編輯:關於Android編程
原文地址:https://developer.android.com/training/gestures/scale.html
這節課主要學習如何使用觸摸手勢來拖動、放大屏幕上的對象。
如果你的重點在Android 3.0以上的版本,那麼你可以使用內置的拖拽事件監聽器View.OnDragListener。
觸摸手勢最常見的操作就是使用它來拖動屏幕上的對象。下面的代碼允許用戶拖動屏幕上的圖像。要注意以下幾點:
在拖動操作中,APP會一直保持手指拖動的軌跡,就算是另一只手指觸到屏幕也是。舉個例子,想象一根手指在拖動著一張圖像,這時用戶將第二根手指放置到屏幕上,如果APP只是追蹤單根手指的軌跡,那麼它會將第二根手指作為默認位置,並會將圖像移動到這個位置。 為了防止這樣的事件發生,APP需要區分第一根手指與其它手指。為此,需要追蹤 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時候由onTouchEvent()方法傳回。 在ACTION_POINTER_UP的情況下,示例提取了這個事件的索引,並確保當前活動的指針不是那個已經不在屏幕上的指針。如果是那個指針的話,那麼APP會選擇一個不同的指針使其活動並保存它的X及Y的位置。一旦這個值被保存下來,那麼APP將會使用正確指針的數據一直計算剩余移動的距離。下面的代碼允許用戶在屏幕上拖動對象。它記錄了當前活動指針的初始位置,計算了它所位移的距離,並將對象移動到新的位置上。
這裡要注意,代碼段使用了getActionMasked()方法。你應該一直使用這個方法來接收MotionEvent對象的活動。與getAction()方法不同,getActionMasked()工作於多點觸控模式下。它會返回被執行的掩飾活動,不包括指針的索引比特。
// 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; }
上面的部分展示了如何在屏幕上拖動對象。另一個通用的場景就是平移了,平移的意思是:用戶的拖動動作引起的x及y軸方向上的滾動。上面的代碼直接將MotionEvent攔截實現拖動。這部分的代碼將會采用另一種更具有優勢的方法,以便支持通用手勢。它重寫了GestureDetector.SimpleOnGestureListener的onScroll()方法。
只有用戶在使用手指移動內容時,onScroll()才會被調用。onScroll()只有在手指按下的時候才會調用,一旦手指離開屏幕,那麼平移手勢也隨之終止。
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()方法的實現,它主要實現了移動內容的邏輯:
/** * 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); }
在Detecting Common Gestures中,我們討論到GestureDetector可以幫助我們來檢測比如滑動、滾動、長按等手勢。而對於縮放,Android提供了ScaleGestureDetector. GestureDetector 以及 ScaleGestureDetector。
為了可以反饋檢測到的手勢事件,手勢探測器使用了監聽器對象ScaleGestureDetector.OnScaleGestureListener。如果你只關心部分手勢的話,Android還提供了ScaleGestureDetector.SimpleOnScaleGestureListener,你可以通過重寫它的方法來使用。
下面的代碼是縮放所需要的基礎:
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; } }
下面是一個稍微復雜一點的示例,它摘自與這節課所提供的示例InteractiveChart(PS:示例工程請參見原網頁)。InteractiveChart同時支持平移、縮放,它使用了ScaleGestureDetector的“平移”(getCurrentSpanX/Y)及“焦點” (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; } };
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// 表示事件是否攔截, 返回false表示不攔截 @Override public boolean onInterceptTouchEvent(Motion