我正在开发canvas应用程序和Undo＆amp; amp;重做功能现在工作正常我想使用重置按钮重置画布。我尝试使用一些参考但无法找到适当的解决方案，这是我的代码
public class MainActivity extends Activity {
/** Called when the activity is first created. */
private ArrayList<Path> undonePaths = new ArrayList<Path>();
private ArrayList<Path> paths = new ArrayList<Path>();
FrameLayout frmLayout;
Canvas canvas;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
frmLayout=(FrameLayout)findViewById(R.id.frameLayout);
final DrawingPanel dp = new DrawingPanel(this);
frmLayout.addView(dp);
((Button) findViewById(R.id.Clear))
.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
Path path = new Path();
Paint clearPaint = new Paint();
clearPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
canvas.drawRect(0, 0, 0, 0, clearPaint);
}
});
((Button) findViewById(R.id.Undo))
.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
if (paths.size() > 0) {
undonePaths.add(paths
.remove(paths.size() -1));
dp.invalidate();
}
}
});
((Button) findViewById(R.id.Redo))
.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
if (undonePaths.size()>0) {
paths.add(undonePaths.remove(undonePaths.size()-1));
dp.invalidate();
}
}
});
}
public class DrawingPanel extends View implements OnTouchListener {
private Canvas mCanvas;
private Path mPath;
private Paint mPaint, circlePaint, outercirclePaint;
// private ArrayList<Path> undonePaths = new ArrayList<Path>();
private float xleft, xright, xtop, xbottom;
public DrawingPanel(Context context) {
super(context);
setFocusable(true);
setFocusableInTouchMode(true);
this.setOnTouchListener(this);
circlePaint = new Paint();
mPaint = new Paint();
outercirclePaint = new Paint();
outercirclePaint.setAntiAlias(false);
circlePaint.setAntiAlias(false);
mPaint.setAntiAlias(false);
mPaint.setColor(0xFF000000);
outercirclePaint.setColor(0x44FFF000);
circlePaint.setColor(0xF57F35);
outercirclePaint.setStyle(Paint.Style.FILL_AND_STROKE);
circlePaint.setStyle(Paint.Style.FILL);
mPaint.setStyle(Paint.Style.STROKE);
mPaint.setStrokeJoin(Paint.Join.MITER);
mPaint.setStrokeCap(Paint.Cap.ROUND);
mPaint.setStrokeWidth(20);
outercirclePaint.setStrokeWidth(15);
mCanvas = new Canvas();
mPath = new Path();
paths.add(mPath);
}
public void colorChanged(int color) {
mPaint.setColor(color);
}
@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
super.onSizeChanged(w, h, oldw, oldh);
}
@Override
protected void onDraw(Canvas canvas) {
for (Path p : paths) {
canvas.drawPath(p, mPaint);
}
}
private float mX, mY;
private static final float TOUCH_TOLERANCE = 0;
private void touch_start(float x, float y) {
mPath.reset();
mPath.moveTo(x, y);
mX = x;
mY = y;
}
private void touch_move(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;
}
}
private void touch_up() {
mPath.lineTo(mX, mY);
// commit the path to our offscreen
mCanvas.drawPath(mPath, mPaint);
// kill this so we don't double draw
mPath = new Path();
paths.add(mPath);
}
@Override
public boolean onTouch(View arg0, MotionEvent event) {
float x = event.getX();
float y = event.getY();
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
// if (x <= cx+circleRadius+5 && x>= cx-circleRadius-5) {
// if (y<= cy+circleRadius+5 && cy>= cy-circleRadius-5){
// paths.clear();
// return true;
// }
// }
touch_start(x, y);
invalidate();
break;
case MotionEvent.ACTION_MOVE:
touch_move(x, y);
invalidate();
break;
case MotionEvent.ACTION_UP:
touch_up();
invalidate();
break;
}
return true;
}
}
}
现在，我想重置画布。有人可以帮助我吗？
错误
by using this code got error 02-05 11:50:03.169: E/AndroidRuntime(7542): FATAL EXCEPTION: main
02-05 11:50:03.169: E/AndroidRuntime(7542): java.lang.NullPointerException
02-05 11:50:03.169: E/AndroidRuntime(7542): at com.example.undoredo.MainActivity$1.onClick(MainActivity.java:52)
02-05 11:50:03.169: E/AndroidRuntime(7542): at android.view.View.performClick(View.java:4204)
02-05 11:50:03.169: E/AndroidRuntime(7542): at android.view.View$PerformClick.run(View.java:17355)
02-05 11:50:03.169: E/AndroidRuntime(7542): at android.os.Handler.handleCallback(Handler.java:725)
02-05 11:50:03.169: E/AndroidRuntime(7542): at android.os.Handler.dispatchMessage(Handler.java:92)
02-05 11:50:03.169: E/AndroidRuntime(7542): at android.os.Looper.loop(Looper.java:137)
02-05 11:50:03.169: E/AndroidRuntime(7542): at android.app.ActivityThread.main(ActivityThread.java:5041)
02-05 11:50:03.169: E/AndroidRuntime(7542): at java.lang.reflect.Method.invokeNative(Native Method)
02-05 11:50:03.169: E/AndroidRuntime(7542): at java.lang.reflect.Method.invoke(Method.java:511)
02-05 11:50:03.169: E/AndroidRuntime(7542): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
答案 0 :(得分：0)
Path path = new Path();
Paint clearPaint = new Paint();
clearPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
canvas.drawRect(0, 0, 0, 0, clearPaint);
答案 1 :(得分：0)
根据文档尝试调用canvas.restore()：
<强>恢复
此调用会平衡之前对save（）的调用，并用于删除 自上次保存调用以来对矩阵/剪辑状态的所有修改。 调用restore（）比调用save（）的次数多出错。
保存
此调用会平衡之前对save（）的调用，并用于删除 自上次保存调用以来对矩阵/剪辑状态的所有修改。 调用restore（）比调用save（）的次数多出错。
所以你最初可以调用canvas.save（），然后在清除时只调用restore。