navigator.md

Navigator implementation

As we've seen, our MainActivity currently has adding JobListFragment when it's first created hardcoded into it. Let's make some changes to get it ready for navigation between our Fragments.

The architecture provides a Navigator interface for us to implement, which will be very easy to invoke later from Fragments. It looks something like this:

interface Navigator {

    fun add(fragment: Fragment)
    fun pop()

}

This version of it has just these two operations so that we can easily implement it. (You could of course add others onto it later!)

We'll think about our app as a stack of screens (not the Fragment backstack, a real stack). Through this interface, we can:

• add: put a screen on the top of the stack,
• pop: remove the screen currently on top.

We'll add the Navigator interface to MainActivity's supertypes, and generate the necessary methods.

For add, we can use the implementation already in onCreate, modified to add the Fragment we get as a parameter, and not just a JobListFragment every time:

override fun add(fragment: Fragment) {
    supportFragmentManager
            .beginTransaction()
            .replace(R.id.mainContent, fragment)
            .addToBackStack(null)
            .commit()
}

With this done, we can update our onCreate method to use add:

if (savedInstanceState == null) {
   add(JobListFragment())
}

From here, pop is very simple to implement:

supportFragmentManager.popBackStack()

Finally, we should handle back presses nicely as well:

override fun onBackPressed() {
    if (supportFragmentManager.backStackEntryCount > 1) {
        pop()
    } else {
        finish()
    }
}

If we forgot this, we could remove our first Fragment and be left with a blank MainActivity still open!

Continue...

Time to create some state.