Conductor isn't good at handling view attaching or detaching, it delegates the task to ControllerChangeHandler. But ControllerChangeHandler doesn't do it well, some views which can't be seen are still attached.

For Example

We have a Route with a NormalController as root. The Controllers under NormalController can't be seen.

ControllerStack	Can be Seen	Attached
NormalController	yes	yes

Then we push a DialogController. The Controller under DialogController can be seen. So the removeFromViewOnPush must be set to false.

ControllerStack	Can be Seen	Attached
DialogController	yes	yes
NormalController	yes	yes

Then we push another NormalController.

ControllerStack	Can be Seen	Attached
NormalController	yes	yes
DialogController	no	no
NormalController	no	yes

The root NormalController can't be seen, but it's still attached now.

Another Example

We have a Route with a NormalController as root.

ControllerStack	Can be Seen	Attached
NormalController	yes	yes

Then we push a SwipeToFinishController. The Controller under SwipeToFinishController can be seen when the SwipeToFinishController is being swiping. So the removeFromViewOnPush must be set to false.

ControllerStack	Can be Seen	Attached
SwipeToFinishController	yes	yes
NormalController	yes	yes

Then we push another SwipeToFinishController.

ControllerStack	Can be Seen	Attached
SwipeToFinishController	yes	yes
SwipeToFinishController	yes	yes
NormalController	no	yes

Then we push another SwipeToFinishController.

ControllerStack	Can be Seen	Attached
SwipeToFinishController	yes	yes
SwipeToFinishController	yes	yes
SwipeToFinishController	no	yes
NormalController	no	yes

That's worse. The Controller under the SwipeToFinishController can be seen only if this SwipeToFinishController is the top Controller. But all Controllers are attached.

Principle

Only keep the views which can be seen attached.

Solution

1. Add a new property to Controller. The property describes how the Controller affects the visibility of Controllers below.
2. Let Route decide which views should be attached or detached according to the property of them.

In this project, I name the property opacity. It can be one of three values.

• TRANSPARENT - The Controller which is under this Controller can always be seen.
• TRANSLUCENT - The Controller which is under this Controller can only be seen if this Controller is the top Controller.
• OPAQUE - The Controller which is under current Controller will never be seen.

I do step 2 in a Controller.LifecycleListener named ControllerAttacher.

All Controllers need implement ControllerOpacity to tell ControllerAttacher its opacity.