This course introduces you to the design and implementation of Android applications for mobile devices. You will develop an app from scratch, assuming a basic knowledge of Java, and learn how to set up Android Studio, work with various Activities and create simple user interfaces to make your apps run smoothly.
The Android Development Environment - Part 2
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来自 University of Maryland, College Park 的课程
Programming Mobile Applications for Android Handheld Systems: Part 1
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University of Maryland, College Park
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从本节课中
Android Platform and Development Environment
与讲师见面
Dr. Adam PorterProfessor
Computer Science
[MUSIC]
The IDE also comes with a debugger that gives developers powerful tools for
examining the internal state of a running application.
And this can be useful of course when you have a bug in your application and
you're not sure exactly what's causing it.
So let's use the debugger to examine an application.
Here I'm showing you a simple Android application I've named, the answer.
And this app is supposed to show you the answer to life, the universe and
everything which, as The Hitchhiker's Guide to the Galaxy tells us, is 42.
As you can see however,
instead of displaying 42, this app says, we may never know.
So clearly there's a bug somewhere.
Let's use the debugger to track that bug down.
And here we are in the IDE.
First I'll open up the answer application and show you it's source code.
Now, the truth is that this is a pretty silly program and
a bug would be fairly obvious once you've looked at the code a bit.
But my goal here is to show you how the debugger works, so just between you and
us, let's pretend that we don't know what's going on.
And you can see that there's an array called mAnswers through
which the program will search looking for the answer 42.
Now here's the onCreate method which is the starting point when this class is
executed.
Inside onCreate there's a call to a method called findAnswer.
And the code for the findAnswer method is listed down below.
Now findAnswer is supposed to search the mAnswers array and
return the number 42 if it's in there or minus one, if it's not.
Now going back up, the program stores findAnswers return value in the variable
val, and then uses the variable to compute a string called output.
Then finally,
the program calls the setText method to display the output string on the screen.
So let's say that,
since I don't know where the bug is, I decide to debug this program.
The first thing I'll do then is to set a break point that stops the program
when it enters the onCreate method.
Now, instead of running the program, I'll debug it by selecting Run, and
then debug app from the application bar.
And I could have also just typed Ctrl+D as well.
I'll click OK to run this on my phone.
Now when the application reaches onCreate, it stops and
shows me the debug view with the current line highlighted in the source code.
At this point, I'll step through the code line by line, examining the program state.
To step forward, I'll select Step Over from the debugger toolbar,
which advances the program one line.
Let me do that a few more times.
And I'm now at the line that calls findAnswer.
And store the return result in the variable val.
Let me step over this line.
And once I step over this line, you'll see that the value of the variable val
is now minus 10, which is already a problem.
It should have been 42.
Now if I advance again, I see that the output string is set to We may never know.
Okay, so the problem is certainly down in the findAnswer method, so
let's look more closely at the findAnswer method.
To do that, I'll put a break point at the findAnswer method,
and then restart the application.
Now this time when the application stops at onCreate I'll just hit the resume
button which let's the application continue executing until the findAnswer
break point is hit.
And I can now step forward in this method and examine the values of different
variables to determine that 42 is indeed in the array,
and that findAnswer sees it, but considers it the wrong answer.
Anyway, as I'm sure you've figured out long ago, the problem is that I'm
accidentally checking for not equals rather than for equals, equals.
So, I'll fix that.
And now I'll start over.
And this time we'll see the correct behavior.
And there it goes.
And yes, the answer to life, the universe and everything is indeed 42.
And I think the debuggers help most when you have a reasonably good
idea of where your problem is, and you can focus in on just a few lines of code.
But, in many other situations, that kind of low level instruction by
instruction examination that debuggers are so good at, is a lot less helpful.
Instead it may be better to use more general tools
that instrument your system and let you monitor its behavior over time.
Android provides a number of these monitoring tools
in its Dalvik Debug Monitor Service, or DDMS.
Now today I'll talk about just four of these tools.
The file explorer, for viewing a devices file system, logcat, for
logging and displaying runtime events, traceview for displaying method
execution traces and the hierarchy viewer for examining user interface layouts.
Let's look at these tools and see how you can access them from the IDE.
First let's start the device monitor, which exposes a number of panes and
buttons for accessing the DDMS tools.
Now here you can see a devices pane.
That shows the devices that are currently connected and
shows a list of processes running on those devices.
The first tool we'll look at, is the file explorer.
This view shows you the files on the selected devices file system.
For example, here we see that this device has a slash data, slash data directory,
in which applications typically store persistent data.
This device also has an external memory card mounted under slash storage,
slash SD card.
And there are some standard directories there for
storing music, and movies, and other things.
The next tool I'll talk about shows the logcat view.
Android logs many events by default.
And that is, as the software runs,
it spits out information about the things that are happening on the device.
And this information is stored and can be displayed in the logcat view.
Android also provides methods that let developers log their own information.
Another tool is called traceview.
Traceview allows you to trace the methods your application calls.
Let's do this for the anstrap application I showed you earlier.
So this is the source code for the application.
To simplify my demonstration I'll first put a breakpoint in the onCreate method.
And now I'll start debugging it.
As you can see, it stopped at the breakpoint.
And now when I start profiling, so I'll open the DDMS
tool window by typing Cmd+6, or Alt+6 in Windows.
And then I'll click on the button that shows a clock and
displays the text, Start Method Tracing.
Now I'll return to the debug view, and resume the application.
I want to program run for a while, and now, I'll return back to the DDMS tool
window, and press that same button, but this time, stop the method profiling.
And this point, traceview will process the trace information, and
show it to me graphically.
The traceview display has two panels.
One is a timeline view showing the method call stack across time.
The other is a textual view which shows execution
statistics such as the number of times a method was called and
the amount of time spent exclusively in that method, and the amount of time spent
in that method including any time spent in methods called from the current one.
Additionally, you can search for
a particular method by typing Cmd+F or Ctrl+F.
For instance, let's search for our old friend, findAnswer.
Now here I'll start to type in its name.
And you can see that the methods are filtered until only findAnswer is left.
And now I can zoom and pan on the timeline to see more information, and
surrounding context.
The last tool I'll talk about is the UI hierarchy viewer.
This tool allows you to examine your application's user interface and
see how it's organized.
For example, let's look again at the Hello Android application.
First, let's run the Hello Android application.
Once it's up and running, go to the menu bar, and
click on the Android device monitor window.
And this will bring up the device monitor.
And at this point,
you can select the device on which the application is running.
And then click on the Window > Open Perspective, and
then Hierarchy View, to open a hierarchy viewer.
Go to the Windows panel and
double-click the entry corresponding to, to the Hello Android application.
And it's the one that has the words Hello Android in it.
Now double-clicking on this entry should bring up a graphical
representation of Hello Android's user interface.
Which like most user interfaces is organized as a tree.
For instance,
there's an outer most application window which holds the entire user interface.
And it contains two children.
A title or action bar.
And then the larger tangular body where the application is seen,
each of these in turn, contain further elements.
So let's look at some of these, some of these other elements.
When I click on the outer most window, I see what that entire window looks like,
and I can inspect detailed information about it.
Such as its layout parameters, which shows for example how big the window is,
in pixels.
The region occupied by each element is shown in a layout view in the bottom right
corner.
Now, for instance, I can see this information for the title bar.
And I can see the same information for the body.
And for the text view it contains.
