递三个参数,分别是:最新计算的值,上次计算的值(假如是第一次的话,则传递新值),最后一个参数是当前作用域实例,这里有一个设置外loop的条件值，那就是dirty = true,也就是说只要内loop执行了一次watch,则外loop还要接着执行，这是为了保证所有的model都能监测一次，虽然这个有点浪费性能,不过超过ttl设置的值后,dirty check会强制关闭，并抛出异常

if((dirty || asyncQueue.length) && !(ttl--)) {    clearPhase();    throw $rootScopeMinErr('infdig',        '{0} $digest() iterations reached. Aborting!\n' +        'Watchers fired in the last 5 iterations: {1}',        TTL, toJson(watchLog));}

这里的watchLog日志对象是在内loop里,当ttl低于5的时候开始记录的

if (ttl < 5) {    logIdx = 4 - ttl;    if (!watchLog[logIdx]) watchLog[logIdx] = [];    logMsg = (isFunction(watch.exp))        ? 'fn: ' + (watch.exp.name || watch.exp.toString())        : watch.exp;    logMsg += '; newVal: ' + toJson(value) + '; oldVal: ' + toJson(last);    watchLog[logIdx].push(logMsg);}

当检查完一个作用域内的所有watch之后，则开始深度遍历当前作用域的子级或者父级,虽然这有些影响性能，就像这里的注释写的那样yes, this code is a bit crazy

// Insanity Warning: scope depth-first traversal// yes, this code is a bit crazy, but it works and we have tests to prove it!// this piece should be kept in sync with the traversal in $broadcastif (!(next = (current.$$childHead ||      (current !== target && current.$$nextSibling)))) {    while(current !== target && !(next = current.$$nextSibling)) {      current = current.$parent;    }}

上面的代码其实就是不断的查找当前作用域的子级,没有子级，则开始查找兄弟节点,最后查找它的父级节点,是一个深度遍历查找.只要next有值,则内loop则一直执行

while ((current = next))

不过内loop也有跳出的情况,那就是当前watch跟最后一次检查的watch相等时就退出内loop.

else if (watch === lastDirtyWatch) {    // If the most recently dirty watcher is now clean, short circuit since the remaining watchers    // have already been tested.    dirty = false;    break traverseScopesLoop;}

注意这个内loop同时也是一个label(标签)语句,这个可以在loop中执行跳出操作就像上面的break

正常执行完两个loop之后,清除当前的阶段标识clearPhase();,然后开始执行postDigestQueue队列里的表达式.

while(postDigestQueue.length) {    try {      postDigestQueue.shift()();    } catch (e) {      $exceptionHandler(e);    }}

接下来说说,用的也比较多的$apply方法

$apply

这个方法一般用在，不在ng的上下文中执行js代码的情况,比如原生的DOM事件中执行想改变ng中某些model的值,这个时候就要使用$apply方法了

$apply: function(expr) {    try {      beginPhase('$apply');      return this.$eval(expr);    } catch (e) {      $exceptionHandler(e);    } finally {      clearPhase();      try {        $rootScope.$digest();      } catch (e) {        $exceptionHandler(e);        throw e;      }    }}

代码中，首先让当前阶段标识为$apply,这个可以防止使用$apply方法时检查是否已经在这个阶段了,然后就是执行$eval方法, 这个方法上面有讲到,最后执行$digest方法,来使ng中的M或者VM改变.

接下来说说scope中event模块,它的api跟一般的event事件模块比较像,提供有$on,$emit,$broadcast,这三个很实用的方法

$on

这个方法是用来定义事件的,这里用到了两个实例变量$$listeners, $$listenerCount,分别用来保存事件,以及事件数量计数

$on: function(name, listener) {        var namedListeners = this.$$listeners[name];        if (!namedListeners) {          this.$$listeners[name] = namedListeners = [];        }        namedListeners.push(listener);        var current = this;        do {          if (!current.$$listenerCount[name]) {            current.$$listenerCount[name] = 0;          }          current.$$listenerCount[name]++;        } while ((current = current.$parent));        var self = this;        return function() {          namedListeners[indexOf(namedListeners, listener)] = null;          decrementListenerCount(self, 1, name);        };      }

分析上面的代码,可以看出每当定义一个事件的时候,都会向$$listeners对象中添加以name为key的属性,值就是事件执行函数,注意这里有个事件计数,只要有父级，则也给父级的$$listenerCount添加以name为key的属性,并且值+1,这个$$listenerCount
会在广播事件的时候用到,最后这个方法返回一个取消事件的函数,先设置$$listeners中以name为key的值为null,然后调用decrementListenerCount来使该事件计数-1.

$emit

这个方法是用来触发$on定义的事件,原理就是loop$$listeners属性,检查是否有值,有的话，则执行,然后依次往上检查父级,这个方法有点类似冒泡执行事件.

$emit: function(name, args) {        var empty = [],            namedListeners,            scope = this,            stopPropagation = false,            event = {              name: name,              targetScope: scope,              stopPropagation: function() {stopPropagation = true;},              preventDefault: function() {                event.defaultPrevented = true;              },              defaultPrevented: false            },            listenerArgs = concat([event], arguments, 1),            i, length;        do {          namedListeners = scope.$$listeners[name] || empty;          event.currentScope = scope;          for (i=0, length=namedListeners.length; i<length; i++) {            // if listeners were deregistered, defragment the array            if (!namedListeners[i]) {              namedListeners.splice(i, 1);              i--;              length--;              continue;            }            try {              //allow all listeners attached to the current scope to run              namedListeners[i].apply(null, listenerArgs);            } catch (e) {              $exceptionHandler(e);            }          }          //if any listener on the current scope stops propagation, prevent bubbling          if (stopPropagation) return event;          //traverse upwards          scope = scope.$parent;        } while (scope);        return event;      }

上面的代码比较简单,首先定义一个事件参数,然后开启一个loop，只要scope有值,则一直执行,这个方法的事件链是一直向上传递的,不过当在事件函数执行stopPropagation方法，就会停止向上传递事件.

$broadcast

这个是$emit的升级版，广播事件,即能向上传递,也能向下传递,还能平级传递,核心原理就是利用深度遍历当前作用域

$broadcast: function(name, args) {    var target = this,        current = target,        next = target,        event = {          name: name,          targetScope: target,          preventDefault: function() {            event.defaultPrevented = true;          },          defaultPrevented: false        },        listenerArgs = concat([event], arguments, 1),        listeners, i, length;    //down while you can, then up and next sibling or up and next sibling until back at root    while ((current = next)) {      event.currentScope = current;      listeners = current.$$listeners[name] || [];      for (i=0, length = listeners.length; i<length; i++) {        // if listeners were deregistered, defragment the array        if (!listeners[i]) {          listeners.splice(i, 1);          i--;          length--;          continue;        }        try {          listeners[i].apply(null, listenerArgs);        } catch(e) {          $exceptionHandler(e);        }      }      // Insanity Warning: scope depth-first traversal      // yes, this code is a bit crazy, but it works and we have tests to prove it!      // this piece should be kept in sync with the traversal in $digest      // (though it differs due to having the extra check for $$listenerCount)      if (!(next = ((current.$$listenerCount[name] && current.$$childHead) ||          (current !== target && current.$$nextSibling)))) {        while(current !== target && !(next = current.$$nextSibling)) {          current = current.$parent;        }      }    }    return event;}

代码跟$emit差不多，只是跟它不同的时,这个是不断的取next值,而next的值则是通过深度遍历它的子级节点,兄弟节点,父级节点,依次查找可用的以name为key的事件.注意这里的注释,跟$digest里的差不多，都是通过深度遍历查找,所以$broadcast方法也不能常用，性能不是很理想

$destroy

这个方法是用来销毁当前作用域,代码主要是清空当前作用域内的一些实例属性,以免执行digest,$emit,$broadcast时会关联到

$destroy: function() {    // we can't destroy the root scope or a scope that has been already destroyed    if (this.$$destroyed) return;    var parent = this.$parent;    this.$broadcast('$destroy');    this.$$destroyed = true;    if (this === $rootScope) return;    forEach(this.$$listenerCount, bind(null, decrementListenerCount, this));    // sever all the references to parent scopes (after this cleanup, the current scope should    // not be retained by any of our references and should be eligible for garbage collection)    if (parent.$$childHead == this) parent.$$childHead = this.$$nextSibling;    if (parent.$$childTail == this) parent.$$childTail = this.$$prevSibling;    if (this.$$prevSibling) this.$$prevSibling.$$nextSibling = this.$$nextSibling;    if (this.$$nextSibling) this.$$nextSibling.$$prevSibling = this.$$prevSibling;    // All of the code below is bogus code that works around V8's memory leak via optimized code    // and inline caches.    //    // see:    // - https://code.google.com/p/v8/issues/detail?id=2073#c26    // - https://github.com/angular/angular.js/issues/6794#issuecomment-38648909    // - https://github.com/angular/angular.js/issues/1313#issuecomment-10378451    this.$parent = this.$$nextSibling = this.$$prevSibling = this.$$childHead =        this.$$childTail = this.$root = null;    // don't reset these to null in case some async task tries to register a listener/watch/task    this.$$listeners = {};    this.$$watchers = this.$$asyncQueue = this.$$postDigestQueue = [];    // prevent NPEs since these methods have references to properties we nulled out    this.$destroy = this.$digest = this.$apply = noop;    this.$on = this.$watch = function() { return noop; };}

代码比较简单,先是通过foreach来清空$$listenerCount实例属性,然后再设置$parent,$$nextSibling,$$prevSibling,$$childHead,$$childTail,$root为null,清空$$listeners,$$watchers,$$asyncQueue,$$postDigestQueue,最后就是重罢方法为noop占位函数

总结

rootScope说完了,这是个使用比例非常高的核心provider,分析的比较简单,有啥错误的地方，希望大家能够指出来，大家一起学习学习,下次有空接着分析别的.