原文地址：https://developer.android.com/training/gestures/scale.html
这节课主要学习如何使用触摸手势来拖动、放大屏幕上的对象。
拖动对象
如果你的重点在Android 3.0以上的版本，那么你可以使用内置的拖拽事件监听器View.OnDragListener。
触摸手势最常见的操作就是使用它来拖动屏幕上的对象。下面的代码允许用户拖动屏幕上的图像。要注意以下几点：
- 在拖动操作中，APP会一直保持手指拖动的轨迹，就算是另一只手指触到屏幕也是。举个例子，想象一根手指在拖动着一张图像，这时用户将第二根手指放置到屏幕上，如果APP只是追踪单根手指的轨迹，那么它会将第二根手指作为默认位置，并会将图像移动到这个位置。
- 为了防止这样的事件发生，APP需要区分第一根手指与其它手指。为此，需要追踪 ACTION_POINTER_DOWN 及 ACTION_POINTER_UP 。ACTION_POINTER_DOWN 及 ACTION_POINTER_UP在第二根手指落下或抬起的时候由onTouchEvent()方法传回。
- 在ACTION_POINTER_UP的情况下，示例提取了这个事件的索引，并确保当前活动的指针不是那个已经不在屏幕上的指针。如果是那个指针的话，那么APP会选择一个不同的指针使其活动并保存它的X及Y的位置。一旦这个值被保存下来，那么APP将会使用正确指针的数据一直计算剩余移动的距离。
下面的代码允许用户在屏幕上拖动对象。它记录了当前活动指针的初始位置，计算了它所位移的距离，并将对象移动到新的位置上。
这里要注意，代码段使用了getActionMasked()方法。你应该一直使用这个方法来接收MotionEvent对象的活动。与getAction()方法不同，getActionMasked()工作于多点触控模式下。它会返回被执行的掩饰活动，不包括指针的索引比特。
// 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;
}平移
上面的部分展示了如何在屏幕上拖动对象。另一个通用的场景就是平移了，平移的意思是：用户的拖动动作引起的x及y轴方向上的滚动。上面的代码直接将MotionEvent拦截实现拖动。这部分的代码将会采用另一种更具有优势的方法，以便支持通用手势。它重写了GestureDetector.SimpleOnGestureListener的onScroll()方法。
只有用户在使用手指移动内容时，onScroll()才会被调用。onScroll()只有在手指按下的时候才会调用，一旦手指离开屏幕，那么平移手势也随之终止。
下面是onScroll()的使用摘要：
// 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;
}下面是setViewportBottomLeft()方法的实现，它主要实现了移动内容的逻辑：
/**
* 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);
}缩放
在Detecting Common Gestures中，我们讨论到GestureDetector可以帮助我们来检测比如滑动、滚动、长按等手势。而对于缩放，Android提供了ScaleGestureDetector. GestureDetector 以及 ScaleGestureDetector。
为了可以反馈检测到的手势事件，手势探测器使用了监听器对象ScaleGestureDetector.OnScaleGestureListener。如果你只关心部分手势的话，Android还提供了ScaleGestureDetector.SimpleOnScaleGestureListener，你可以通过重写它的方法来使用。
缩放基础示例
下面的代码是缩放所需要的基础：
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;
}
}稍微复杂点的示例
下面是一个稍微复杂一点的示例，它摘自与这节课所提供的示例InteractiveChart(PS:示例工程请参见原网页)。InteractiveChart同时支持平移、缩放，它使用了ScaleGestureDetector的“平移”(getCurrentSpanX/Y)及“焦点” (getFocusX/Y)特性：
@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;
}
};时间： 2024-10-01 07:22:08