InteractiveChart.zip
This lesson describes how to use touch gestures to drag and scale on-screen objects, using {@link android.view.View#onTouchEvent onTouchEvent()} to intercept touch events.
If you are targeting Android 3.0 or higher, you can use the built-in drag-and-drop event listeners with {@link android.view.View.OnDragListener}, as described in Drag and Drop.
A common operation for a touch gesture is to use it to drag an object across the screen. The following snippet lets the user drag an on-screen image. Note the following:
The following snippet enables a user to drag an object around on the screen. It records the initial position of the active pointer, calculates the distance the pointer traveled, and moves the object to the new position. It correctly manages the possibility of additional pointers, as described above.
Notice that the snippet uses the {@link android.view.MotionEvent#getActionMasked getActionMasked()} method. You should always use this method (or better yet, the compatability version {@link android.support.v4.view.MotionEventCompat#getActionMasked MotionEventCompat.getActionMasked()}) to retrieve the action of a {@link android.view.MotionEvent}. Unlike the older {@link android.view.MotionEvent#getAction getAction()} method, {@link android.support.v4.view.MotionEventCompat#getActionMasked getActionMasked()} is designed to work with multiple pointers. It returns the masked action being performed, without including the pointer index bits.
// The ‘active pointer’ is the one currently moving our object.
private int mActivePointerId = INVALID_POINTER_ID;
@Override
public boolean onTouchEvent(MotionEvent ev) {
// Let the ScaleGestureDetector inspect all events.
mScaleDetector.onTouchEvent(ev);
final int action = MotionEventCompat.getActionMasked(ev);
switch (action) {
case MotionEvent.ACTION_DOWN: {
final int pointerIndex = MotionEventCompat.getActionIndex(ev);
final float x = MotionEventCompat.getX(ev, pointerIndex);
final float y = MotionEventCompat.getY(ev, pointerIndex);
// Remember where we started (for dragging)
mLastTouchX = x;
mLastTouchY = y;
// Save the ID of this pointer (for dragging)
mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
break;
}
case MotionEvent.ACTION_MOVE: {
// Find the index of the active pointer and fetch its position
final int pointerIndex =
MotionEventCompat.findPointerIndex(ev, mActivePointerId);
final float x = MotionEventCompat.getX(ev, pointerIndex);
final float y = MotionEventCompat.getY(ev, pointerIndex);
// Calculate the distance moved
final float dx = x - mLastTouchX;
final float dy = y - mLastTouchY;
mPosX += dx;
mPosY += dy;
invalidate();
// Remember this touch position for the next move event
mLastTouchX = x;
mLastTouchY = y;
break;
}
case MotionEvent.ACTION_UP: {
mActivePointerId = INVALID_POINTER_ID;
break;
}
case MotionEvent.ACTION_CANCEL: {
mActivePointerId = INVALID_POINTER_ID;
break;
}
case MotionEvent.ACTION_POINTER_UP: {
final int pointerIndex = MotionEventCompat.getActionIndex(ev);
final int pointerId = MotionEventCompat.getPointerId(ev, pointerIndex);
if (pointerId == mActivePointerId) {
// This was our active pointer going up. Choose a new
// active pointer and adjust accordingly.
final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
mLastTouchX = MotionEventCompat.getX(ev, newPointerIndex);
mLastTouchY = MotionEventCompat.getY(ev, newPointerIndex);
mActivePointerId = MotionEventCompat.getPointerId(ev, newPointerIndex);
}
break;
}
}
return true;
}
The previous section showed an example of dragging an object around the screen. Another common scenario is panning, which is when a user's dragging motion causes scrolling in both the x and y axes. The above snippet directly intercepted the {@link android.view.MotionEvent} actions to implement dragging. The snippet in this section takes advantage of the platform's built-in support for common gestures. It overrides {@link android.view.GestureDetector.OnGestureListener#onScroll onScroll()} in {@link android.view.GestureDetector.SimpleOnGestureListener}.
To provide a little more context, {@link android.view.GestureDetector.OnGestureListener#onScroll onScroll()} is called when a user is dragging his finger to pan the content. {@link android.view.GestureDetector.OnGestureListener#onScroll onScroll()} is only called when a finger is down; as soon as the finger is lifted from the screen, the gesture either ends, or a fling gesture is started (if the finger was moving with some speed just before it was lifted). For more discussion of scrolling vs. flinging, see Animating a Scroll Gesture.
Here is the snippet for {@link android.view.GestureDetector.OnGestureListener#onScroll onScroll()}:
// The current viewport. This rectangle represents the currently visible
// chart domain and range.
private RectF mCurrentViewport =
new RectF(AXIS_X_MIN, AXIS_Y_MIN, AXIS_X_MAX, AXIS_Y_MAX);
// The current destination rectangle (in pixel coordinates) into which the
// chart data should be drawn.
private Rect mContentRect;
private final GestureDetector.SimpleOnGestureListener mGestureListener
= new GestureDetector.SimpleOnGestureListener() {
...
@Override
public boolean onScroll(MotionEvent e1, MotionEvent e2,
float distanceX, float distanceY) {
// Scrolling uses math based on the viewport (as opposed to math using pixels).
// Pixel offset is the offset in screen pixels, while viewport offset is the
// offset within the current viewport.
float viewportOffsetX = distanceX * mCurrentViewport.width()
/ mContentRect.width();
float viewportOffsetY = -distanceY * mCurrentViewport.height()
/ mContentRect.height();
...
// Updates the viewport, refreshes the display.
setViewportBottomLeft(
mCurrentViewport.left + viewportOffsetX,
mCurrentViewport.bottom + viewportOffsetY);
...
return true;
}
The implementation of {@link android.view.GestureDetector.OnGestureListener#onScroll onScroll()} scrolls the viewport in response to the touch gesture:
/**
* Sets the current viewport (defined by mCurrentViewport) to the given
* X and Y positions. Note that the Y value represents the topmost pixel position,
* and thus the bottom of the mCurrentViewport rectangle.
*/
private void setViewportBottomLeft(float x, float y) {
/*
* Constrains within the scroll range. The scroll range is simply the viewport
* extremes (AXIS_X_MAX, etc.) minus the viewport size. For example, if the
* extremes were 0 and 10, and the viewport size was 2, the scroll range would
* be 0 to 8.
*/
float curWidth = mCurrentViewport.width();
float curHeight = mCurrentViewport.height();
x = Math.max(AXIS_X_MIN, Math.min(x, AXIS_X_MAX - curWidth));
y = Math.max(AXIS_Y_MIN + curHeight, Math.min(y, AXIS_Y_MAX));
mCurrentViewport.set(x, y - curHeight, x + curWidth, y);
// Invalidates the View to update the display.
ViewCompat.postInvalidateOnAnimation(this);
}
As discussed in Detecting Common Gestures, {@link android.view.GestureDetector} helps you detect common gestures used by Android such as scrolling, flinging, and long press. For scaling, Android provides {@link android.view.ScaleGestureDetector}. {@link android.view.GestureDetector} and {@link android.view.ScaleGestureDetector} can be used together when you want a view to recognize additional gestures.
To report detected gesture events, gesture detectors use listener objects passed to their constructors. {@link android.view.ScaleGestureDetector} uses {@link android.view.ScaleGestureDetector.OnScaleGestureListener}. Android provides {@link android.view.ScaleGestureDetector.SimpleOnScaleGestureListener} as a helper class that you can extend if you don’t care about all of the reported events.
Here is a snippet that illustrates the basic ingredients involved in scaling.
private ScaleGestureDetector mScaleDetector;
private float mScaleFactor = 1.f;
public MyCustomView(Context mContext){
...
// View code goes here
...
mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
// Let the ScaleGestureDetector inspect all events.
mScaleDetector.onTouchEvent(ev);
return true;
}
@Override
public void onDraw(Canvas canvas) {
super.onDraw(canvas);
canvas.save();
canvas.scale(mScaleFactor, mScaleFactor);
...
// onDraw() code goes here
...
canvas.restore();
}
private class ScaleListener
extends ScaleGestureDetector.SimpleOnScaleGestureListener {
@Override
public boolean onScale(ScaleGestureDetector detector) {
mScaleFactor *= detector.getScaleFactor();
// Don't let the object get too small or too large.
mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f));
invalidate();
return true;
}
}
Here is a more complex example from the {@code InteractiveChart} sample provided with this class. The {@code InteractiveChart} sample supports both scrolling (panning) and scaling with multiple fingers, using the {@link android.view.ScaleGestureDetector} "span" ({@link android.view.ScaleGestureDetector#getCurrentSpanX getCurrentSpanX/Y}) and "focus" ({@link android.view.ScaleGestureDetector#getFocusX getFocusX/Y}) features:
@Override
private RectF mCurrentViewport =
new RectF(AXIS_X_MIN, AXIS_Y_MIN, AXIS_X_MAX, AXIS_Y_MAX);
private Rect mContentRect;
private ScaleGestureDetector mScaleGestureDetector;
...
public boolean onTouchEvent(MotionEvent event) {
boolean retVal = mScaleGestureDetector.onTouchEvent(event);
retVal = mGestureDetector.onTouchEvent(event) || retVal;
return retVal || super.onTouchEvent(event);
}
/**
* The scale listener, used for handling multi-finger scale gestures.
*/
private final ScaleGestureDetector.OnScaleGestureListener mScaleGestureListener
= new ScaleGestureDetector.SimpleOnScaleGestureListener() {
/**
* This is the active focal point in terms of the viewport. Could be a local
* variable but kept here to minimize per-frame allocations.
*/
private PointF viewportFocus = new PointF();
private float lastSpanX;
private float lastSpanY;
// Detects that new pointers are going down.
@Override
public boolean onScaleBegin(ScaleGestureDetector scaleGestureDetector) {
lastSpanX = ScaleGestureDetectorCompat.
getCurrentSpanX(scaleGestureDetector);
lastSpanY = ScaleGestureDetectorCompat.
getCurrentSpanY(scaleGestureDetector);
return true;
}
@Override
public boolean onScale(ScaleGestureDetector scaleGestureDetector) {
float spanX = ScaleGestureDetectorCompat.
getCurrentSpanX(scaleGestureDetector);
float spanY = ScaleGestureDetectorCompat.
getCurrentSpanY(scaleGestureDetector);
float newWidth = lastSpanX / spanX * mCurrentViewport.width();
float newHeight = lastSpanY / spanY * mCurrentViewport.height();
float focusX = scaleGestureDetector.getFocusX();
float focusY = scaleGestureDetector.getFocusY();
// Makes sure that the chart point is within the chart region.
// See the sample for the implementation of hitTest().
hitTest(scaleGestureDetector.getFocusX(),
scaleGestureDetector.getFocusY(),
viewportFocus);
mCurrentViewport.set(
viewportFocus.x
- newWidth * (focusX - mContentRect.left)
/ mContentRect.width(),
viewportFocus.y
- newHeight * (mContentRect.bottom - focusY)
/ mContentRect.height(),
0,
0);
mCurrentViewport.right = mCurrentViewport.left + newWidth;
mCurrentViewport.bottom = mCurrentViewport.top + newHeight;
...
// Invalidates the View to update the display.
ViewCompat.postInvalidateOnAnimation(InteractiveLineGraphView.this);
lastSpanX = spanX;
lastSpanY = spanY;
return true;
}
};