ember.js - Emberjs DefaultTo 计算属性

Question

通常，如果未设置实际模型属性，我希望我的 View 默认为特定值。此占位符文本/值仅供查看，因此不应放置在模型 imo 中。

所以，这就是我最终要做的:

// Sample 'Model' for illustration purposes only.
var myModel = Ember.Object.extend({
  title: null,

  description: null,

  thumbUrl: null
});

/**
 * Sample View
 * Render view properties which are actually 
 * computed of the actual 'content' properties
 */
var myView = Ember.View.extend({
  template: Ember.Handlebars.compile('<p>Title: {{view.title}}</p> <p>Description: {{view.description}}</p> <p>Image: <img {{bindAttr src="view.thumbUrl"}}/></p>'),

  title: function () {
    return this.get('content.title') || 'Title goes here';// placeholder 'title' text
  }.property('content.title'),

  description: function () {
    return this.get('content.description') || 'This is your description'; // placeholder 'description'
  }.property('content.description'),

  thumbUrl: function () {
    return this.get('content.thumbUrl') || 'http://placehold.it/100x100';
  }.property('content.thumbUrl')
});

关于如何减少在所有这些属性(即“title”、“description”和“thumbUrl”)上定义默认值的样板文件的任何建议？

我查看了 Ember.computed.defaultTo 但未能理解如何使用它。这就是我设想的实际情况:

var myView = Ember.View.extend({
  template: Ember.Handlebars.compile('<p>Title: {{view.title}}</p> <p>Description: {{view.description}}</p> <p>Image: <img {{bindAttr src="view.thumbUrl"}}/></p>'),

  title: Ember.computed.defaultTo('content.title', 'Title goes here'),

  description: Ember.computed.defaultTo('content.description', 'This is your description'),

  thumbUrl: Ember.computed.defaultTo('content.thumbUrl', 'http://placehold.it/100x100')
});

那么如何做到这一点呢？

如果有更好的方法来做这类事情，我希望在评论中听到它们。

另外，指向 Ember.computed.defaultTo 所做的事情的指针也会非常有帮助。

Answer 1

这不是 Ember.computed.defaultTo 的目的。 Ember.computed.defaultTo 采用单个 defaultPath 参数。来自文档:

computed property which acts like a standard getter and setter, but defaults to the value from defaultPath.

如果您阅读 test , 它揭示了一些光。

testBoth('Ember.computed.defaultTo', function(get, set) {
  var obj = { source: 'original source value' };
  Ember.defineProperty(obj, 'copy', Ember.computed.defaultTo('source'));

  equal(get(obj, 'copy'), 'original source value');

  set(obj, 'copy', 'new copy value');
  equal(get(obj, 'source'), 'original source value');
  equal(get(obj, 'copy'), 'new copy value');

  set(obj, 'source', 'new source value');
  equal(get(obj, 'copy'), 'new copy value');

  set(obj, 'copy', null);
  equal(get(obj, 'copy'), 'new source value');
});

相反，您可以像这样编写自己的助手:

Ember.computed.defaultValue = function(dependentKey, defaultValue) {
  return Ember.computed(dependentKey, function() {
    return Ember.get(this, dependentKey) || defaultValue;
  });
};

var myView = Ember.View.extend({
  template: Ember.Handlebars.compile('<p>Title: {{view.title}}</p> <p>Description: {{view.description}}</p> <p>Image: <img {{bindAttr src="view.thumbUrl"}}/></p>'),

  title: Ember.computed.defaultValue('content.title', 'Title goes here'),

  description: Ember.computed.defaultValue('content.description', 'This is your description'),

  thumbUrl: Ember.computed.defaultValue('content.thumbUrl', 'http://placehold.it/100x100')
});

如果您不想创建自己的辅助函数，另一种方法是为默认值使用单独的属性，然后使用 Ember.computed.any。

var myView = Ember.View.extend({
  template: Ember.Handlebars.compile('<p>Title: {{view.title}}</p> <p>Description: {{view.description}}</p> <p>Image: <img {{bindAttr src="view.thumbUrl"}}/></p>'),

  defaultTitle: 'Title goes here',
  title: Ember.computed.any('content.title', 'defaultTitle'),

  defaultDescription: 'This is your description',
  description: Ember.computed.any('content.description', 'defaultDescription'),

  defaultThumbUrl: 'http://placehold.it/100x100',
  thumbUrl: Ember.computed.any('content.thumbUrl', 'defaultThumbUrl')
});