我正在开发绘图应用程序，但面临一些撤消问题。编码如下：
public class DoodleView extends View
{
Context context_new;
private static final float TOUCH_TOLERANCE = 5;
private Bitmap bitmap; // drawing area for display or saving
private Canvas bitmapCanvas; // used to draw on bitmap
private Paint paintScreen; // use to draw bitmap onto screen
private Paint paintLine; // used to draw lines onto bitmap
private Path mPath;
private ArrayList<Path> paths = new ArrayList<Path>();
private ArrayList<Path> undonePaths = new ArrayList<Path>();
private float mX, mY;
// DoodleView constructor initializes the DoodleView
public DoodleView(Context context, AttributeSet attrs)
{
super(context, attrs); // pass context to View's constructor
this.context_new=context;
paintScreen = new Paint(); // used to display bitmap onto screen
// set the initial display settings for the painted line
paintLine = new Paint();
paintLine.setAntiAlias(true); // smooth edges of drawn line
paintLine.setColor(Color.BLACK); // default color is black
paintLine.setStyle(Paint.Style.STROKE); // solid line
mPath = new Path();
paths.add(mPath);
} // end DoodleView constructor
OnSizeChanged：
@Override
public void onSizeChanged(int w, int h, int oldW, int oldH)
{
super.onSizeChanged(w, h, oldW, oldH);
DoodlzViewWidth = w;
DoodlzViewHeight = h;
bitmap = Bitmap.createBitmap(getWidth(), DoodlzViewHeight, Bitmap.Config.ARGB_8888);
bitmapCanvas = new Canvas(bitmap);
bitmap.eraseColor(Color.WHITE); // erase the BitMap with white
}
绘制：
@Override
protected void onDraw(Canvas canvas)
{
canvas.drawBitmap(bitmap, 0, 0, paintScreen);
// for each path currently being drawn
for (Path p : paths){canvas.drawPath(p, paintLine);}
}
触摸事件：
@Override
public boolean onTouchEvent(MotionEvent event)
{
float x = event.getX();
float y = event.getY();
switch (event.getAction())
{
case MotionEvent.ACTION_DOWN:
touchStarted(x, y);
invalidate();
break;
case MotionEvent.ACTION_MOVE:
touchMoved(x, y);
invalidate();
break;
case MotionEvent.ACTION_UP:
touchEnded();
invalidate();
break;
}
return true;
}
触摸开始：
private void touchStarted(float x, float y)
{
mPath.reset();
mPath.moveTo(x, y);
mX = x;
mY = y;
}
触摸移动：
private void touchMoved(float x, float y)
{
float dx = Math.abs(x - mX);
float dy = Math.abs(y - mY);
if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE)
{
mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
mX = x;
mY = y;
}
}
触摸结束：
private void touchEnded()
{
mPath.lineTo(mX, mY);
bitmapCanvas.drawPath(mPath, paintLine);
mPath = new Path();
paths.add(mPath);
Toast.makeText(getContext(), "touchEnded" + paths.size(), Toast.LENGTH_SHORT).show();
}
撤消：
public void onClickUndo()
{
Toast.makeText(getContext(), "before undo button pressed " + paths.size(), Toast.LENGTH_SHORT).show();
if (paths.size()>0)
{
undonePaths.add(paths.remove(paths.size()-1));
Toast.makeText(getContext(), "after undo button pressed " + paths.size(), Toast.LENGTH_SHORT).show();
Log.i("UNDOING", "PREPARE INVALIDATE");
invalidate();
Log.i("UNDOING", "FINISH INVALIDATE");
}
else Toast.makeText(getContext(), "nothing to undo" + paths.size(), Toast.LENGTH_SHORT).show();
}
问题：
以上内容来源于网上搜索的其他示例。不知道为什么需要在实现时设置 path.reset() touchStarted？
Q1。当我按下撤消按钮时，它会正确显示已toast按下撤消按钮，并报告path.size()为 0，因此不会删除之前立即绘制的线。我真的不知道为什么它是0？它不是已经添加到路径数组中了吗？如何修改代码？
** 代码在采纳 android-developer 的建议后修改！谢谢！！它现在正确显示 path.size()。对不起，愚蠢的错过了！*但是之前绘制的线仍然无法删除？=(
Q2。当手指在屏幕上移动并立即显示线条时应用程序正常运行，当我按下撤消按钮时，除了上面没有删除上一行之外，线条在按下后进一步绘制到屏幕上直到手指抬起按钮才会显示出来。
回答 Q2：
将以下 2 行touchEnded()从touchStarted()
mPath = new Path();
paths.add(mPath);
.
private void touchStarted(float x, float y)
{
mPath.reset();
mPath = new Path();
paths.add(mPath);
mPath.moveTo(x, y);
mX = x;
mY = y;
}
private void touchEnded()
{
mPath.lineTo(mX, mY);
bitmapCanvas.drawPath(mPath, paintLine);// commit the path to our offscreen
Toast.makeText(getContext(), "touchEnded" + paths.size(), Toast.LENGTH_SHORT).show();
}
谢谢！！！