编写：Andrwyw
项目：Android Training中文版
原文：http://developer.android.com/training/gestures/scale.html
本节课程讲述，如何使用触摸手势拖拽、缩放屏幕上的对象，使用onTouchEvent()来截获触摸事件。
如果你的目标版本为3.0或以上，你可以使用View.OnDragListener监听内置的drag-and-drop事件，拖拽与释放中有更多描述。
使用触摸手势在屏幕上拖拽一个对象是很常见的操作。接下来的代码段让用户可以拖拽屏幕上的图片。需要注意以下几点：
下面的代码段允许用户拖拽屏幕上的对象。它会记录操作中的点（active pointer）的初始位置，计算触摸点移动过的距离，再把对象移动到新的位置。如上所述，它也正确地处理了额外触摸点的可能。
需要注意的是，代码段中使用了getActionMasked()函数。你应该始终使用这个函数（或者更好用MotionEventCompat.getActionMasked()这个兼容版本）来获得MotionEvent对应的动作(action)。不像旧的getAction()函数，getActionMasked()就是设计用来处理多点触摸的。它会返回执行过的动作的掩码值，不包括该点的索引位。
// 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;
}
前一部分展示了一个拖拽屏幕上对象的例子。另一个常见的场景是平移（panning），是指用户通过拖拽移动引起x、y轴方向发生滚动(scrolling)。上面的代码段直接截获了MotionEvent动作来实现拖拽。这一部分的代码段，利用了平台对常用手势的内置支持。它重写了GestureDetector.SimpleOnGestureListener的onScroll()函数。
更详细地说，当用户拖拽手指来平移内容时，onScroll()函数就会被调用。onScroll()函数只会在手指按下的情况下被调用，一旦手指离开屏幕了，要么手势终止，要么快速滑动(fling)手势开始（如果手指在离开屏幕前快速移动了一段距离）。关于滚动与快速滑动的更多讨论，可以查看Scroll手势动画章节。
这里是onScroll()的相关代码段：
// 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;
}
onScroll()函数中滑动视窗(viewport)来响应触摸手势的实现：
/**
* 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);
}
如同检测常用手势章节中提到的，GestureDetector可以帮助你检测Android中的常见手势，例如滚动，快速滚动以及长按。对于缩放，Android也提供了ScaleGestureDetector类。当你想让view能识别额外的手势时，你可以配合使用GestureDetector和ScaleGestureDetector类。
为了报告检测到的手势事件，手势检测需要使用作为构造函数参数的listener对象。ScaleGestureDetector使用ScaleGestureDetector.OnScaleGestureListener。Android提供了ScaleGestureDetector.SimpleOnScaleGestureListener类作为帮助类，如果你不是关注所有的手势事件，你可以自行拓展(extend)它。
下面的代码段展示了缩放功能中的基本部分。
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;
}
}
这是本章节提供的InteractiveChart样例中一个更复杂的示范。通过使用ScaleGestureDetector中的”span”(getCurrentSpanX/Y)和”focus”(getFocusX/Y)，InteractiveChart样例支持滚动（平移）以及多指缩放。
@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;
}
};