In HTML5, you can use the Cancel button to prevent users from uploading a file. File length is zero, and the cancel button is displayed when the user does not click it. The code below combines this code with the code in the previous section to detect a click outside of an element. This method is useful for file upload control, input validation, and cloud storage for Firebase plugins.
[lwptoc]Detecting a click outside of an element
You can attach a click event handler to an area outside the element. For example, if a user clicks on a document, you can use jQuery’s closest method to check whether that element was clicked. Depending on the target, this may be an empty document or an entire document. In any case, detecting a click outside of an element is a simple way to add a custom event to a file.
A common JavaScript pattern is to detect a click outside of an element. It can also be used to close a non-modal user interface component. In vanilla JavaScript, this technique is implemented by going up the DOM from the clicked target element and then searching for an ancestor of that element that is part of a flyout container. If a click occurs outside the element, the underlying file is closed, and the file is not opened.
HTML5 for input validation
One of the most common problems in web development is how to validate input when a user clicks on the Cancel button. HTML5 outlines built-in functionality for common input types and the code to use it may be part of your HTML5 page. You may also need to add some scripting to the page to perform custom validation. You must notify your users in a way that is easy for them to understand. Performing input validation on the client-side does not guarantee security, so you will need to use scripting or server-side validation to secure the data.
The HTML5 for error messages is much simpler and more stable than the old IE7. You can customize the error message with the invalid event handler using CSS presentation styles or JavaScript programming codes. File input in an HTML form is a little tricky to the program since you can include the entire file. This is not a simple problem, though. Depending on your browser, it might also display notes about filtering.
HTML5 for file upload control
When a user clicks the “cancel” button on a file input, a dialog box is shown. A listener can detect the change and cancel the dialog. If a user clicks the cancel button, the file upload will be stopped. It will also display an alert box if the user does not wish to continue. The following code demonstrates the use of HTML5 to detect the cancel button.
An IE6 browser isn’t designed to support this behavior, so you should add a code snippet to your HTML5 page that implements this feature. File upload control detection is a standard HTML5 feature. It is possible to detect the change in an input value by implementing jQuery input handlers. But this method is limited to hidden inputs. It also has a small drawback. In order to detect a change in the value of an input, you have to handle the ‘onchange’ event for the file.
Cloud Storage for Firebase plugin
The Cloud Storage for Firebase plugin lets you upload files directly into your Firebase bucket. You must set up Firebase Authentication in order to use it, but you can also allow unauthenticated access. Since your App Engine app and Firebase app share a common data store, you’ll want to limit the scope of access. This plugin supports the state-changed observer, which is useful for handling errors and progress events.
If a user clicks the cancel button before selecting a file, the upload will fail. This happens because the file input field displays a cancel box. If the user clicks this box, the file selection dialog will appear within a thousand milliseconds. Additionally, the cancel button will block the browser window. This dialog is not supported in all browsers. To fix this problem, you must remove the cancel button from your HTML.