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In this video we'll see how to use the debugger to monitor our app in real-time. We'll see how we can execute one line of code at a time, and how the debugger helps us investigate our variables as the app is running!
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So far, we've looked at two ways
to post information from our app
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either to the screen or to the log.
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Now, we're going to go even further and
learn how to monitor our app in real time.
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Our Android tools included debugging tool
called the Dalvik Debug Monitor Server or
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DDMS for short.
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DDMS provides a ton of insight into how
a connected device or emulator is running.
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And we're going to see how to use
it to debug our code line by line.
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Instead of just running
our app like we have been,
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we now need to run our app
with the debugger attached.
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But the first thing we need to do
is set a breakpoint in our code.
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A breakpoint is a point in our code where
the debugger will break out of the normal
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computation and then wait for
instructions from us on how to proceed.
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Its a stopping point where we can
take out time to look at the current
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state of our app.
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Then, if we want, we can continue
processing the codes step by step or
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presume running the app.
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Let's start at the beginning of onCreate.
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Now technically, there's some code that
runs before we get to our onCreate method,
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but we're not worried about that code.
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Let's set our first breakpoint
on the first line of onCreate.
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We can add breakpoints
in a few different ways.
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First, let's click on the line
where we want to add a breakpoint.
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Then click on the Run menu, And
select Toggle Line Breakpoint.
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Notice that a little red circle
representing the breakpoint appears to
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the left of this line of code.
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This area is known as the gutter.
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To delete a breakpoint make sure
that line is still selected, and
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then select Run, Toggle Line Breakpoint
to toggle it back off.
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We can also click in the gutter
next to a line of code to add or
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delete a breakpoint like this.
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We can add breakpoints all over our
code to stop anywhere we like, but
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let's start with just this one.
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Now to run our app with the debugger, we
want to use the debug button at the top.
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Which is just to the right
of the run button and
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looks like a little bug with
a play button on it, click it.
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Then choose to run the app.
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And once they have starts,
it should stop at this breakpoint and
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open the debug perspective.
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Check it out, we now have a new debug
view at the bottom of our screen.
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And the line where we inserted our
breakpoint is now highlighted with blue
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to show where the apps stop.
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Our app is in a pause state at this point.
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If we switch to the emulator,
we'll it's just a blank screen.
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That's because we still haven't run any
of the code that sets up our layout.
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While it's paused, we can look at
all sorts of things about our code.
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We can hover over variables
to see their values.
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Let's scroll up and
see what's in our factBook.
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And then from this little pop-up, we can
click the + sign to take a further look.
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And look, our factBook has already been
loaded and has all of our facts in it.
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Since these variables are part of
the class, they get created first,
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even before the onCreate method.
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But let's take a look at what happens if
we look at these variables that are after
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the current line where we stopped.
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Nothing happens if we hover over fact or
color.
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That's because these variables
haven't been initialized yet.
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They get initialized here in the
onClickListener, but we haven't gotten to
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that point of the execution yet,
we're still up here at the blue line.
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All right,
now let's step through this line by line.
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There's a toolbar down here
in the lower left, and
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it contains controls on how to debug.
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Let's quickly go over each one.
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The first one is the resume button, which
resumes normal execution of the code.
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If there are any further
breakpoints in the code,
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then it will run until it
gets to the next breakpoint.
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Then there are pause and stop buttons,
which are pretty self-explanatory.
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Notice that pause is currently grayed
out because we're already paused.
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The next one down here
with the multiple circles
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shows all the breakpoints we
currently have in the app.
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This can be especially useful when
we're debugging more complex tasks.
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Now let's jump over here to
these buttons at the top.
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These control how we walkthrough the code.
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The first one takes us to
the current line of execution.
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So if we had opened up a different
class like color wheel,
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we could use this button to get
back to where we were paused.
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These next ones are used to step
through the code in different ways.
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The first one is called step over.
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Followed by two different step into
buttons, and then a step out button.
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Step over, the first one,
will step to the next line to be
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executed after this current line is
all done, which for me is line 24.
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Clicking on this step over button
would process everything in
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the super.onCreate method, and
then it would pause again right here.
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Step into means that we will step
into the current method call,
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provided there is one on the current line.
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In this case, clicking step into means
that we would take the debugger into
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the onCreate method of the superclass, and
we would continue debugging from there.
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Eventually, we'd end up back here,
just like if we clicked step over, but
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that could be 1 or 1,000 lines later.
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The last button, step out,
means that we'll automatically process all
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the code until the end of the current
method, which in this case is OnCreate.
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All right, let's step through our
OnCreate code using the step over button.
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Click it twice to get down here to
where we initialize factTextView.
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Now this line hasn't been executed yet,
so we still can't look at factTextView.
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Let's hit step over again, and
now that that line is done,
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we can hover over factTextView and see
a little pop up like we've seen before.
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Hitting the + sign, let us see
a lot more information about it.
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And if we scroll down to mText,
there's our fun fact.
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Also, in the debug pane, if we expand
this which is our fun facts activity,
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we can see that factTextView
now has a value.
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Whereas, showFactButton and
relativeLayout are still null.
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But if we step over two more times,
Now all three views have been initialized.
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Finally, if we hit the resume button,
we can skip the rest of the code, and
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if we switch to our emulator,
our app is up and running.
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And if we click the button,
it just continues as normal.
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Now if we add a new breakpoint here,
In the onClickListener,
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then go back to the app and
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click the button, We're brought right
back into the debugger, awesome.
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This should be enough to get you started.
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You can use the debugger
to troubleshoot or
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also just understand how
your program is running.
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It's a super useful tool.
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