Android简明开发教程十九：线程 Bezier曲线

Android中使用线程Thread的方法和Java SE相同。和大多数OS系统一样，Android中也有称为UI Thread的主线程。UI Thread 主要用来给相应的Widget分发消息，包括绘制（Drawing）事件。UI Thread 也是用来处理用户交互事件的线程。比如：如果你按下屏幕上某个按钮，UI 线程则将Touch 事件通知对应的控件（Widgets)，Widget 则将其状态设置成“按下”，并把“重绘”（Invalidate）事件发到Event Queue中去。 UI线程从Event Queue中读取事件后通知Widgets重画自身。

如果你的应用设计不好的话， UI线程的这种单线程模式就会导致非常差的用户响应性能。特别是你将一些费时的操作如网络访问或数据库访问也放在UI线程中，这些操作会造成用户界面无反应，最糟糕的是，如果UI线程阻塞超过几秒（5秒），著名的ANR对话框就会出现：

所以在设计应用时，需要把一些费时的任务使用单独的工作线程来运行避免阻塞UI线程，但是如果在工作线程中想更新UI线程的话，不能直接在工作线程中更新UI，这是因为UI线程不是“Thread Safe”。因此所有UI相关的操作一般必须在UI Thread中进行。

Android OS提供了多种方法可以用在非UI线程访问UI线程。

• Activity.runOnUiThread(Runnable)
• View.post(Runnable)
• View.postDelayed(Runnable, long)
• Handler

Bezier 示例动态显示Bezier曲线，使用了Activity.runOnUiThread 来更新屏幕，完整代码如下：

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public class Bezier extends Graphics2DActivity implements OnClickListener,Runnable{ /** * The animation thread. */ private Thread thread; private volatile boolean stopThread=false; private boolean stopOrNot=false; boolean drawn; /** * The random number generator. */ static java.util.Random random = new java.util.Random(); /** * The animated path */ Path path = new Path(); /** * Red brush used to fill the path. */ SolidBrush brush = new SolidBrush(Color.RED); private static final int NUMPTS = 6; private int animpts[] = new int[NUMPTS * 2]; private int deltas[] = new int[NUMPTS * 2]; long startt, endt; private Button btnOptions; @Override protected void drawImage() { drawDemo(100, 100); } public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.beziers); graphic2dView = (GuidebeeGraphics2DView) findViewById(R.id.graphics2dview); btnOptions = (Button) findViewById(R.id.btnStopStart); btnOptions.setOnClickListener(this); reset(100,100); if (thread == null) { thread = new Thread(this); thread.start(); } } @Override public void onClick(View view) { if(!stopOrNot){ btnOptions.setText("Start"); stopThread=true; } else{ stopThread=false; btnOptions.setText("Stop"); if (thread == null) { thread = new Thread(this); thread.start(); } } stopOrNot=!stopOrNot; } /** * Generates new points for the path. */ private void animate(int[] pts, int[] deltas, int i, int limit) { int newpt = pts[i] + deltas[i]; if (newpt <= 0) { newpt = -newpt; deltas[i] = (random.nextInt() & 0x00000003) + 2; } else if (newpt >= limit) { newpt = 2 * limit - newpt; deltas[i] = -((random.nextInt() & 0x00000003) + 2); } pts[i] = newpt; } /** * Resets the animation data. */ private void reset(int w, int h) { for (int i = 0; i < animpts.length; i += 2) { animpts[i + 0] = (random.nextInt() & 0x00000003) * w / 2; animpts[i + 1] = (random.nextInt() & 0x00000003) * h / 2; deltas[i + 0] = (random.nextInt() & 0x00000003) * 6 + 4; deltas[i + 1] = (random.nextInt() & 0x00000003) * 6 + 4; if (animpts[i + 0] > w / 2) { deltas[i + 0] = -deltas[i + 0]; } if (animpts[i + 1] > h / 2) { deltas[i + 1] = -deltas[i + 1]; } } } final Runnable updateCanvas = new Runnable() { public void run() { int offsetX = (graphic2dView.getWidth() - SharedGraphics2DInstance.CANVAS_WIDTH) / 2; int offsetY = (graphic2dView.getHeight() - SharedGraphics2DInstance.CANVAS_HEIGHT) / 2; graphic2dView.invalidate(offsetX,offsetY, offsetX+100,offsetY+100); } }; /** * Sets the points of the path and draws and fills the path. */ private void drawDemo(int w, int h) { for (int i = 0; i < animpts.length; i += 2) { animate(animpts, deltas, i + 0, w); animate(animpts, deltas, i + 1, h); } //Generates the new pata data. path.reset(); int[] ctrlpts = animpts; int len = ctrlpts.length; int prevx = ctrlpts[len - 2]; int prevy = ctrlpts[len - 1]; int curx = ctrlpts[0]; int cury = ctrlpts[1]; int midx = (curx + prevx) / 2; int midy = (cury + prevy) / 2; path.moveTo(midx, midy); for (int i = 2; i <= ctrlpts.length; i += 2) { int x1 = (curx + midx) / 2; int y1 = (cury + midy) / 2; prevx = curx; prevy = cury; if (i < ctrlpts.length) { curx = ctrlpts[i + 0]; cury = ctrlpts[i + 1]; } else { curx = ctrlpts[0]; cury = ctrlpts[1]; } midx = (curx + prevx) / 2; midy = (cury + prevy) / 2; int x2 = (prevx + midx) / 2; int y2 = (prevy + midy) / 2; path.curveTo(x1, y1, x2, y2, midx, midy); } path.closePath(); // clear the clipRect area before production graphics2D.clear(Color.WHITE); graphics2D.fill(brush, path); this.runOnUiThread(updateCanvas); } public void run() { Thread me = Thread.currentThread(); if (!drawn) { synchronized (this) { graphics2D.clear(Color.WHITE); graphics2D.fill(brush, path); graphic2dView.refreshCanvas(); drawn = true; } } while (thread == me && !stopThread) { drawDemo(100,100); } thread = null; } }

除了上述的方法外，Android还提供了AsyncTask类以简化工作线程与UI线程之间的通信。这里不详述。此外，上面Bezier曲线动画在屏幕上显示时有闪烁的现象，这是动态显示图像的一个常见问题，后面将专门讨论。