我正在开发一个 Android 应用程序，它应该允许用户在画布上画一个圆圈并拖动它。我已经可以在上面画线和圆了。但是我似乎不能拖动它。
是否可以在 Android 中拖动画布上绘制的对象？例如，如果我检测到圆圈内有触摸，如何从先前位置删除圆圈并将其移动到当前位置？ invalidate()我试过
【问题讨论】：
您似乎在处理多点触控/绘图时遇到问题。在Android Developer site 和Android Blog 上有一些有用的教程。
基于此，我能够创建一个示例，我认为该示例与您尝试实现的示例非常相似（没有完整的圆形绘图-单次触摸即可生成圆形）：
public class CirclesDrawingView extends View {
private static final String TAG = "CirclesDrawingView";
/** Main bitmap */
private Bitmap mBitmap = null;
private Rect mMeasuredRect;
/** Stores data about single circle */
private static class CircleArea {
int radius;
int centerX;
int centerY;
CircleArea(int centerX, int centerY, int radius) {
this.radius = radius;
this.centerX = centerX;
this.centerY = centerY;
}
@Override
public String toString() {
return "Circle[" + centerX + ", " + centerY + ", " + radius + "]";
}
}
/** Paint to draw circles */
private Paint mCirclePaint;
private final Random mRadiusGenerator = new Random();
// Radius limit in pixels
private final static int RADIUS_LIMIT = 100;
private static final int CIRCLES_LIMIT = 3;
/** All available circles */
private HashSet<CircleArea> mCircles = new HashSet<CircleArea>(CIRCLES_LIMIT);
private SparseArray<CircleArea> mCirclePointer = new SparseArray<CircleArea>(CIRCLES_LIMIT);
/**
* Default constructor
*
* @param ct {@link android.content.Context}
*/
public CirclesDrawingView(final Context ct) {
super(ct);
init(ct);
}
public CirclesDrawingView(final Context ct, final AttributeSet attrs) {
super(ct, attrs);
init(ct);
}
public CirclesDrawingView(final Context ct, final AttributeSet attrs, final int defStyle) {
super(ct, attrs, defStyle);
init(ct);
}
private void init(final Context ct) {
// Generate bitmap used for background
mBitmap = BitmapFactory.decodeResource(ct.getResources(), R.drawable.up_image);
mCirclePaint = new Paint();
mCirclePaint.setColor(Color.BLUE);
mCirclePaint.setStrokeWidth(40);
mCirclePaint.setStyle(Paint.Style.FILL);
}
@Override
public void onDraw(final Canvas canv) {
// background bitmap to cover all area
canv.drawBitmap(mBitmap, null, mMeasuredRect, null);
for (CircleArea circle : mCircles) {
canv.drawCircle(circle.centerX, circle.centerY, circle.radius, mCirclePaint);
}
}
@Override
public boolean onTouchEvent(final MotionEvent event) {
boolean handled = false;
CircleArea touchedCircle;
int xTouch;
int yTouch;
int pointerId;
int actionIndex = event.getActionIndex();
// get touch event coordinates and make transparent circle from it
switch (event.getActionMasked()) {
case MotionEvent.ACTION_DOWN:
// it's the first pointer, so clear all existing pointers data
clearCirclePointer();
xTouch = (int) event.getX(0);
yTouch = (int) event.getY(0);
// check if we've touched inside some circle
touchedCircle = obtainTouchedCircle(xTouch, yTouch);
touchedCircle.centerX = xTouch;
touchedCircle.centerY = yTouch;
mCirclePointer.put(event.getPointerId(0), touchedCircle);
invalidate();
handled = true;
break;
case MotionEvent.ACTION_POINTER_DOWN:
Log.w(TAG, "Pointer down");
// It secondary pointers, so obtain their ids and check circles
pointerId = event.getPointerId(actionIndex);
xTouch = (int) event.getX(actionIndex);
yTouch = (int) event.getY(actionIndex);
// check if we've touched inside some circle
touchedCircle = obtainTouchedCircle(xTouch, yTouch);
mCirclePointer.put(pointerId, touchedCircle);
touchedCircle.centerX = xTouch;
touchedCircle.centerY = yTouch;
invalidate();
handled = true;
break;
case MotionEvent.ACTION_MOVE:
final int pointerCount = event.getPointerCount();
Log.w(TAG, "Move");
for (actionIndex = 0; actionIndex < pointerCount; actionIndex++) {
// Some pointer has moved, search it by pointer id
pointerId = event.getPointerId(actionIndex);
xTouch = (int) event.getX(actionIndex);
yTouch = (int) event.getY(actionIndex);
touchedCircle = mCirclePointer.get(pointerId);
if (null != touchedCircle) {
touchedCircle.centerX = xTouch;
touchedCircle.centerY = yTouch;
}
}
invalidate();
handled = true;
break;
case MotionEvent.ACTION_UP:
clearCirclePointer();
invalidate();
handled = true;
break;
case MotionEvent.ACTION_POINTER_UP:
// not general pointer was up
pointerId = event.getPointerId(actionIndex);
mCirclePointer.remove(pointerId);
invalidate();
handled = true;
break;
case MotionEvent.ACTION_CANCEL:
handled = true;
break;
default:
// do nothing
break;
}
return super.onTouchEvent(event) || handled;
}
/**
* Clears all CircleArea - pointer id relations
*/
private void clearCirclePointer() {
Log.w(TAG, "clearCirclePointer");
mCirclePointer.clear();
}
/**
* Search and creates new (if needed) circle based on touch area
*
* @param xTouch int x of touch
* @param yTouch int y of touch
*
* @return obtained {@link CircleArea}
*/
private CircleArea obtainTouchedCircle(final int xTouch, final int yTouch) {
CircleArea touchedCircle = getTouchedCircle(xTouch, yTouch);
if (null == touchedCircle) {
touchedCircle = new CircleArea(xTouch, yTouch, mRadiusGenerator.nextInt(RADIUS_LIMIT) + RADIUS_LIMIT);
if (mCircles.size() == CIRCLES_LIMIT) {
Log.w(TAG, "Clear all circles, size is " + mCircles.size());
// remove first circle
mCircles.clear();
}
Log.w(TAG, "Added circle " + touchedCircle);
mCircles.add(touchedCircle);
}
return touchedCircle;
}
/**
* Determines touched circle
*
* @param xTouch int x touch coordinate
* @param yTouch int y touch coordinate
*
* @return {@link CircleArea} touched circle or null if no circle has been touched
*/
private CircleArea getTouchedCircle(final int xTouch, final int yTouch) {
CircleArea touched = null;
for (CircleArea circle : mCircles) {
if ((circle.centerX - xTouch) * (circle.centerX - xTouch) + (circle.centerY - yTouch) * (circle.centerY - yTouch) <= circle.radius * circle.radius) {
touched = circle;
break;
}
}
return touched;
}
@Override
protected void onMeasure(final int widthMeasureSpec, final int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
mMeasuredRect = new Rect(0, 0, getMeasuredWidth(), getMeasuredHeight());
}
}
Activity只包含setContentView(R.layout.main)里面的main.xml如下：
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_height="match_parent"
android:layout_width="match_parent"
android:id="@+id/scroller">
<com.example.TestApp.CirclesDrawingView
android:layout_width="match_parent"
android:layout_height="match_parent" />
</RelativeLayout>
【讨论】：