Resume a frozen download in android chrome

Many developers treat the unload event as a guaranteed callback and use it as an end-of-session signal to save state and send analytics data, but doing this is extremely unreliable , especially on mobile!

The unload event does not fire in many typical unload situations, including closing a tab from the tab switcher on mobile or closing the browser app from the app switcher. For this reason, it's always better to rely on the visibilitychange event to determine when a session ends, and consider the hidden state the last reliable time to save app and user data.

Furthermore, the mere presence of a registered unload event handler via either onunload or addEventListener can prevent browsers from being able to put pages in the Back-Forward Cache for faster back and forward loads. In all modern browsers including IE11 , it's recommended to always use the pagehide event to detect possible page unloads a.

If you need to support Internet Explorer versions 10 and lower, you should feature detect the pagehide event and only use unload if the browser doesn't support pagehide :. The beforeunload event has a similar problem to the unload event, in that when present it prevents browsers from caching the page in their Back-Forward Cache.

The difference between beforeunload and unload , though, is that there are legitimate uses of beforeunload. For instance, when you want to warn the user that they have unsaved changes they'll lose if they continue unloading the page. Since there are valid reasons to use beforeunload but using it prevents pages from being added to the Back-Forward Cache, it's recommended that you only add beforeunload listeners when a user has unsaved changes and then remove them immediately after the unsaved changes are saved.

In other words, don't do this since it adds a beforeunload listener unconditionally :. Instead do this since it only adds the beforeunload listener when it's needed, and removes it when it's not :. There are lots of legitimate reasons web pages shouldn't be frozen while running in the hidden state. The most obvious example is an app that plays music. There are also situations where it would be risky for Chrome to discard a page, like if it contains a form with unsubmitted user input, or if it has a beforeunload handler that warns when the page is unloading.

For the moment, Chrome is going to be conservative when discarding pages and only do so when it's confident it won't affect users. For example, pages that have been observed to do any of the following while in the hidden state will not be discarded unless under extreme resource constraints:. The Back-Forward Cache or bfcache or Page Cache is a term used to describe a navigation optimization some browsers implement that makes using the back and forward buttons faster.

When a user navigates away from a page, these browsers freeze a version of that page so that it can be quickly resumed in case the user navigates back using the back or forward buttons. Remember that adding a beforeunload or unload event handler prevents this optimization from being possible. Since a page can be loaded in either the active, passive, or hidden state, a separate loading state does not make sense, and since the load and DOMContentLoaded events don't signal a lifecycle state change, they're not relevant to this API.

In frozen and terminated states, freezable tasks in a page's task queues are suspended, which means asynchronous and callback-based APIs such as IndexedDB cannot be reliably used.

In the future, we will add a commit method to IDBTransaction objects, which will give developers a way to perform what are effectively write-only transactions that don't require callbacks. In other words, if the developer is just writing data to IndexedDB and not performing a complex transaction consisting of reads and writes, the commit method will be able to finish before task queues are suspended assuming the IndexedDB database is already open.

This allows you to ensure your page correctly handles the freeze and resume events as well as the document. Developers who want to respect the system resources of their user's devices should build their apps with Page Lifecycle states in mind. It's critical that web pages are not consuming excessive system resources in situations that the user wouldn't expect.

In addition, the more developers start implementing the new Page Lifecycle APIs, the safer it will be for browsers to freeze and discard pages that aren't being used. This means browsers will consume less memory, CPU, battery, and network resources, which is a win for users. Lastly, developers who want to implement the best practices described in this article but don't want to memorize all the possible state and event transitions paths can use PageLifecycle.

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