拖曳與縮放

嘗試 Compose 方法
Jetpack Compose 是 Android 推薦的 UI 工具包。瞭解如何在 Compose 中使用觸控和輸入功能。

本文件說明如何使用觸控手勢拖曳和縮放螢幕上的物件,以便使用 onTouchEvent()攔截觸控事件。

拖曳物件

觸控手勢的常見作業是使用手勢在螢幕上拖曳物件。

在拖曳或捲動作業中,即使多指輕觸螢幕,應用程式也必須追蹤原始指標。舉例來說,假設使用者在拖曳圖片時將第二指放在觸控螢幕上,並放開第一指。如果應用程式只會追蹤個別指標,則會忽略第二個指標做為預設值,並將圖片移至該位置。

為避免發生這種情況,您的應用程式需要區分原始指標和任何後續指標。為此,系統會按照「處理多點觸控手勢」中的說明,追蹤 ACTION_POINTER_DOWNACTION_POINTER_UP 事件。每當次要指標向下或向上移動時,系統就會將 ACTION_POINTER_DOWNACTION_POINTER_UP 傳遞至 onTouchEvent() 回呼。

ACTION_POINTER_UP 的案例中,您可以擷取這個索引,並確保使用中的指標 ID 並未參照不再碰觸螢幕的指標。如果已啟用,您可以選取其他指標加以啟用,並儲存目前的 X 和 Y 位置。請使用 ACTION_MOVE 案例中的這個儲存位置,計算螢幕上物件移動的距離。這樣一來,應用程式一律會使用正確指標的資料來計算移動距離。

下列程式碼片段可讓使用者拖曳畫面中的物件。會記錄有效指標的初始位置、計算指標移動的距離,並將物件移至新位置。它也能正確管理其他指標的可能性。

程式碼片段使用 getActionMasked() 方法。一律使用此方法擷取 MotionEvent 的動作。

Kotlin

// The "active pointer" is the one moving the object.
private var mActivePointerId = INVALID_POINTER_ID

override fun onTouchEvent(ev: MotionEvent): Boolean {
    // Let the ScaleGestureDetector inspect all events.
    mScaleDetector.onTouchEvent(ev)

    val action = MotionEventCompat.getActionMasked(ev)

    when (action) {
        MotionEvent.ACTION_DOWN -> {
            MotionEventCompat.getActionIndex(ev).also { pointerIndex ->
                // Remember where you start for dragging.
                mLastTouchX = MotionEventCompat.getX(ev, pointerIndex)
                mLastTouchY = MotionEventCompat.getY(ev, pointerIndex)
            }

            // Save the ID of this pointer for dragging.
            mActivePointerId = MotionEventCompat.getPointerId(ev, 0)
        }

        MotionEvent.ACTION_MOVE -> {
            // Find the index of the active pointer and fetch its position.
            val (x: Float, y: Float) =
                    MotionEventCompat.findPointerIndex(ev, mActivePointerId).let { pointerIndex ->
                        // Calculate the distance moved.
                        MotionEventCompat.getX(ev, pointerIndex) to
                                MotionEventCompat.getY(ev, pointerIndex)
                    }

            mPosX += x - mLastTouchX
            mPosY += y - mLastTouchY

            invalidate()

            // Remember this touch position for the next move event.
            mLastTouchX = x
            mLastTouchY = y
        }
        MotionEvent.ACTION_UP, MotionEvent.ACTION_CANCEL -> {
            mActivePointerId = INVALID_POINTER_ID
        }
        MotionEvent.ACTION_POINTER_UP -> {

            MotionEventCompat.getActionIndex(ev).also { pointerIndex ->
                MotionEventCompat.getPointerId(ev, pointerIndex)
                        .takeIf { it == mActivePointerId }
                        ?.run {
                            // This is the active pointer going up. Choose a new
                            // active pointer and adjust it accordingly.
                            val newPointerIndex = if (pointerIndex == 0) 1 else 0
                            mLastTouchX = MotionEventCompat.getX(ev, newPointerIndex)
                            mLastTouchY = MotionEventCompat.getY(ev, newPointerIndex)
                            mActivePointerId = MotionEventCompat.getPointerId(ev, newPointerIndex)
                        }
            }
        }
    }
    return true
}

Java

// The "active pointer" is the one moving the 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 the starting position of the pointer.
        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 is the active pointer going up. Choose a new
            // active pointer and adjust it 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;
}

拖曳即可平移

上一節顯示拖曳畫面中物件的範例。另一個常見情況是平移,也就是使用者的拖曳動作會同時在 X 軸和 Y 軸上捲動。上述程式碼片段會直接攔截 MotionEvent 動作,以實作拖曳。本節中的程式碼片段可覆寫 GestureDetector.SimpleOnGestureListener 中的 onScroll(),並利用平台的常見手勢支援。

如要提供更多背景資訊,當使用者拖曳手指來平移內容時,系統會呼叫 onScroll()。系統只會在手指向下滑動時呼叫 onScroll()。手指離開螢幕後,如果手指在舉起前以一定速度移動,手勢就會結束或進入快速滑過手勢。如要進一步瞭解捲動與快速滑過動作,請參閱「為捲動手勢設定動畫效果」。

onScroll() 的程式碼片段如下:

Kotlin

// The current viewport. This rectangle represents the visible
// chart domain and range.
private val mCurrentViewport = 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 must be drawn.
private val mContentRect: Rect? = null

private val mGestureListener = object : GestureDetector.SimpleOnGestureListener() {
    ...
    override fun onScroll(
            e1: MotionEvent,
            e2: MotionEvent,
            distanceX: Float,
            distanceY: Float
    ): Boolean {
        // Scrolling uses math based on the viewport, as opposed to math using
        // pixels.

        mContentRect?.apply {
            // Pixel offset is the offset in screen pixels, while viewport offset is the
            // offset within the current viewport.
            val viewportOffsetX = distanceX * mCurrentViewport.width() / width()
            val viewportOffsetY = -distanceY * mCurrentViewport.height() / height()


            // Updates the viewport and refreshes the display.
            setViewportBottomLeft(
                    mCurrentViewport.left + viewportOffsetX,
                    mCurrentViewport.bottom + viewportOffsetY
            )
        }

        return true
    }
}

Java

// The current viewport. This rectangle represents the 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 must 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() 實作會配合觸控手勢捲動可視區域:

Kotlin

/**
 * Sets the current viewport, defined by mCurrentViewport, to the given
 * X and Y positions. The Y value represents the topmost pixel position,
 * and thus the bottom of the mCurrentViewport rectangle.
 */
private fun setViewportBottomLeft(x: Float, y: Float) {
    /*
     * Constrains within the scroll range. The scroll range is the viewport
     * extremes, such as AXIS_X_MAX, minus the viewport size. For example, if
     * the extremes are 0 and 10 and the viewport size is 2, the scroll range
     * is 0 to 8.
     */

    val curWidth: Float = mCurrentViewport.width()
    val curHeight: Float = mCurrentViewport.height()
    val newX: Float = Math.max(AXIS_X_MIN, Math.min(x, AXIS_X_MAX - curWidth))
    val newY: Float = Math.max(AXIS_Y_MIN + curHeight, Math.min(y, AXIS_Y_MAX))

    mCurrentViewport.set(newX, newY - curHeight, newX + curWidth, newY)

    // Invalidates the View to update the display.
    ViewCompat.postInvalidateOnAnimation(this)
}

Java

/**
 * 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 the viewport
     * extremes, such as AXIS_X_MAX, minus the viewport size. For example, if
     * the extremes are 0 and 10 and the viewport size is 2, the scroll range
     * is 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);
}

使用觸控操作縮放

如「偵測常見手勢」一節所述,請使用 GestureDetector 偵測 Android 使用的常用手勢,例如捲動、快速滑過、按住。針對資源調度,Android 提供 ScaleGestureDetector。如果您想讓檢視畫面識別其他手勢,可以搭配使用 GestureDetectorScaleGestureDetector

如要回報偵測到的手勢事件,手勢偵測工具會使用傳遞至其建構函式的事件監聽器物件。ScaleGestureDetector 使用 ScaleGestureDetector.OnScaleGestureListener。Android 提供 ScaleGestureDetector.SimpleOnScaleGestureListener 做為輔助類別,如果不需要所有回報的事件,則可加以擴充。

基本資源調度範例

下列程式碼片段說明縮放作業的基本元素。

Kotlin

private var mScaleFactor = 1f

private val scaleListener = object : ScaleGestureDetector.SimpleOnScaleGestureListener() {

    override fun onScale(detector: ScaleGestureDetector): Boolean {
        mScaleFactor *= detector.scaleFactor

        // Don't let the object get too small or too large.
        mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f))

        invalidate()
        return true
    }
}

private val mScaleDetector = ScaleGestureDetector(context, scaleListener)

override fun onTouchEvent(ev: MotionEvent): Boolean {
    // Let the ScaleGestureDetector inspect all events.
    mScaleDetector.onTouchEvent(ev)
    return true
}

override fun onDraw(canvas: Canvas?) {
    super.onDraw(canvas)

    canvas?.apply {
        save()
        scale(mScaleFactor, mScaleFactor)
        // onDraw() code goes here.
        restore()
    }
}

Java

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;
    }
}

更複雜的資源調度範例

以下為「為捲動手勢設定動畫」中 InteractiveChart 範例中較為複雜的範例。InteractiveChart 範例支援使用 ScaleGestureDetector Span (getCurrentSpanXgetCurrentSpanY) 和「聚焦」(getFocusXgetFocusY) 功能,透過多指捲動、平移和縮放。

Kotlin

private val mCurrentViewport = RectF(AXIS_X_MIN, AXIS_Y_MIN, AXIS_X_MAX, AXIS_Y_MAX)
private val mContentRect: Rect? = null
...
override fun onTouchEvent(event: MotionEvent): Boolean {
    return mScaleGestureDetector.onTouchEvent(event)
            || mGestureDetector.onTouchEvent(event)
            || super.onTouchEvent(event)
}

/**
 * The scale listener, used for handling multi-finger scale gestures.
 */
private val mScaleGestureListener = object : ScaleGestureDetector.SimpleOnScaleGestureListener() {

    /**
     * This is the active focal point in terms of the viewport. It can be a
     * local variable, but keep it here to minimize per-frame allocations.
     */
    private val viewportFocus = PointF()
    private var lastSpanX: Float = 0f
    private var lastSpanY: Float = 0f

    // Detects new pointers are going down.
    override fun onScaleBegin(scaleGestureDetector: ScaleGestureDetector): Boolean {
        lastSpanX = scaleGestureDetector.currentSpanX
        lastSpanY = scaleGestureDetector.currentSpanY
        return true
    }

    override fun onScale(scaleGestureDetector: ScaleGestureDetector): Boolean {
        val spanX: Float = scaleGestureDetector.currentSpanX
        val spanY: Float = scaleGestureDetector.currentSpanY

        val newWidth: Float = lastSpanX / spanX * mCurrentViewport.width()
        val newHeight: Float = lastSpanY / spanY * mCurrentViewport.height()

        val focusX: Float = scaleGestureDetector.focusX
        val focusY: Float = scaleGestureDetector.focusY
        // Ensures the chart point is within the chart region.
        // See the sample for the implementation of hitTest().
        hitTest(focusX, focusY, viewportFocus)

        mContentRect?.apply {
            mCurrentViewport.set(
                    viewportFocus.x - newWidth * (focusX - left) / width(),
                    viewportFocus.y - newHeight * (bottom - focusY) / height(),
                    0f,
                    0f
            )
        }
        mCurrentViewport.right = mCurrentViewport.left + newWidth
        mCurrentViewport.bottom = mCurrentViewport.top + newHeight
        // Invalidates the View to update the display.
        ViewCompat.postInvalidateOnAnimation(this@InteractiveLineGraphView)

        lastSpanX = spanX
        lastSpanY = spanY
        return true
    }
}

Java

private RectF mCurrentViewport =
        new RectF(AXIS_X_MIN, AXIS_Y_MIN, AXIS_X_MAX, AXIS_Y_MAX);
private Rect mContentRect;
private ScaleGestureDetector mScaleGestureDetector;
...
@Override
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. It can be a
     * local variable, but keep it here to minimize per-frame allocations.
     */
    private PointF viewportFocus = new PointF();
    private float lastSpanX;
    private float lastSpanY;

    // Detects 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();
        // Ensures 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;
    }
};

其他資源

如要進一步瞭解輸入事件、感應器和互動式自訂檢視畫面,請參閱下列參考資料。