下拉刷新的简单实现

看了Taurus的下拉刷新代码，参考它的主要思想自己也动手写一个下拉刷新。

首先看一下实现了之后，demo项目中的布局

<com.yzx.TestPullToRefreshView
    android:id="@+id/pull_to_refresh"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ListView
        android:id="@+id/list_view"
        android:divider="@null"
        android:dividerHeight="0dp"
        android:fadingEdge="none"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</com.yzx.TestPullToRefreshView>

这里的TestPullToRefreshView就是我们的下拉刷新自定义view。主要的思路就是在TestPullToRefreshView中添加一个RefreshView作为刷新的View。然后通过onTouchEvent来计算位移，再通过offsetTopAndBottom函数来控制ListView和RefreshView所显示的位置。

@Override
    public boolean onTouchEvent(MotionEvent ev) {
        final int action = MotionEventCompat.getActionMasked(ev);

        switch (action) {
            case MotionEventCompat.ACTION_POINTER_DOWN:
                final int index = MotionEventCompat.getActionIndex(ev);
                mActivePointerId = MotionEventCompat.getPointerId(ev, index);

                final float initialMotionY = getMotionEventY(ev, mActivePointerId);
                if (initialMotionY == -1) {
                    return false;
                }
                mInitialMotionY = initialMotionY;
                break;
            case MotionEvent.ACTION_MOVE: {
                int offset = calOffset(ev);
                if (offset == -1) {
                    return false;
                }

                setTargetOffsetTop(offset);
                break;
            }
            case MotionEventCompat.ACTION_POINTER_UP:
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL: {
                return false;
            }
        }

        return true;
    }

calOffset方法根据下拉距离计算RefreshView显示大小，setTargetOffsetTop用来实现ListView的上下滑动。
下面方法是根据Taurus中的计算方法来计算位移值

private int calOffset(MotionEvent ev) {
        final int pointerIndex = MotionEventCompat.findPointerIndex(ev, mActivePointerId);
        if (pointerIndex < 0) {
            return -1;
        }

        final float y = MotionEventCompat.getY(ev, pointerIndex);
        final float yDiff = y - mInitialMotionY;
        final float scrollTop = yDiff * DRAG_RATE;
        mCurrentDragPercent = scrollTop / mTotalDragDistance;
        if (mCurrentDragPercent < 0) {
            return -1;
        }
        float boundedDragPercent = Math.min(1f, Math.abs(mCurrentDragPercent));
        float extraOS = Math.abs(scrollTop) - mTotalDragDistance;
        float slingshotDist = mTotalDragDistance;
        float tensionSlingshotPercent = Math.max(0, Math.min(extraOS, slingshotDist * 2) / slingshotDist);
        float tensionPercent = (float) ((tensionSlingshotPercent / 4) - Math.pow(
                (tensionSlingshotPercent / 4), 2)) * 2f;
        float extraMove = (slingshotDist) * tensionPercent / 2;
        int targetY = (int) ((slingshotDist * boundedDragPercent) + extraMove);

        return targetY - mCurrentOffsetTop;
    }

下面方法是具体条用offsetTopAndBottom的方法。

private void setTargetOffsetTop(int offset, boolean requiresUpdate) {
    mTarget.offsetTopAndBottom(offset);
    mTestDrawable.offsetTopAndBottom(offset);
    mCurrentOffsetTop = mTarget.getTop();
    if (requiresUpdate && android.os.Build.VERSION.SDK_INT < 11) {
        invalidate();
    }
}