拖拽與縮放
編寫:Andrwyw - 原文:http://developer.android.com/training/gestures/scale.html
本節課程講述，使用onTouchEvent()截獲觸摸事件後，如何使用觸摸手勢拖拽、縮放屏幕上的對象。
拖拽一個對象
如果我們的目標版本為3.0或以上，我們可以使用View.OnDragListener監聽內置的拖放（drag-and-drop）事件，拖拽與釋放中有更多相關描述。
對於觸摸手勢來說，一個很常見的操作是在屏幕上拖拽一個對象。接下來的代碼段讓用戶可以拖拽屏幕上的圖片。需要注意以下幾點：
- 拖拽操作時，即使有額外的手指放置到屏幕上了，app也必須保持對最初的點（手指）的追蹤。比如，想象在拖拽圖片時，用戶放置了第二根手指在屏幕上，並且擡起了第一根手指。如果我們的app只是單獨地追蹤每個點，它會把第二個點當做默認的點，並且把圖片移到該點的位置。
- 為了防止這種情況發生，我們的app需要區分初始點以及隨後任意的觸摸點。要做到這一點，它需要追蹤處理多觸摸手勢章節中提到過的 ACTION_POINTER_DOWN 和 ACTION_POINTER_UP 事件。每當第二根手指按下或拿起時，ACTION_POINTER_DOWN 和 ACTION_POINTER_UP 事件就會傳遞給
onTouchEvent()回調函數。 - 當ACTION_POINTER_UP事件發生時，示例程序會移除對該點的索引值的引用，確保操作中的點的ID(the active pointer ID)不會引用已經不在觸摸屏上的觸摸點。這種情況下，app會選擇另一個觸摸點來作為操作中(active)的點，並保存它當前的x、y值。由於在ACTION_MOVE事件時，這個保存的位置會被用來計算屏幕上的對象將要移動的距離，所以app會始終根據正確的觸摸點來計算移動的距離。
下面的代碼段允許用戶拖拽屏幕上的對象。它會記錄操作中的點（active pointer）的初始位置，計算觸摸點移動過的距離，再把對象移動到新的位置。如上所述，它也正確地處理了額外觸摸點的可能。
需要注意的是，代碼段中使用了getActionMasked()函數。我們應該始終使用這個函數（或者最好用MotionEventCompat.getActionMasked()這個兼容版本）來獲得MotionEvent對應的動作(action)。不像舊的getAction()函數，getActionMasked()就是設計用來處理多點觸摸的。它會返回執行過的動作的掩碼值，不包括該點的索引位。
// 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;
}
通過拖拽平移
前一節展示了一個，在屏幕上拖拽對象的例子。另一個常見的場景是平移（panning），平移是指用戶通過拖拽移動引起x、y軸方向發生滾動(scrolling)。上面的代碼段直接截獲了MotionEvent動作來實現拖拽。這一部分的代碼段，利用了平臺對常用手勢的內置支持。它重寫了GestureDetector.SimpleOnGestureListener的onScroll()函數。
更詳細地說，當用戶拖拽手指來平移內容時，onScroll()函數就會被調用。onScroll()函數只會在手指按下的情況下被調用，一旦手指離開屏幕了，要麼手勢終止，要麼快速滑動(fling)手勢開始（如果手指在離開屏幕前快速移動了一段距離）。關於滾動與快速滑動的更多討論，可以查看滾動手勢動畫章節。
這裡是onScroll()的相關代碼段：
// 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()函數中滑動視窗(viewport)來響應觸摸手勢的實現：
/**
* 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);
}
使用觸摸手勢進行縮放
如同檢測常用手勢章節中提到的，GestureDetector可以幫助我們檢測Android中的常見手勢，例如滾動，快速滾動以及長按。對於縮放，Android也提供了ScaleGestureDetector類。當我們想讓view能識別額外的手勢時，我們可以同時使用GestureDetector和ScaleGestureDetector類。
為了報告檢測到的手勢事件，手勢檢測需要一個作為構造函數參數的listener對象。ScaleGestureDetector使用ScaleGestureDetector.OnScaleGestureListener。Android提供了ScaleGestureDetector.SimpleOnScaleGestureListener類作為幫助類，如果我們不是關注所有的手勢事件，我們可以繼承(extend)它。
基本的縮放示例
下面的代碼段展示了縮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;
}
}
更加複雜的縮放示例
這是本章節提供的InteractiveChart示例中一個更復雜的示範。通過使用ScaleGestureDetector中的"span"(getCurrentSpanX/Y)和"focus"(getFocusX/Y)功能，InteractiveChart示例同時支持滾動（平移）以及多指縮放。
@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;
}
};
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