问题
I'm working on a drawing app, where the user can pan & zoom to a specific portion of the screen and start drawing with zoom applied. To be more specific, I'm looking for a way to implement zoom, pinch & pan gesture in Canvas (horizontal and vertical scrolling with moveable x, y coordinates).
Right now, I've successfully developed the zoom only feature but it's not accurate and the pan option is not working.
See my sample code here,
public class DrawingView extends View {
//canvas
private Canvas drawCanvas;
//canvas bitmap
private Bitmap canvasBitmap;
private boolean dragged = false;
private float displayWidth;
private float displayHeight;
//These two constants specify the minimum and maximum zoom
private static float MIN_ZOOM = 1f;
private static float MAX_ZOOM = 5f;
private float scaleFactor = 1.f;
private ScaleGestureDetector detector;
//These constants specify the mode that we're in
private static int NONE = 0;
private static int DRAG = 1;
private static int ZOOM = 2;
private int mode;
public DrawingView(Context context, AttributeSet attrs) {
super(context, attrs);
ctx = context;
setupDrawing();
}
private void setupDrawing() {
detector = new ScaleGestureDetector(getContext(), new ScaleListener());
WindowManager wm = (WindowManager) ctx.getSystemService(Context.WINDOW_SERVICE);
Display display = wm.getDefaultDisplay();
DisplayMetrics metrics = new DisplayMetrics();
display.getMetrics(metrics);
displayWidth = metrics.widthPixels;
displayHeight = metrics.heightPixels;
setFocusable(true);
setFocusableInTouchMode(true);
}
class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {
@Override
public boolean onScale(ScaleGestureDetector detector) {
scaleFactor *= detector.getScaleFactor();
scaleFactor = Math.max(MIN_ZOOM, Math.min(scaleFactor, MAX_ZOOM));
return true;
}
}
@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
//view given size
super.onSizeChanged(w, h, oldw, oldh);
canvasBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
drawCanvas = new Canvas(canvasBitmap);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
//detect user touch
float x = event.getX();
float y = event.getY();
switch (event.getAction() & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_DOWN:
//
downx = event.getX();
downy = event.getY();
mode = DRAG;
//We assign the current X and Y coordinate of the finger to startX and startY minus the previously translated
//amount for each coordinates This works even when we are translating the first time because the initial
//values for these two variables is zero.
startX = event.getX() - previousTranslateX;
startY = event.getY() - previousTranslateY;
break;
case MotionEvent.ACTION_MOVE:
translateX = event.getX() - startX;
translateY = event.getY() - startY;
//We cannot use startX and startY directly because we have adjusted their values using the previous translation values.
//This is why we need to add those values to startX and startY so that we can get the actual coordinates of the finger.
double distance = Math.sqrt(Math.pow(event.getX() - (startX + previousTranslateX), 2) + Math.pow(event.getY() - (startY + previousTranslateY), 2));
if (distance > 0) {
dragged = true;
}
break;
case MotionEvent.ACTION_POINTER_DOWN:
mode = ZOOM;
break;
ase MotionEvent.ACTION_UP:
//
upx = event.getX();
upy = event.getY();
mode = NONE;
dragged = false;
//All fingers went up, so let's save the value of translateX and translateY into previousTranslateX and
//previousTranslate
previousTranslateX = translateX;
previousTranslateY = translateY;
break;
case MotionEvent.ACTION_POINTER_UP:
mode = DRAG;
//This is not strictly necessary; we save the value of translateX and translateY into previousTranslateX
//and previousTranslateY when the second finger goes up
previousTranslateX = translateX;
previousTranslateY = translateY;
break;
detector.onTouchEvent(event);
//We redraw the canvas only in the following cases:
//
// o The mode is ZOOM
// OR
// o The mode is DRAG and the scale factor is not equal to 1 (meaning we have zoomed) and dragged is
// set to true (meaning the finger has actually moved)
if ((mode == DRAG && scaleFactor != 1f && dragged) || mode == ZOOM) {
invalidate();
}
}
@Override
protected void onDraw(Canvas canvas) {
canvas.save();
//If translateX times -1 is lesser than zero, let's set it to zero. This takes care of the left bound
if ((translateX * -1) < 0) {
translateX = 0;
}
//This is where we take care of the right bound. We compare translateX times -1 to (scaleFactor - 1) * displayWidth.
//If translateX is greater than that value, then we know that we've gone over the bound. So we set the value of
//translateX to (1 - scaleFactor) times the display width. Notice that the terms are interchanged; it's the same
//as doing -1 * (scaleFactor - 1) * displayWidth
else if ((translateX * -1) > (scaleFactor - 1) * displayWidth) {
translateX = (1 - scaleFactor) * displayWidth;
}
if (translateY * -1 < 0) {
translateY = 0;
}
//We do the exact same thing for the bottom bound, except in this case we use the height of the display
else if ((translateY * -1) > (scaleFactor - 1) * displayHeight) {
translateY = (1 - scaleFactor) * displayHeight;
}
//We need to divide by the scale factor here, otherwise we end up with excessive panning based on our zoom level
//because the translation amount also gets scaled according to how much we've zoomed into the canvas.
canvas.translate(translateX / scaleFactor, translateY / scaleFactor);
}
//We're going to scale the X and Y coordinates by the same amount
//canvas.scale(scaleFactor, scaleFactor);
canvas.scale(this.scaleFactor, this.scaleFactor, this.detector.getFocusX(), this.detector.getFocusY());
}
来源：https://stackoverflow.com/questions/39155545/pan-zoom-and-scale-a-custom-view-for-canvas-drawing-in-android