运动小球demo，UI

绘制一个运动的小球，小球有4个运动方向，分别是45度、135度、225度和315度。开始时小球位于任意位置，并以4个方向中的任意一个方向开始运动，当遇到区域边缘（屏幕边缘）时，以相反角度（镜面反射）回弹。

/* AUTO-GENERATED FILE.  DO NOT MODIFY.
 *
 * This class was automatically generated by the
 * gradle plugin from the resource data it found. It
 * should not be modified by hand.
 */
package android.arch.core;

public final class R {
    private R（） {}

}

 属性            大小     日期    时间   名称
----------- ---------  ---------- -----  ----
     目录           0  2018-10-19 19:15  Chapter\
     文件         187  2018-10-19 19:15  Chapter\.gitignore
     目录           0  2018-10-19 19:15  Chapter\.gradle\
     目录           0  2018-10-19 19:25  Chapter\.gradle\4.6\
     目录           0  2018-10-19 19:15  Chapter\.gradle\4.6\fileChanges\
     文件           1  2018-10-20 15:33  Chapter\.gradle\4.6\fileChanges\last-build.bin
     目录           0  2018-10-19 19:25  Chapter\.gradle\4.6\fileContent\
     文件          17  2018-10-20 15:33  Chapter\.gradle\4.6\fileContent\fileContent.lock
     目录           0  2018-10-19 19:15  Chapter\.gradle\4.6\fileHashes\
     文件      154733  2018-10-20 15:02  Chapter\.gradle\4.6\fileHashes\fileHashes.bin
     文件          17  2018-10-20 15:33  Chapter\.gradle\4.6\fileHashes\fileHashes.lock
     文件       19653  2018-10-19 19:25  Chapter\.gradle\4.6\fileHashes\resourceHashesCache.bin
     目录           0  2018-10-19 19:25  Chapter\.gradle\4.6\javaCompile\
     文件      650258  2018-10-20 15:02  Chapter\.gradle\4.6\javaCompile\classAnalysis.bin
     文件      248471  2018-10-19 19:25  Chapter\.gradle\4.6\javaCompile\jarAnalysis.bin
     文件          17  2018-10-20 15:02  Chapter\.gradle\4.6\javaCompile\javaCompile.lock
     文件      626790  2018-10-20 15:02  Chapter\.gradle\4.6\javaCompile\taskHistory.bin
     文件       20967  2018-10-20 15:02  Chapter\.gradle\4.6\javaCompile\taskJars.bin
     目录           0  2018-10-19 19:15  Chapter\.gradle\4.6\taskHistory\
     文件      826160  2018-10-20 15:02  Chapter\.gradle\4.6\taskHistory\taskHistory.bin
     文件          17  2018-10-20 15:33  Chapter\.gradle\4.6\taskHistory\taskHistory.lock
     目录           0  2018-10-19 19:15  Chapter\.gradle\buildOutputCleanup\
     文件          17  2018-10-20 15:33  Chapter\.gradle\buildOutputCleanup\buildOutputCleanup.lock
     文件          51  2018-10-19 19:15  Chapter\.gradle\buildOutputCleanup\cache.properties
     文件       20729  2018-10-20 15:02  Chapter\.gradle\buildOutputCleanup\outputFiles.bin
     目录           0  2018-10-19 19:15  Chapter\.gradle\vcsWorkingDirs\
     文件           0  2018-10-19 19:15  Chapter\.gradle\vcsWorkingDirs\gc.properties
     目录           0  2018-10-20 19:19  Chapter\.idea\
     目录           0  2018-10-19 19:15  Chapter\.idea\caches\
     文件         535  2018-10-20 13:14  Chapter\.idea\caches\build_file_checksums.ser
     目录           0  2018-10-19 19:15  Chapter\.idea\codestyles\
............此处省略1231个文件信息