在过去的几周里，我一直在寻找合适的源代码，展示如何在自定义视图上启用缩放和平移功能。我找到的所有解决方案都存在一些问题。例如，在缩放（双指）操作之后释放一个手指时，移动/缩放不够平滑或者视图跳跃。所以我想出了一个我希望与您分享的修改后的解决方案。欢迎提出建议和改进。
有什么不同？
在每个单个事件之间计算任何交互（平移，缩放）的差异（距离向量）并使用它来设置新值是不好的做法。如果这样做，操作看起来不平滑，视图可能会闪烁（在某些像素中跳转）。更好的方法是在动作开始时（onScaleBegin，touch-down）记住值，并计算每个事件与这些起始值相比的距离。
您可以在onTouchEvent中处理手指索引，以更好地区分平移/移动和缩放/缩放交互。
public class CanvasView extends SurfaceView implements SurfaceHolder.Callback {
final static String TAG = "CanvasView";
Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
ScaleGestureDetector mScaleDetector;
InteractionMode mode;
Matrix mMatrix = new Matrix();
float mScaleFactor = 1.f;
float mTouchX;
float mTouchY;
float mTouchBackupX;
float mTouchBackupY;
float mTouchDownX;
float mTouchDownY;
Rect boundingBox = new Rect();
public CanvasView(Context context) {
super(context);
// we need to get a call for onSurfaceCreated
SurfaceHolder sh = this.getHolder();
sh.addCallback(this);
// for zooming (scaling) the view with two fingers
mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
boundingBox.set(0, 0, 1024, 768);
paint.setColor(Color.GREEN);
paint.setStyle(Style.STROKE);
setFocusable(true);
// initial center/touch point of the view (otherwise the view would jump
// around on first pan/move touch
DisplayMetrics metrics = context.getResources().getDisplayMetrics();
mTouchX = metrics.widthPixels / 2;
mTouchY = metrics.heightPixels / 2;
}
@Override
public boolean onTouchEvent(MotionEvent event) {
mScaleDetector.onTouchEvent(event);
if (!this.mScaleDetector.isInProgress()) {
switch (event.getAction()) {
case MotionEvent.ACTION_UP:
// similar to ScaleListener.onScaleEnd (as long as we don't
// handle indices of touch events)
mode = InteractionMode.None;
case MotionEvent.ACTION_DOWN:
Log.d(TAG, "Touch down event");
mTouchDownX = event.getX();
mTouchDownY = event.getY();
mTouchBackupX = mTouchX;
mTouchBackupY = mTouchY;
// pan/move started
mode = InteractionMode.Pan;
break;
case MotionEvent.ACTION_MOVE:
// make sure we don't handle the last move event when the first
// finger is still down and the second finger is lifted up
// already after a zoom/scale interaction. see
// ScaleListener.onScaleEnd
if (mode == InteractionMode.Pan) {
Log.d(TAG, "Touch move event");
// get current location
final float x = event.getX();
final float y = event.getY();
// get distance vector from where the finger touched down to
// current location
final float diffX = x - mTouchDownX;
final float diffY = y - mTouchDownY;
mTouchX = mTouchBackupX + diffX;
mTouchY = mTouchBackupY + diffY;
CalculateMatrix(true);
}
break;
}
}
return true;
}
@Override
public void onDraw(Canvas canvas) {
int saveCount = canvas.getSaveCount();
canvas.save();
canvas.concat(mMatrix);
canvas.drawColor(Color.BLACK);
canvas.drawRect(boundingBox, paint);
canvas.restoreToCount(saveCount);
}
@Override
public void surfaceChanged(SurfaceHolder arg0, int arg1, int arg2, int arg3) {
}
@Override
public void surfaceCreated(SurfaceHolder arg0) {
// otherwise onDraw(Canvas) won't be called
this.setWillNotDraw(false);
}
@Override
public void surfaceDestroyed(SurfaceHolder arg0) {
}
void CalculateMatrix(boolean invalidate) {
float sizeX = this.getWidth() / 2;
float sizeY = this.getHeight() / 2;
mMatrix.reset();
// move the view so that it's center point is located in 0,0
mMatrix.postTranslate(-sizeX, -sizeY);
// scale the view
mMatrix.postScale(mScaleFactor, mScaleFactor);
// re-move the view to it's desired location
mMatrix.postTranslate(mTouchX, mTouchY);
if (invalidate)
invalidate(); // re-draw
}
private class ScaleListener extends
ScaleGestureDetector.SimpleOnScaleGestureListener {
float mFocusStartX;
float mFocusStartY;
float mZoomBackupX;
float mZoomBackupY;
public ScaleListener() {
}
@Override
public boolean onScaleBegin(ScaleGestureDetector detector) {
mode = InteractionMode.Zoom;
mFocusStartX = detector.getFocusX();
mFocusStartY = detector.getFocusY();
mZoomBackupX = mTouchX;
mZoomBackupY = mTouchY;
return super.onScaleBegin(detector);
}
@Override
public void onScaleEnd(ScaleGestureDetector detector) {
mode = InteractionMode.None;
super.onScaleEnd(detector);
}
@Override
public boolean onScale(ScaleGestureDetector detector) {
if (mode != InteractionMode.Zoom)
return true;
Log.d(TAG, "Touch scale event");
// get current scale and fix its value
float scale = detector.getScaleFactor();
mScaleFactor *= scale;
mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f));
// get current focal point between both fingers (changes due to
// movement)
float focusX = detector.getFocusX();
float focusY = detector.getFocusY();
// get distance vector from initial event (onScaleBegin) to current
float diffX = focusX - mFocusStartX;
float diffY = focusY - mFocusStartY;
// scale the distance vector accordingly
diffX *= scale;
diffY *= scale;
// set new touch position
mTouchX = mZoomBackupX + diffX;
mTouchY = mZoomBackupY + diffY;
CalculateMatrix(true);
return true;
}
}
}