这通过注册一个emHeightSource名为every 的观察者来工作$digest。它会更新该__height属性，该属性又将在监视emHeightTarget：
/*
* Get notified when height changes and change margin-top
*/
.directive( 'emHeightTarget', function() {
return {
link: function( scope, elem, attrs ) {
scope.$watch( '__height', function( newHeight, oldHeight ) {
elem.attr( 'style', 'margin-top: ' + (58 + newHeight) + 'px' );
} );
}
}
} )
/*
* Checks every $digest for height changes
*/
.directive( 'emHeightSource', function() {
return {
link: function( scope, elem, attrs ) {
scope.$watch( function() {
scope.__height = elem.height();
} );
}
}
} )
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