 <p>The observer is notified when an observed key path changes it's value. The ´change` dictionary contains information related to how the observed key path has changed. This dictionary is only filled with the values according to the options that you provide when setting</p> <ul> <li><code>NSKeyValueObservingOptionNew</code> - Specifies that you want to have access to the new value that the key path changed into. </li> <li><code>NSKeyValueObservingOptionOld</code> - Specifies that you want to have access to the old value that the key path changed from.</li> </ul> <p>If specified to be sent these old and/or new values are accessible from the <code>change</code> dictionary using these keys:</p> <ul> <li><code>NSKeyValueChangeNewKey</code> - To access the new value.</li> <li><code>NSKeyValueChangeOldKey</code> - To access the old/previous value.</li> </ul>