Modification of android.support.v4.view.PagerTitleStrip preserving only primary page indicator, see http://www.thedroidsonroids.com/blog/android/pagertitlestrip-with-current-page-indicator-only/ for more info.

SinglePagerTitleStrip.java

/*
 * Copyright (C) 2011 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.
 *
 * Copyright (c) 2015 Karol Wrótniak, Droids On Roids
 */

package android.support.v4.view;

import android.content.Context;
import android.content.res.TypedArray;
import android.database.DataSetObserver;
import android.graphics.drawable.Drawable;
import android.text.TextUtils.TruncateAt;
import android.util.AttributeSet;
import android.util.TypedValue;
import android.view.Gravity;
import android.view.ViewGroup;
import android.view.ViewParent;
import android.widget.TextView;

import java.lang.ref.WeakReference;

/**
 * SinglePagerTitleStrip is a non-interactive indicator of the current page
 * of a {@link ViewPager}. It is intended to be used as a
 * child view of a ViewPager widget in your XML layout.
 * Add it as a child of a ViewPager in your layout file and set its
 * android:layout_gravity to TOP or BOTTOM to pin it to the top or bottom
 * of the ViewPager. The title from each page is supplied by the method
 * {@link PagerAdapter#getPageTitle(int)} in the adapter supplied to
 * the ViewPager.
 *
 * This is modification of {@link PagerTitleStrip}. All stuff related to
 * non-primary pages was removed.
 * 
 * <p>For an interactive indicator, see {@link PagerTabStrip}.</p>
 */
public class SinglePagerTitleStrip extends ViewGroup implements ViewPager.Decor {

    ViewPager mPager;
    TextView mCurrText;

    private int mLastKnownCurrentPage = -1;
    private float mLastKnownPositionOffset = -1;
    private int mScaledTextSpacing;
    private int mGravity;

    private boolean mUpdatingText;
    private boolean mUpdatingPositions;

    private final PageListener mPageListener = new PageListener();

    private WeakReference<PagerAdapter> mWatchingAdapter;

    private static final int[] ATTRS = new int[] {
        android.R.attr.textAppearance,
        android.R.attr.textSize,
        android.R.attr.textColor,
        android.R.attr.gravity
    };

    private static final int[] TEXT_ATTRS = new int[] {
        0x0101038c // android.R.attr.textAllCaps
    };

    private static final int TEXT_SPACING = 16; // dip


    interface PagerTitleStripImpl {
        void setSingleLineAllCaps(TextView text);
    }

    static class PagerTitleStripImplBase implements PagerTitleStripImpl {
        public void setSingleLineAllCaps(TextView text) {
            text.setSingleLine();
        }
    }

    static class PagerTitleStripImplIcs implements PagerTitleStripImpl {
        public void setSingleLineAllCaps(TextView text) {
            PagerTitleStripIcs.setSingleLineAllCaps(text);
        }
    }

    private static final PagerTitleStripImpl IMPL;
    static {
        if (android.os.Build.VERSION.SDK_INT >= 14) {
            IMPL = new PagerTitleStripImplIcs();
        } else {
            IMPL = new PagerTitleStripImplBase();
        }
    }

    private static void setSingleLineAllCaps(TextView text) {
        IMPL.setSingleLineAllCaps(text);
    }

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

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

        addView(mCurrText = new TextView(context));

        final TypedArray a = context.obtainStyledAttributes(attrs, ATTRS);
        final int textAppearance = a.getResourceId(0, 0);
        if (textAppearance != 0) {
            mCurrText.setTextAppearance(context, textAppearance);
        }
        final int textSize = a.getDimensionPixelSize(1, 0);
        if (textSize != 0) {
            setTextSize(TypedValue.COMPLEX_UNIT_PX, textSize);
        }
        if (a.hasValue(2)) {
            final int textColor = a.getColor(2, 0);
            mCurrText.setTextColor(textColor);
        }
        mGravity = a.getInteger(3, Gravity.BOTTOM);
        a.recycle();

        mCurrText.setEllipsize(TruncateAt.END);

        boolean allCaps = false;
        if (textAppearance != 0) {
            final TypedArray ta = context.obtainStyledAttributes(textAppearance, TEXT_ATTRS);
            allCaps = ta.getBoolean(0, false);
            ta.recycle();
        }

        if (allCaps) {
            setSingleLineAllCaps(mCurrText);
        } else {
            mCurrText.setSingleLine();
        }

        final float density = context.getResources().getDisplayMetrics().density;
        mScaledTextSpacing = (int) (TEXT_SPACING * density);
    }

    /**
     * Set the required spacing between title segments.
     *
     * @param spacingPixels Spacing between each title displayed in pixels
     */
    public void setTextSpacing(int spacingPixels) {
        mScaledTextSpacing = spacingPixels;
        requestLayout();
    }

    /**
     * @return The required spacing between title segments in pixels
     */
    public int getTextSpacing() {
        return mScaledTextSpacing;
    }

    /**
     * Set the color value used as the base color for all displayed page titles.
     * Alpha will be ignored for non-primary page titles. See {@link #setNonPrimaryAlpha(float)}.
     *
     * @param color Color hex code in 0xAARRGGBB format
     */
    public void setTextColor(int color) {
        mCurrText.setTextColor(color);
    }

    /**
     * Set the default text size to a given unit and value.
     * See {@link TypedValue} for the possible dimension units.
     *
     * <p>Example: to set the text size to 14px, use
     * setTextSize(TypedValue.COMPLEX_UNIT_PX, 14);</p>
     *
     * @param unit The desired dimension unit
     * @param size The desired size in the given units
     */
    public void setTextSize(int unit, float size) {
        mCurrText.setTextSize(unit, size);
    }

    /**
     * Set the {@link Gravity} used to position text within the title strip.
     * Only the vertical gravity component is used.
     *
     * @param gravity {@link Gravity} constant for positioning title text
     */
    public void setGravity(int gravity) {
        mGravity = gravity;
        requestLayout();
    }

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();

        final ViewParent parent = getParent();
        if (!(parent instanceof ViewPager)) {
            throw new IllegalStateException(
                    "PagerTitleStrip must be a direct child of a ViewPager.");
        }

        final ViewPager pager = (ViewPager) parent;
        final PagerAdapter adapter = pager.getAdapter();

        pager.setInternalPageChangeListener(mPageListener);
        pager.setOnAdapterChangeListener(mPageListener);
        mPager = pager;
        updateAdapter(mWatchingAdapter != null ? mWatchingAdapter.get() : null, adapter);
    }

    @Override
    protected void onDetachedFromWindow() {
        super.onDetachedFromWindow();
        if (mPager != null) {
            updateAdapter(mPager.getAdapter(), null);
            mPager.setInternalPageChangeListener(null);
            mPager.setOnAdapterChangeListener(null);
            mPager = null;
        }
    }

    void updateText(int currentItem, PagerAdapter adapter) {
        final int itemCount = adapter != null ? adapter.getCount() : 0;
        mUpdatingText = true;

        mCurrText.setText(adapter != null && currentItem < itemCount ?
                adapter.getPageTitle(currentItem) : null);

        // Measure everything
        final int width = getWidth() - getPaddingLeft() - getPaddingRight();
        final int childHeight = getHeight() - getPaddingTop() - getPaddingBottom();
        final int childWidthSpec = MeasureSpec.makeMeasureSpec((int) (width * 0.8f),
                MeasureSpec.AT_MOST);
        final int childHeightSpec = MeasureSpec.makeMeasureSpec(childHeight, MeasureSpec.AT_MOST);
        mCurrText.measure(childWidthSpec, childHeightSpec);

        mLastKnownCurrentPage = currentItem;

        if (!mUpdatingPositions) {
            updateTextPositions(currentItem, mLastKnownPositionOffset, false);
        }

        mUpdatingText = false;
    }

    @Override
    public void requestLayout() {
        if (!mUpdatingText) {
            super.requestLayout();
        }
    }

    void updateAdapter(PagerAdapter oldAdapter, PagerAdapter newAdapter) {
        if (oldAdapter != null) {
            oldAdapter.unregisterDataSetObserver(mPageListener);
            mWatchingAdapter = null;
        }
        if (newAdapter != null) {
            newAdapter.registerDataSetObserver(mPageListener);
            mWatchingAdapter = new WeakReference<PagerAdapter>(newAdapter);
        }
        if (mPager != null) {
            mLastKnownCurrentPage = -1;
            mLastKnownPositionOffset = -1;
            updateText(mPager.getCurrentItem(), newAdapter);
            requestLayout();
        }
    }

    void updateTextPositions(int position, float positionOffset, boolean force) {
        if (position != mLastKnownCurrentPage) {
            updateText(position, mPager.getAdapter());
        } else if (!force && positionOffset == mLastKnownPositionOffset) {
            return;
        }

        mUpdatingPositions = true;

        final int currWidth = mCurrText.getMeasuredWidth();
        final int halfCurrWidth = currWidth / 2;

        final int stripWidth = getWidth();
        final int stripHeight = getHeight();
        final int paddingLeft = getPaddingLeft();
        final int paddingRight = getPaddingRight();
        final int paddingTop = getPaddingTop();
        final int paddingBottom = getPaddingBottom();
        final int textPaddedLeft = paddingLeft + halfCurrWidth;
        final int textPaddedRight = paddingRight + halfCurrWidth;
        final int contentWidth = stripWidth - textPaddedLeft - textPaddedRight;

        float currOffset = positionOffset + 0.5f;
        if (currOffset > 1.f) {
            currOffset -= 1.f;
        }
        final int currCenter = stripWidth - textPaddedRight - (int) (contentWidth * currOffset);
        final int currLeft = currCenter - currWidth / 2;
        final int currRight = currLeft + currWidth;

        final int alignedCurrHeight =  mCurrText.getMeasuredHeight();

        final int vgrav = mGravity & Gravity.VERTICAL_GRAVITY_MASK;

        int currTop;
        switch (vgrav) {
            default:
            case Gravity.TOP:
                currTop = paddingTop;
                break;
            case Gravity.CENTER_VERTICAL:
                final int paddedHeight = stripHeight - paddingTop - paddingBottom;
                final int centeredTop = (paddedHeight - alignedCurrHeight) / 2;
                currTop = centeredTop ;
                break;
            case Gravity.BOTTOM:
                final int bottomGravTop = stripHeight - paddingBottom - alignedCurrHeight;
                currTop = bottomGravTop;
                break;
        }

        mCurrText.layout(currLeft, currTop, currRight,
                currTop + mCurrText.getMeasuredHeight());

        mLastKnownPositionOffset = positionOffset;
        mUpdatingPositions = false;
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        final int heightSize = MeasureSpec.getSize(heightMeasureSpec);

        if (widthMode != MeasureSpec.EXACTLY) {
            throw new IllegalStateException("Must measure with an exact width");
        }

        int childHeight = heightSize;
        int minHeight = getMinHeight();
        int padding = 0;
        padding = getPaddingTop() + getPaddingBottom();
        childHeight -= padding;

        final int childWidthSpec = MeasureSpec.makeMeasureSpec((int) (widthSize * 0.8f),
                MeasureSpec.AT_MOST);
        final int childHeightSpec = MeasureSpec.makeMeasureSpec(childHeight, MeasureSpec.AT_MOST);

        mCurrText.measure(childWidthSpec, childHeightSpec);

        if (heightMode == MeasureSpec.EXACTLY) {
            setMeasuredDimension(widthSize, heightSize);
        } else {
            int textHeight = mCurrText.getMeasuredHeight();
            setMeasuredDimension(widthSize, Math.max(minHeight, textHeight + padding));
        }
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        if (mPager != null) {
            final float offset = mLastKnownPositionOffset >= 0 ? mLastKnownPositionOffset : 0;
            updateTextPositions(mLastKnownCurrentPage, offset, true);
        }
    }

    int getMinHeight() {
        int minHeight = 0;
        final Drawable bg = getBackground();
        if (bg != null) {
            minHeight = bg.getIntrinsicHeight();
        }
        return minHeight;
    }

    private class PageListener extends DataSetObserver implements ViewPager.OnPageChangeListener,
            ViewPager.OnAdapterChangeListener {
        private int mScrollState;

        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            if (positionOffset > 0.5f) {
                // Consider ourselves to be on the next page when we're 50% of the way there.
                position++;
            }
            updateTextPositions(position, positionOffset, false);
        }

        @Override
        public void onPageSelected(int position) {
            if (mScrollState == ViewPager.SCROLL_STATE_IDLE) {
                // Only update the text here if we're not dragging or settling.
                updateText(mPager.getCurrentItem(), mPager.getAdapter());

                final float offset = mLastKnownPositionOffset >= 0 ? mLastKnownPositionOffset : 0;
                updateTextPositions(mPager.getCurrentItem(), offset, true);
            }
        }

        @Override
        public void onPageScrollStateChanged(int state) {
            mScrollState = state;
        }

        @Override
        public void onAdapterChanged(PagerAdapter oldAdapter, PagerAdapter newAdapter) {
            updateAdapter(oldAdapter, newAdapter);
        }

        @Override
        public void onChanged() {
            updateText(mPager.getCurrentItem(), mPager.getAdapter());

            final float offset = mLastKnownPositionOffset >= 0 ? mLastKnownPositionOffset : 0;
            updateTextPositions(mPager.getCurrentItem(), offset, true);
        }
    }
}