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Logging the Lifecycle Methods
5:40We've learned a lot about the activity lifecycle. In this video we'll go through a sample app and see the activity lifecycle in action!
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All right, now that we've got a pretty
good idea of what the activity life cycle
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is and when each of the life
cycle methods will be called,
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let's jump into a sample app and
run a few tests to see these in action.
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Here's the app we'll be using to
test out the activity life cycle.
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Before we get started let's take
a minute to see how the app works.
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First, there's a custom activity
called LoggingActivity.
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Every activity that
extends LoggingActivity
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will log a message each time
a lifecycle method is called.
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Next we have MainActivity.
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MainActivity is the entry
point of the application.
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And has two buttons that launch
the other two activities.
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One button launches opaque activity and
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the other button launches
transparent activity.
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MainActivity, OpaqueActivity, and
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TransparentActivity all
extend the LoggingActivity.
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So we should see a log message for
each and
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every life cycle method that gets
called for these activities.
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Let's run the app and
see what gets called.
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Before we start parsing the log cat, let's
filter it by our tag, life cycle example.
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Just as we would expect from the diagram,
the methods called so
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far are onCreate, onStart and
onResume from MainActivity.
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And MainActivity is currently in a running
state at the top of the activity stack.
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What do we expect to happen
if we launch OpaqueActivity?
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If OpaqueActivity is launched,
MainActivity won't be visible anymore.
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So we would expect to see onPause and
onStop from MainActivity.
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And for
OpaqueActivity to be the running activity,
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we would expect to see onCreate, onStart,
and onResume from OpaqueActivity.
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But what will be the order
of these methods?
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Take a minute to think about it.
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Feel free to pause me, I'll be right here.
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All right, got your guess?
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Let's find out.
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If you guessed MainActivity is onPause,
followed by onCreate,
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onStart, and
onResume from OpaqueActivity, and
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finishing up with MainActivity is onStop,
then you're right.
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Let's take a minute to
see why this is the case.
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OnPause is called when you can no
longer interact with an activity.
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This being called first, means main
activity is removed from the top of
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the activity stack, which is necessary if
we're going to have a new activity on top.
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Next comes onCreate, onStart.
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And onResume, from OpaqueActivity.
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OpaqueActivity is now on top of
the activity stack, and ready for
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user interaction.
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Lastly, MainActivities on stop is called.
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OnStop is called when the activity
is no longer visible.
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Since OpaqueActivity isn't
visible until after onStart and
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since onResume will
immediately follow onStart.
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MainActivity's onStop method
won't be called until after
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OpaqueActivity's onResume method.
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Now that we know which life cycle
methods were called to get us here,
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let's hit the back button and
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see which life cycle methods get called
when we go back to Main Activity.
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Looks like it's the same
calls as last time,
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except we've switched MainActivity and
OpaqueActivity.
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The only difference is the call
to onDestroy and OpaqueActivity.
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Leaving an activity by hitting
the back button is our way of telling
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Android that we're
finished with an activity.
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That's why Android destroys OpaqueActivity
instead of leaving it in a stopped state.
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Now that we're back in MainActivity,
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let's see what happens when we
launch the TransparentActivity.
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Go ahead and pause me if you'd like
to take a guess at what happens.
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Here we go.
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Turns out, it's exactly like when
we launched the OpaqueActivity
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except OnStop isn't
called in MainActivity.
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Since we can still see main activity
behind our TransparentActivity.
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MainActivity is on a pause state and
onStop isn't called.
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Cool.
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For the last test, let's see what
happens when we hit the home button.
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First, TransparentActivity is pause.
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And then, both MainActivity and
TransparentActivity are stopped.
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When we hit the Home button,
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we're telling Android that we're
moving on to something else.
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Android doesn't know if we'll be back or
not, so
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it keeps our activities around in
a stopped state, in case we return.
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Keep in mind that Android may kill
stopped activities to free up memory.
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You've learned a lot about
the activity lifecycle.
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In the next video we'll learn how we
can hook into our activities lifecycle
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to save data using shared preferences.
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Also, if you'd like to try these tests on
your own, or are curious about how this
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app is programmed there's a link to
the GitHub project in the teacher's notes.
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And it's included below in
the project files as well.
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