Ember.js：如何防止多次击中该属性观察器？

Question

In my controller I have a property and a function observing that property. When the property changes once, the observer is being hit three times - twice with the old data and once with the new data. You can see it happen in the console window of the jsbin I just created:

jsbin

Usage: click one of the books (not the first one), and look in the console window at the results.

In my actual app, the work to be performed requires an asynchronous download. This observer is downloading the wrong content twice and the correct content once because of the three hits. Making this problem more obvious is that the asynchronous responses do not come back in sequence. :)

An interim solution has been to schedule the download code to run later.

Answer 1

I'm not sure why this is happening, but the guides give a way to fix it. They suggest something like this:

bookObserver: function() {
    Ember.run.once(this, 'bookWasChanged');
}.observes('book'),

bookWasChanged: function() {
    // This will only run once per run loop
}

Personally, I always make the assumption that observers could fire even when I don't want them to. For instance, with this code, I would do the following:

bookObserver: function() {
    var book = this.get('book');
    var lastBook = this.get('lastBook');

    if (book !== lastBook) {
        this.set('lastBook', book);
        doSomethingWithBook(book);
    }
}.observes('book')

This way, even if Ember calls your observer 1000 times, you're only going to do the work you want when the book actually changes.