Question

我正在阅读有关范围方法$watch here的文档。该方法接受： $watch(watchExpression, [listener]; 然后他们提供了例子：

// let's assume that scope was dependency injected as the $rootScope
var scope = $rootScope;
scope.name = 'misko';
scope.counter = 0;

expect(scope.counter).toEqual(0);
scope.$watch('name', function(newValue, oldValue) {
 scope.counter = scope.counter + 1;
});
expect(scope.counter).toEqual(0);

scope.$digest();
// the listener is always called during the first $digest loop after it was registered
expect(scope.counter).toEqual(1);

scope.$digest();
// but now it will not be called unless the value changes
expect(scope.counter).toEqual(1);

scope.name = 'adam';
scope.$digest();
expect(scope.counter).toEqual(2);



// Using a listener function
var food;
scope.foodCounter = 0;
expect(scope.foodCounter).toEqual(0);
scope.$watch(
 // This is the listener function --------- WHY ?????????????
 function() { return food; },
 // This is the change handler ---- THIS SHOULD BE A LISTNER FUNCTION
 function(newValue, oldValue) {
   if ( newValue !== oldValue ) {
     // Only increment the counter if the value changed
     scope.foodCounter = scope.foodCounter + 1;
   }
 }
);
// No digest has been run so the counter will be zero
expect(scope.foodCounter).toEqual(0);

// Run the digest but since food has not changed count will still be zero
scope.$digest();
expect(scope.foodCounter).toEqual(0);

// Update food and run digest.  Now the counter will increment
food = 'cheeseburger';
scope.$digest();
expect(scope.foodCounter).toEqual(1);

我不理解的是，如果这是function() { return food; } // This is the listener function的函数，他们在第二个示例中将should return the value that will be watched.称为watchExpression的原因？

Answer 1

评论有点误导，您可能会提出问题或提出改进请求。

正如你所说，$watch(watchExpression, [listener])

有2个参数

watchExpression
每次调用$ digest（）时都会调用watchExpression，并且应该返回将要监视的值监视表达式可以是string或function。如果你指定一个函数，这是一个不的监听器，而是一个多次调用的比较函数，所以不要在这里做任何花哨的东西;）
该比较功能用于角度脏检查关于这方面的更多细节可以在angularJs的创建者的视频的这一部分中找到：https://www.youtube.com/watch?feature=player_detailpage&v=ZhfUv0spHCY#t=1988
listener
只要watchExpression的值发生变化，就会调用此函数因此，这是更新模型和实现业务逻辑的理想场所

$ watch API描述 - 第一个函数描述为监听器功能，为什么？

1 个答案: