State persistence - making your app “always alive”

One of the challenges when writing mobile applications is to manage the application lifecycle. To preserve battery life and performance, apps may be suspended as the user moves to other applications. Furthermore, your application may be terminated to free more memory if this is required.

One aspect of making your application feel “always alive” is to ensure that if it is later relaunched then the user is taken to exactly the same point that they were previously viewing. The Okra App Framework simplifies this process by persisting the navigation stack in addition to allowing for page specific state.

Persisting the navigation stack

By default the navigation stack is not automatically persisted. To enable this you should add the following code to your AppBootstrapper.SetupServices() method.

public class AppBootstrapper : OkraBootstrapper
{
    protected override void SetupServices()
    {
        NavigationManager.NavigationStorageType = NavigationStorageType.Local;
    }
}

There are three possible NavigationStorageType values,

Preserving page state

In some cases you may want to go a step further and store some page state. Imagine for example you are writing a comment in a social networking app, then change application. When you return you should expect to continue from where you left off. Other things you may want to preserve are selected search filters, selected items, position in a view, etc.

To allow a page to persist state the view-model should implement the IActivatable interface.

public interface IActivatable 
{ 
    void Activate(PageInfo pageInfo); 
    void SaveState(PageInfo pageInfo); 
}

When the page is initiated, the Activate(…) method is called with any page state encapsulated in a PageInfo object. If this is the first navigation to this page then no state will available, and calls to the PageInfo.TryGetState<T>(string key, out T value) method will return false.

If however the Okra App Framework needs to suspend the page for any reason, the SaveState(…) method will be called. In this case the view-model can persist any state using the PageInfo.SetState<T>(string key, T value) method. When the page is reactivated the PageInfo.TryGetState<T>(string key, out T value) method will return true, with the persisted state returned as ‘value’.

Example code

For example, in our hypothetical social networking application you can comment on the current post. In this case we may wish to persist a partially written comment when the user switches to another application, such that if the application is terminated it can be restored when the user returns. In this case the “ViewPost” view-model has a ‘Comment’ property that is data-bound to the UI.

[ViewModelExport("ViewPost")]
public class ViewPostViewModel : MyViewModelBase, IActivatable
{
    public string Comment { ... }

    ...
 
    public void Activate(PageInfo pageInfo)
    {
        ...

        string persistedComment;

        if (pageInfo.TryGetState("Comment", out persistedComment))
            this.Comment = persistedComment;
    }
 
    public void SaveState(PageInfo pageInfo)
    {
        pageInfo.SetState("Comment", this.Comment);
    }
}