问题
In this document describing the lifecycle of a Windows 10 UWP app, it states:
Users now expect your app to remember its state as they multitask on their device. For example, they expect the page to be scrolled to the same position and all of the controls to be in the same state as before. By understanding the application lifecycle of launching, suspending, and resuming, you can provide this kind of seamless behavior.
However, there doesn't appear to be much documentation on how this is actually achieved. I gather that everything is to be manually saved by the app developer, and then recreated from scratch on resume using whatever data you stashed away when the app was suspending, all in order to create the illusion that the exact memory state of the app never changed.
I'm trying to puzzle through this using just a minimal example, a XAML page containing nothing other than a TextBox. Even this situation, though, I'm struggling a bit to understand how to achieve the goal. I'll provide more general thoughts, but my concrete question simply is how do you save and then restore a simple TextBox for resume from termination? I'm working in C++/CX but will take any help I can get.
Here are my thoughts on this so far:
- At minimum, obviously the text of the TextBox has to be saved.
- This could be saved into the
ApplicationData::Current->LocalSettings. - One issue I see immediately is that the document I cited above on lifecycles states that apps must take care of their saving within 5 seconds of the suspend signal or face termination. A Textbox could potentially hold a lot of data, causing a save to potentially be cutoff in the face of busy IO, particularly if we start scaling beyond the trivial single TextBox situation.
- Fortunately, the document states, "We recommended that you use the application data APIs for this purpose because they are guaranteed to complete before the app enters the Suspended state. For more info, see Accessing app data with the UWP app." Unfortunately, when you follow that link, there is nothing relevant there providing any more detail, and I can't find anything documenting this behavior in the API's. By saving into
ApplicationData::Current->LocalSettingsare we safe from being cut off with corrupted or lost data?
- This could be saved into the
- Once the minimum has been taken care of, next we'll probably need extras like cursor and window position.
- We can get the cursor position with
TextBox->SelectionStart, which as far as I can tell, is undocumented in the API of this usage of returning the current cursor position. This seems an easy fit to also store as anintintoApplicationData::Current->LocalSettings. - How can we get, save, and restore the scroll position of the TextBox window?
- We can get the cursor position with
- Now that we've got the extras, how about the hard stuff, like undo history? I'm assuming this is impossible as my question on Stackoverflow on how to access the TextBox's undo facility has gotten no answers. Nonetheless, it does seem like a poor user-experience if they swap to another app, come back thinking that the app never closed due to the beautiful and seamless restore from termination we implemented, and their undo history has been wiped out.
- Is there anything else that would need to be saved for the TextBox to create the ideal user-experience? Am I missing something or is there an easier way to do all this? How would something like Microsoft's Edge Browser handle the complex case where there are dozens of tabs, form inputs, scroll positions, etc. that all need to be saved in 5 seconds?
回答1:
The App lifecyle document you reference has been updated for Windows 10, but seems to have lost some of the important bits that you are wondering about.
I found an old blog post, Managing app lifecycle so your apps feel "always alive", that seems to be the inspiration for your link.
In the blog post, there is a paragraph towards the end that reads:
Save the right data at the right time
Always save important data incrementally throughout the life of your app. Because your app has only up to five seconds to run suspending event handler code, you need to ensure that your important app data has been saved to persistent storage by the time it is suspended.
There are two types of data for you to manage as you write your app: session data and user data. Session data is temporary data that is relevant to the user’s current experience in your app. For example, the current stock the user is viewing, the current page the user is reading in an eBook or the scroll position in a long list of items. User data is persistent and must always be accessible to the user, no matter what. For example, an in-progress document that the user is typing, a photo that was taken in the app or the user’s progress in your game.
Given the above, I'll attempt to answer your questions:
how do you save and then restore a simple TextBox for resume from termination?
As the end user is typing in the TextBox, the app saves the contents in the background to the data store. To borrow from how word processing software works, you auto-save the textbox "document". I would consider the textbox content to be what the blog post above describes as "user data". Since the save is done outside of suspension, there is no time window to worry about.
When your app resumes from termination, it checks the data store and loads any data into the textbox.
Once the minimum has been taken care of, next we'll probably need extras like cursor and window position.
I would consider these items "session data" and would save them during suspension. After all there is no need to keep track of this info while the app is active. The user doesn't care where the cursor was 10 minutes ago when he started typing - he only cares about the cursor position at the time of suspension.
how about the hard stuff, like undo history?
I would consider undo history to be "user data" and would save it while it is happening (outside of suspension). In other words, as the user types in content, your app should be saving the information necessary to undo.
来源：https://stackoverflow.com/questions/31977234/uwp-winrt-how-to-save-and-then-restore-a-simple-textbox-for-resume-from-termina