Android MotionEvent

Class Overview

Object used to report movement (mouse, pen, finger, trackball) events. Motion events may hold either absolute or relative movements and other data, depending on the type of device.
Overview

Motion events describe movements in terms of an action code and a set of axis values. The action code specifies the state change that occurred such as a pointer going down or up. The axis values describe the position and other movement properties.

For example, when the user first touches the screen, the system delivers a touch event to the appropriate View with the action code ACTION_DOWN and a set of axis values that include the X and Y coordinates of the touch and information about the pressure, size and orientation of the contact area.

Some devices can report multiple movement traces at the same time. Multi-touch screens emit one movement trace for each finger. The individual fingers or other objects that generate movement traces are referred to as pointers. Motion events contain information about all of the pointers that are currently active even if some of them have not moved since the last event was delivered.

The number of pointers only ever changes by one as individual pointers go up and down, except when the gesture is canceled.

Each pointer has a unique id that is assigned when it first goes down (indicated by ACTION_DOWN or ACTION_POINTER_DOWN). A pointer id remains valid until the pointer eventually goes up (indicated by ACTION_UP or ACTION_POINTER_UP) or when the gesture is canceled (indicated by ACTION_CANCEL).

The MotionEvent class provides many methods to query the position and other properties of pointers, such as getX(int), getY(int), getAxisValue(int), getPointerId(int), getToolType(int), and many others. Most of these methods accept the pointer index as a parameter rather than the pointer id. The pointer index of each pointer in the event ranges from 0 to one less than the value returned by getPointerCount().

The order in which individual pointers appear within a motion event is undefined. Thus the pointer index of a pointer can change from one event to the next but the pointer id of a pointer is guaranteed to remain constant as long as the pointer remains active. Use the getPointerId(int) method to obtain the pointer id of a pointer to track it across all subsequent motion events in a gesture. Then for successive motion events, use the findPointerIndex(int) method to obtain the pointer index for a given pointer id in that motion event.

Mouse and stylus buttons can be retrieved using getButtonState(). It is a good idea to check the button state while handling ACTION_DOWN as part of a touch event. The application may choose to perform some different action if the touch event starts due to a secondary button click, such as presenting a context menu.
Batching

For efficiency, motion events with ACTION_MOVE may batch together multiple movement samples within a single object. The most current pointer coordinates are available using getX(int) and getY(int). Earlier coordinates within the batch are accessed using getHistoricalX(int, int) and getHistoricalY(int, int). The coordinates are "historical" only insofar as they are older than the current coordinates in the batch; however, they are still distinct from any other coordinates reported in prior motion events. To process all coordinates in the batch in time order, first consume the historical coordinates then consume the current coordinates.

Example: Consuming all samples for all pointers in a motion event in time order.

void printSamples(MotionEvent ev) {
final int historySize = ev.getHistorySize();
final int pointerCount = ev.getPointerCount();
for (int h = 0; h < historySize; h++) {
System.out.printf("At time %d:", ev.getHistoricalEventTime(h));
for (int p = 0; p < pointerCount; p++) {
System.out.printf(" pointer %d: (%f,%f)",
ev.getPointerId(p), ev.getHistoricalX(p, h), ev.getHistoricalY(p, h));
}
}
System.out.printf("At time %d:", ev.getEventTime());
for (int p = 0; p < pointerCount; p++) {
System.out.printf(" pointer %d: (%f,%f)",
ev.getPointerId(p), ev.getX(p), ev.getY(p));
}
}