拖拽操作時(shí)，即使有額外的手指放置到屏幕上了，app也必須保持對(duì)最初的點(diǎn)（手指）的追蹤。比如，想象在拖拽圖片時(shí)，用戶放置了第二根手指在屏幕上，并且抬起了第一根手指。如果我們的app只是單獨(dú)地追蹤每個(gè)點(diǎn)，它會(huì)把第二個(gè)點(diǎn)當(dāng)做默認(rèn)的點(diǎn)，并且把圖片移到該點(diǎn)的位置。
為了防止這種情況發(fā)生，我們的app需要區(qū)分初始點(diǎn)以及隨后任意的觸摸點(diǎn)。要做到這一點(diǎn)，它需要追蹤處理多觸摸手勢(shì)章節(jié)中提到過(guò)的 ACTION_POINTER_DOWN 和 ACTION_POINTER_UP 事件。每當(dāng)?shù)诙种赴聪禄蚰闷饡r(shí)，ACTION_POINTER_DOWN 和 ACTION_POINTER_UP 事件就會(huì)傳遞給onTouchEvent()回調(diào)函數(shù)。
當(dāng)ACTION_POINTER_UP事件發(fā)生時(shí)，示例程序會(huì)移除對(duì)該點(diǎn)的索引值的引用，確保操作中的點(diǎn)的ID(the active pointer ID)不會(huì)引用已經(jīng)不在觸摸屏上的觸摸點(diǎn)。這種情況下，app會(huì)選擇另一個(gè)觸摸點(diǎn)來(lái)作為操作中(active)的點(diǎn)，并保存它當(dāng)前的x、y值。由于在ACTION_MOVE事件時(shí)，這個(gè)保存的位置會(huì)被用來(lái)計(jì)算屏幕上的對(duì)象將要移動(dòng)的距離，所以app會(huì)始終根據(jù)正確的觸摸點(diǎn)來(lái)計(jì)算移動(dòng)的距離。

下面的代碼段允許用戶拖拽屏幕上的對(duì)象。它會(huì)記錄操作中的點(diǎn)（active pointer）的初始位置，計(jì)算觸摸點(diǎn)移動(dòng)過(guò)的距離，再把對(duì)象移動(dòng)到新的位置。如上所述，它也正確地處理了額外觸摸點(diǎn)的可能。

需要注意的是，代碼段中使用了getActionMasked()函數(shù)。我們應(yīng)該始終使用這個(gè)函數(shù)（或者最好用MotionEventCompat.getActionMasked()這個(gè)兼容版本）來(lái)獲得MotionEvent對(duì)應(yīng)的動(dòng)作(action)。不像舊的getAction()函數(shù)，getActionMasked()就是設(shè)計(jì)用來(lái)處理多點(diǎn)觸摸的。它會(huì)返回執(zhí)行過(guò)的動(dòng)作的掩碼值，不包括該點(diǎn)的索引位。

// The ‘a(chǎn)ctive 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(guò)拖拽平移

前一節(jié)展示了一個(gè)，在屏幕上拖拽對(duì)象的例子。另一個(gè)常見(jiàn)的場(chǎng)景是平移（panning），平移是指用戶通過(guò)拖拽移動(dòng)引起x、y軸方向發(fā)生滾動(dòng)(scrolling)。上面的代碼段直接截獲了MotionEvent動(dòng)作來(lái)實(shí)現(xiàn)拖拽。這一部分的代碼段，利用了平臺(tái)對(duì)常用手勢(shì)的內(nèi)置支持。它重寫(xiě)了GestureDetector.SimpleOnGestureListener的onScroll()函數(shù)。

更詳細(xì)地說(shuō)，當(dāng)用戶拖拽手指來(lái)平移內(nèi)容時(shí)，onScroll()函數(shù)就會(huì)被調(diào)用。onScroll()函數(shù)只會(huì)在手指按下的情況下被調(diào)用，一旦手指離開(kāi)屏幕了，要么手勢(shì)終止，要么快速滑動(dòng)(fling)手勢(shì)開(kāi)始（如果手指在離開(kāi)屏幕前快速移動(dòng)了一段距離）。關(guān)于滾動(dòng)與快速滑動(dòng)的更多討論，可以查看滾動(dòng)手勢(shì)動(dòng)畫(huà)章節(jié)。

這里是onScroll()的相關(guān)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(shù)中滑動(dòng)視窗(viewport)來(lái)響應(yīng)觸摸手勢(shì)的實(shí)現(xiàn)：

/**
 * 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(shì)進(jìn)行縮放

如同檢測(cè)常用手勢(shì)章節(jié)中提到的，GestureDetector可以幫助我們檢測(cè)Android中的常見(jiàn)手勢(shì)，例如滾動(dòng)，快速滾動(dòng)以及長(zhǎng)按。對(duì)于縮放，Android也提供了ScaleGestureDetector類。當(dāng)我們想讓view能識(shí)別額外的手勢(shì)時(shí)，我們可以同時(shí)使用GestureDetector和ScaleGestureDetector類。

為了報(bào)告檢測(cè)到的手勢(shì)事件，手勢(shì)檢測(cè)需要一個(gè)作為構(gòu)造函數(shù)參數(shù)的listener對(duì)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(guān)注所有的手勢(shì)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;
    }
}

更加復(fù)雜的縮放示例

這是本章節(jié)提供的InteractiveChart示例中一個(gè)更復(fù)雜的示范。通過(guò)使用ScaleGestureDetector中的"span"(getCurrentSpanX/Y)和"focus"(getFocusX/Y)功能，InteractiveChart示例同時(shí)支持滾動(dòng)（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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