DiscrollView.java

package com.marshalchen.common.uimodule.discrollview;

import android.content.Context;
import android.util.AttributeSet;
import android.view.View;
import android.widget.ScrollView;

/**
 * Layout container for a View hierarchy that can discrollved (ie Discrollvable).
 * For each discrollvables, if it's possible (ie enough space to scroll the whole view),
 * the discrollvable starts to be discrollved when its center reaches the center of the DiscrollView;
 * else, the discrollvables starts to be discrollved when its top reaches the bottom of the DiscrollView
 *
 * @see com.marshalchen.common.uimodule.discrollview.Discrollvable
 * @see com.marshalchen.common.uimodule.discrollview.DiscrollViewContent
 *
 */
public class DiscrollView extends ScrollView {

    private DiscrollViewContent mContent;

    public DiscrollView(Context context) {
        super(context);
    }

    public DiscrollView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public DiscrollView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        setupFirstView();

    }

    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        if(getChildCount() != 1) {
            throw new IllegalStateException("Discrollview must host one child.");
        }
        View content = getChildAt(0);
        if(!(content instanceof DiscrollViewContent)) {
            throw new IllegalStateException("Discrollview must host a DiscrollViewContent.");
        }
        mContent = (DiscrollViewContent) content;
        if(mContent.getChildCount() < 2) {
            throw new IllegalStateException("Discrollview must have at least 2 children.");
        }
    }

    private void setupFirstView() {
        //first first take all the parent height
        View first = mContent.getChildAt(0);
        if(first != null) {
            first.getLayoutParams().height = getHeight();
        }
    }

    @Override
    protected void onScrollChanged(int l, int t, int oldl, int oldt) {
        super.onScrollChanged(l, t, oldl, oldt);
        onScrollChanged(t);
    }

    public static float clamp(float value, float max, float min) {
        return Math.max(Math.min(value, min), max);
    }

    private int getAbsoluteBottom() {
        View last = getChildAt(getChildCount() - 1);
        if(last == null) {
            return 0;
        }
        return last.getBottom();
    }

    private void onScrollChanged(int top) {
        int scrollViewHeight = getHeight();
        int scrollViewBottom = getAbsoluteBottom();
        int scrollViewHalfHeight = scrollViewHeight / 2;

        // starts at 1 because the first View is a static non Discrollvable
        // view.
        for(int index = 1; index < mContent.getChildCount() ; index++) {
            View child = mContent.getChildAt(index);
            if(! (child instanceof Discrollvable)) {
                //it's a static view, doesn't care about
                continue;
            }
            Discrollvable discrollvable = (Discrollvable) child;
            int discrollvableTop = child.getTop();
            int discrollvableHeight = child.getHeight();
            int discrollvableAbsoluteTop = discrollvableTop - top;

            //the Discrollvable is too big to be discrollved when its center is reached
            //the Discrollvable center. Discrollve it by its top.
            if(scrollViewBottom - child.getBottom() < discrollvableHeight + scrollViewHalfHeight) {
                //the Discrollvable top reaches the DiscrollView bottom
                if(discrollvableAbsoluteTop <= scrollViewHeight) {
                    int visibleGap = scrollViewHeight - discrollvableAbsoluteTop;
                    discrollvable.onDiscrollve(clamp(visibleGap / (float) discrollvableHeight, 0.0f, 1.0f));
                } else {
                    discrollvable.onResetDiscrollve();
                }
            } else {
                if(discrollvableAbsoluteTop <= scrollViewHalfHeight) {
                    //the Discrollvable center reaches the DiscrollView center
                    int visibleGap = scrollViewHalfHeight - discrollvableAbsoluteTop;
                    discrollvable.onDiscrollve(clamp(visibleGap / (float) discrollvableHeight, 0.0f, 1.0f));
                } else {
                    discrollvable.onResetDiscrollve();
                }
            }
        }
    }
}