Showing Revision 2 created 08/18/2015 by Udacity Robot.

1. Title:
   05-24 Debug Mode in Android Studio
2. Description:

   05-24 Debug Mode in Android Studio

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   We're making really good progress so
   far.
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   As we continue to build
   more challenging things for
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   our app, we're going to have to
   improve our debugging skills.
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   In this video,
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   we're going to learn about how to
   use a tool called the debugger.
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   We'll access the debugger
   using Android Studio, and
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   the debugger is meant to help us
   identify and fix errors in our code.
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    The great thing about the debugger is
    that we can pause the app at a certain
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    point in time, and then we can inspect
    the whole state of the app more closely.
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    Normally, when the app is
    running on the device,
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    all the code gets executed very rapidly.
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    Within milliseconds, it can respond
    to button clicks, update the screen,
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    calculate the price, and etcetera.
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    But with the debugger, we can pause at
    a specific line of code in our app,
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    then we can step through our code line
    by line as quickly or slowly as we want.
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    If you want to learn more techniques on
    how to debug your app in Android Studio,
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    just search for
    debugging Android Studio.
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    This first link is good,
    it's a developer.android.com page and
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    it has a bunch of techniques
    on how to debug your app.
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    Together we're going to learn about
    how to work with breakpoints.
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    After this course,
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    you can always come back to this
    page to refresh your memory.
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    In this exercise, I want to show you
    that your Android device does run
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    each line of code one at at time,
    from top to bottom, within a method.
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    We'll also verify that clicking
    on the + button does trigger
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    the increment method.
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    And clicking on the- button does
    trigger the decrement method.
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    First add something
    known as a breakpoint.
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    A breakpoint marks
    a specific line of code
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    where the debugger should
    pause when it reaches here.
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    The red circular dot indicates
    that the breakpoint is
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    on this line on line 25 of
    our main activity file.
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    You can add breakpoints to as many
    places as you want in your code.
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    But for now,
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    I'm just going to add one on the first
    line of the increment method.
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    Then I can hit this bug icon
    to run the app in debug mode.
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    The app will only pause
    on these breakpoints,
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    when the app is running in debug mode.
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    If you run the app normally,
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    with the screen play button, then it
    won't pause at these break points.
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    For a brief moment on the device you
    may have seen a dialogue that says,
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    waiting for debugger.
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    There's also a button to force close
    the app, but don't click on that.
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    Just wait for the debugger to attach,
    and the message will go away soon.
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    Then the app starts like normal and
    you can interact with it.
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    Now in debug mode, this window will pop
    up and show the status of the debugger.
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    If you don't see it, you can click
    on this tab down here called, Debug.
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    Now if I hit this + button here,
    I expect the increment method will be
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    triggered and that the app
    will pause at this breakpoint.
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    Let's see what happens.
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    Cool, it stops there.
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    In the Debug pane,
    we see a list of variables.
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    This reflects the current state
    of the app where it was paused.
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    We don't see our quantity variable yet
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    because it hasn't finished
    executing this line of code yet.
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    There's a bunch of options in how
    to proceed with debugging, but
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    I'm going to click on this
    option that says step over.
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    This will step over the current line
    of code to the next line of code.
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    Now line 25 hasn't been executed, so
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    there's a quantity variable showing
    up in the Variables pane here.
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    Now the app is currently
    paused at line 26.
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    We finished executing line 25,
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    so that's why we see the quantity
    variable showing up down here.
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    The value of the variable is set to 3,
    which is expected.
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    This list of variables also shows
    the current value of each variable.
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    We can also verify that the quantity
    variable has a value of 3,
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    which we assigned up here.
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    We know that the app is currently paused
    at this point, because normally when you
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    hit the + button, it would automatically
    update the quantity to be 3.
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    But since the quantity is still at 2,
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    we know that it hasn't finished
    executing the increment method yet.
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    We can click step over
    to go to the next line.
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    At this point the method is done,
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    and we're not really interested in
    how it actually updates this screen.
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    So let's just hit resume program.
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    All we care about is that it
    actually does update the screen.
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    If you're interested, you could have
    clicked on these other options to look
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    at more details of how it
    actually updates the screen.
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    At this point the app
    is running normally.
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    But the debugger is still attached.
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    So if I hit the + button again,
    it will stop at this breakpoint.
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    If I want to detach the debugger so that
    it doesn't always stop on this line when
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    I hit the + button, I can just
    click on this red stop button.
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    Now, whenever I hit the + button
    it responds immediately and
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    updates the value to 3.
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    Now I want you to try
    it on your computer.
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    Go ahead and add a breakpoint to
    the increment and decrement methods.
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    Then run the app in Debug Mode,
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    and then on the device, try to
    trigger each of these breakpoints.
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    Then step through each line of code,
    making sure that the quantity
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    variable shows up with the right
    value in the variables list.
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    You can click on this play button
    to resume execution of the app so
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    that it runs as normal.
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    Or you can hit the stop button
    to detach the debugger.