Save Previous States with a History State

Instructor: [00:00] I've modified the matching machine to wrap the answering state in a quiz state. This allowed me to add a verifying step where you can verify your answers before you choose to submit them or go back and change them again.

[00:14] I've also removed the onDone transition and replaced it with a manual continue event, so the user can decide when they want to move on. You can continue and then change answers will take you back to the answering state. I still need to write a transition to go from the verifying state over to the submitted state.

[00:33] Let's add that in the code here. We'll add a submit event. Because submit it is not a sibling to the verifying state, we need to reference it by ID and add that ID to the submitted state. If we update, the verifying state has a transition directly to the submitted state.

[00:53] There's an issue with this machine, though. Say, we select some items and then continue to the verifying state, and realize we want to change them. When we come back to the answering state, we've returned in the initial unselected state for each of the list. We need some way for the state machine to remember what state it was in. This is where we can use history states.

[01:14] We'll add a hist state as a sibling to the top list and bottom list and give it a type of history. What a history state does, is it keeps track of any transitions that happen to any of its sibling states. Then, when the state machine transitions to that history state, the history state redirects to the most recently entered sibling state.

[01:38] We can specify that the history state is a history state of type shallow, which is the default, or deep, which will remember any transitions to siblings or their sub-states. Now we just update this change answers transition so that it goes to answering.hist, instead of the higher level answering state.

[01:59] Now if we select the top list, but leave the bottom list unselected, and then continue to verifying and return back to the history state, the top list is selected and the bottom list is unselected.

[02:13] Let's add the UI for this verifying state. We'll add a button to change answers and one to submit. I need to reference the verifying state by its parent, saying, quiz.verifying. We'll add a button to the answering state to continue. Let's see what that looks like.

[02:39] We can select R2D2 and Yavin4 and choose continue. Then, if we're not sure, we can go back to change answers and that remembers that we're in a selected state. Then, we can continue and when we are ready, summit our answer.

[02:58] One thing to watch out for with history states is that you almost never want to transition from a sibling state to a history state, because all that would do is transition immediately back to the state you came from. Any transitions into this history state should come from outside of this answering state.