Dr. Dobb's Journal - September 2008 - (Page 47)

d09thom_p3ds 7/14/08 9:34 AM Page 47 simply called getWidth() and getHeight() for this information. Calling the same methods in Android’s View constructor always retrieved values of zero. The reason is that View doesn’t know how the screen is oriented (which affects the View’s dimensions) until onStart() makes it visible. When this occurs, Android immediately invokes an onSizeChanged() method. This method provides values for the screen’s visible dimensions in its new orientation. You override this method when you want to force the View to redraw the screen for the new orientation, but for my application I just grabbed the height and width values. Listing One compares how to determine the mobile phone’s screen dimensions in Java ME and Android. My second programming test was to port over a basic Java ME scribbling program that uses the touchscreen. The Java ME program starts by displaying a blank image. When you touch and draw on the screen with a stylus, the program draws a colored line that tracks the stylus. The MIDlet uses pointerPressed(), pointerDragged(), and pointerRelease() to implement the tracking/drawing operations. A key press paints the image buffer with white, erasing it. The first difference for porting to Android was writing an XML layout file to support the View’s generation of a UI window. The second difference was managing the Bitmap object that both stored and displayed the results of the scribbling. I overrode the onSizeChanged() method so that if the screen’s orientation changed, a new Bitmap would be allocated. Also, to guarantee that work of art you’re creating is safely preserved when another application seizes the foreground, I added code to the onFreeze() method to save the Bitmap into a Bundle, termed an “icicle.” The third difference was that Android uses one lone method, onTouchEvent(), to handle all touchscreen interactions. Fortunately, you can easily break out the types of stylus actions—down, up, and drag—with a switch statement. I used the switch statement to call slightly modified pointerPressed(), pointerDragged(), and pointerRelease() methods. Although onTouchEvent() places all motion events into an array where you can replay the entire track of the pointer’s path, the original pointerDragged() worked by capturing the new point and drawing a line to the previous point, which it had saved previously. The behavior worked the first time in Android. The only changes I made were to how colors were assigned to the event. See Listing Two online for a comparison between the Java ME and Android touch-screen code. The original Java ME program, Simple_Scribble, and the Android application, ScribbleActivity, are also available online. Lessons Learned And Looking Forward If you have Java ME experience, you will have a gentle learning curve when writing Android code. Most of what comprises the curve is determining what classes have familiar methods, and learning Android’s quirks. Using XML for screen layout, which reduces the edit-build-execute cycles to tweak the UI to look right, is a big plus. Android, like the iPhone, has the potential to change the way we work with mobile phones—maybe even make all of their features useful. DDJ #include #define Knots const double standard_rating = 10.0 Knots; double rating; void rate_the_ship( double, double, int, double ); int main() For the big Labor Day { (mixed boat) regatta, rate_the_ship( 64.0, 45.3, 3, standard_rating ); Mike, a better sailor than printf( "%g\n", rating ); programmer, wanted to return 0; apply the latest yacht } racing formulae for the void rate_the_ship( double LWL, double sailarea, competing ships. But int nsails, double rating ) something is going wrong with this test { program. 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Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - September 2008

Dr. Dobb's Journal - September 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Developer’s Notebook A Conversation With Erik Demaine Application Lifecycle Management Meets Model-Driven Development Building a Robust Development Environment Real Users Really Matter Matching Wildcards: An Algorithm The Android Mobile Phone Platform Managing Application Thread Use Signalling Integer Overflows in Java .NET Development & the IBM WebSphere Portal Server The Agile Edge Effective Concurrency Swaine’s Flames

Dr. Dobb's Journal - September 2008

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