触摸时 ACTION_CANCEL

我有以下类，它代表一个可触摸的视图并绘制一个滑动条。

public class SlideBar extends View {
private int progress;
private int max;

private Paint background;
private Paint upground;

private RectF bar;

private boolean firstDraw;

public SlideBar(Context context, AttributeSet attrs) {
    super(context, attrs);
    progress = 0;

    upground = new Paint();
    upground.setColor(Color.parseColor("#C2296C"));

    background = new Paint();
    background.setColor(Color.parseColor("#777777"));
}

private void onFirstDraw() {
    max = getWidth();
    bar = new RectF(0, 19, max, 21);
}

public void onDraw(Canvas canvas) {
    if (!firstDraw) {
        onFirstDraw();
        progress = max;
        firstDraw = true;
    }

    canvas.save();
    canvas.drawRoundRect(bar, 5, 5, background);
    canvas.drawCircle(progress, 20, 9, upground);
    canvas.restore();
}

public void setValue(int value) {
    progress = value;
}

public boolean onTouchEvent(MotionEvent evt) {
    System.out.println(evt.getAction());
    progress = (int) evt.getX();
    invalidate();
    return false;
}
}

但是当触摸并拖动它时，我收到一个 ACTION_DOWN，一些 ACTION_MOVE 然后收到一个 ACTION_CANCEL 并且没有进一步的事件。

为什么会出现这样的情况呢？我不想取消该事件并使其继续拖动栏。

当父容器拦截您的触摸事件时，就会发生这种情况。任何覆盖的 ViewGroupViewGroup.onInterceptTouchEvent(MotionEvent) http://developer.android.com/reference/android/view/ViewGroup.html#onInterceptTouchEvent%28android.view.MotionEvent%29可以做到这一点（例如 ScrollView 或 ListView）。

处理这个问题的正确方法是调用ViewParent.requestDisallowInterceptTouchEvent(布尔值) http://developer.android.com/reference/android/view/ViewParent.html#requestDisallowInterceptTouchEvent%28boolean%29一旦您认为需要保留运动事件，请在您的父视图上使用方法。

这是一个简单的示例（attemptClaimDrag 方法取自 android 源代码）：

/**
 * Tries to claim the user's drag motion, and requests disallowing any
 * ancestors from stealing events in the drag.
 */
private void attemptClaimDrag() {
    //mParent = getParent();
    if (mParent != null) {
        mParent.requestDisallowInterceptTouchEvent(true);
    }
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    if (event.getAction() == MotionEvent.ACTION_DOWN) {
        if (iWantToKeepThisEventForMyself(event)) {
            attemptClaimDrag();
        }
        //your logic here
    } else {
        //your logic here
    }
}