AngularJs双向绑定详解

双向绑定的三个重要方法：

• $scope.$apply()
• $scope.$digest()
• $scope.$watch()

一、$scope.$watch()

　　我理解的$watch就是将对某个数据的监听器对象存储在$scope下。当给$watch指定如下两个函数，就可以创建一个监听器：

• 一个监控函数，我们通常传进去的是一个表达式，比如说“user.firstName”，但框架本身实际上是调用了一个函数，返回指定所关注的那部分数据。
• 一个监听函数，用于在数据变更的时候接受提示。

　　为了实现$watch，我们需要存储监听器对象。在Scope构造函数上添加一个数组：

function Scope() {
  this.$$watchers = [];
}

$$在angular中表示这个变量被当作私有的来考虑，不应当在外部代码中调用。

　　现在我们正式定义$watch()方法，源代码如下所示：

$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
            };

        // 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() {
          arrayRemove(array, watcher);
        };
      },

其中$$watchers就是wo我们上述的scope中存储监听器的数组,$watch()通过unshift()方法将监听器对象加入数组。

二、$scope.$digest()

$digest函数的作用是简而言之就是作用域上遍历所有监听器，也就是$scope.$$watchers，调用每个监听器对象下的监控函数，并且比较它返回的值和上一次返回值的差异。如果不相同，监听器就是脏的，它的监听函数就应当被调用。源代码如下所示：

$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');

        do { // "while dirty" loop
          dirty = false;
          current = target;

          while(asyncQueue.length) {
            try {
              asyncTask = asyncQueue.shift();
              asyncTask.scope.$eval(asyncTask.expression);
            } catch (e) {
              $exceptionHandler(e);
            }
          }

          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 && (value = watch.get(current)) !== (last = watch.last) &&
                      !(watch.eq
                          ? equals(value, last)
                          : (typeof value == 'number' && typeof last == 'number'
                             && isNaN(value) && isNaN(last)))) {
                    dirty = true;
                    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);
                    }
                  }
                } 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 $broadcast
            if (!(next = (current.$$childHead || (current !== target && current.$$nextSibling)))) {
              while(current !== target && !(next = current.$$nextSibling)) {
                current = current.$parent;
              }
            }
          } while ((current = next));

          if(dirty && !(ttl--)) {
            clearPhase();
            throw $rootScopeMinErr('infdig',
                '{0} $digest() iterations reached. Aborting!
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);
          }
        }
      },

三、$scope.$apply()

　　$apply使用函数作参数，它用$eval执行这个函数，然后通过$digest触发digest循环。源代码如下所示：

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

　　可以看到，在apply方法里其实是调用了digest方法的，那么为什么要多增加一个apply来调用digest呢，可以看到这段代码中并没有直接调用digest而是首先进行了对expr的检验，也就是eval方法，这个方法如果校验不通过，是会抛出异常的，而angular并不推荐外部直接调用digest，所以就增加了apply方法来间接调用。

　　利用$apply(),我们可以执行一些与Angular无关的代码，这些代码也还是可以改变作用域上的东西，$apply可以保证作用域上的监听器可以检测这些变更。

四、何时执行和跳出digest loop呢？

　　引用网上一张图来解释

资料引用自：http://my.oschina.net/brant/blog/419641