The host limits the number of templates to display for a given task to a maximum of five, of which the last template must be one of the following types:
NavigationTemplatePaneTemplateMessageTemplateMediaPlaybackTemplateSignInTemplateLongMessageTemplate
Note that this limit applies to the number of templates and not the number of
Screen instances in the stack. For example, if an app sends two templates
while in screen A and then pushes screen B, it can now send three more
templates.
Alternatively, if each screen is structured to send a single template, then the
app can push five screen instances onto the ScreenManager stack.
There are special cases to these restrictions: template refreshes and back and reset operations.
Template refreshes
Certain content updates are not counted toward the template limit. In general,
if an app pushes a new template that is of the same type and contains
the same main content as the previous template, the new template is not
counted against the quota. For example, updating the toggle state of a row in a
ListTemplate does not count against the quota. See the documentation of
individual templates to learn more about what types of content updates can be
considered a refresh.
Back operations
To enable sub-flows within a task, the host detects when an app is popping a
Screen from the ScreenManager stack and updates the remaining quota based on
the number of templates that the app is going backward by.
For example, if the app sends two templates while in screen A, then pushes screen B and sends two more templates, the app has one quota remaining. If the app then pops back to screen A, the host resets the quota to three, because the app has gone backward by two templates.
Note that, when popping back to a screen, an app must send a template that is of the same type as the one last sent by that screen. Sending any other template type causes an error. However, as long as the type remains the same during a back operation, an app can freely modify the contents of the template without affecting the quota.
Reset operations
Certain templates have special semantics that signify the end of a task. For
example, the NavigationTemplate is a view that's expected to remain on
the screen and be refreshed with new turn-by-turn instructions for the user.
When a task reaches one of these templates, the host resets the template quota, treating that template as though it's the first step of a new task. This allows the app to start a new task. To learn more, see the documentation for individual templates to learn which trigger a reset on the host.
If the host receives an intent to start the app from a notification action or from the launcher, the quota is also reset. This mechanism lets an app begin a new task flow from notifications, and it holds true even if an app is already bound and in the foreground.