src/com/android/messaging/ui/ViewPagerTabStrip.java

/*
 * Copyright (C) 2015 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.android.messaging.ui;

import android.content.Context;
import android.content.res.Resources;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;
import android.widget.LinearLayout;

import com.android.messaging.R;
import com.android.messaging.util.OsUtil;

public class ViewPagerTabStrip extends LinearLayout {
    private int mSelectedUnderlineThickness;
    private final Paint mSelectedUnderlinePaint;

    private int mIndexForSelection;
    private float mSelectionOffset;

    public ViewPagerTabStrip(Context context) {
        this(context, null);
    }

    public ViewPagerTabStrip(Context context, AttributeSet attrs) {
        super(context, attrs);

        final Resources res = context.getResources();

        mSelectedUnderlineThickness =
                res.getDimensionPixelSize(R.dimen.pager_tab_underline_selected);
        int underlineColor = res.getColor(R.color.contact_picker_tab_underline);
        int backgroundColor = res.getColor(R.color.action_bar_background_color);

        mSelectedUnderlinePaint = new Paint();
        mSelectedUnderlinePaint.setColor(underlineColor);

        setBackgroundColor(backgroundColor);
        setWillNotDraw(false);
    }

    /**
     * Notifies this view that view pager has been scrolled. We save the tab index
     * and selection offset for interpolating the position and width of selection
     * underline.
     */
    void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
        mIndexForSelection = position;
        mSelectionOffset = positionOffset;
        invalidate();
    }

    @Override
    protected void onDraw(Canvas canvas) {
        int childCount = getChildCount();

        // Thick colored underline below the current selection
        if (childCount > 0) {
            View selectedTitle = getChildAt(mIndexForSelection);
            int selectedLeft = selectedTitle.getLeft();
            int selectedRight = selectedTitle.getRight();
            final boolean isRtl = isRtl();
            final boolean hasNextTab = isRtl ? mIndexForSelection > 0
                    : (mIndexForSelection < (getChildCount() - 1));
            if ((mSelectionOffset > 0.0f) && hasNextTab) {
                // Draw the selection partway between the tabs
                View nextTitle = getChildAt(mIndexForSelection + (isRtl ? -1 : 1));
                int nextLeft = nextTitle.getLeft();
                int nextRight = nextTitle.getRight();

                selectedLeft = (int) (mSelectionOffset * nextLeft +
                        (1.0f - mSelectionOffset) * selectedLeft);
                selectedRight = (int) (mSelectionOffset * nextRight +
                        (1.0f - mSelectionOffset) * selectedRight);
            }

            int height = getHeight();
            canvas.drawRect(selectedLeft, height - mSelectedUnderlineThickness,
                    selectedRight, height, mSelectedUnderlinePaint);
        }
    }

    private boolean isRtl() {
        return OsUtil.isAtLeastJB_MR2() ? getLayoutDirection() == View.LAYOUT_DIRECTION_RTL : false;
    }
}