开源Tab标签PagerSlidingTabStrip的使用及源

这一篇，来写源码解析，其实并不算困难

主要就是PagerSlidingTabStrip这个自定义控件，PagerSlidingTabStrip继承自HorizontalScrollView。首先在成员变量里定义了一些绘图需要的属性，也设置了默认值，比如下面这些：

private final PageListener pageListener = new PageListener();
    // @formatter:on
    public OnPageChangeListener delegatePageListener;
    private LinearLayout.LayoutParams defaultTabLayoutParams;
    private LinearLayout.LayoutParams expandedTabLayoutParams;
    /**
     * HorizontalScrollView里的容器
     */
    private LinearLayout tabsContainer;
    private ViewPager pager;
    private int tabCount;
    private int currentPosition = 0;
    private float currentPositionOffset = 0f;
    private Paint rectPaint;
    private Paint dividerPaint;
    private int indicatorColor = 0xFF666666;
    private int underlineColor = 0x1A000000;
    private int dividerColor = 0x1A000000;
    private boolean shouldExpand = false;
    private boolean textAllCaps = true;
    /**
     * 滑动Tab时之前的Tab移动的距离
     */
    private int scrollOffset = 52;
    private int indicatorHeight = 8;
    private int underlineHeight = 2;
    //竖直分割线的内边矩
    private int dividerPadding = 12;
    private int tabPadding = 24;
    private int dividerWidth = 1;
    private int tabTextSize = 12;
    private int tabTextColor = 0xFF666666;
    private Typeface tabTypeface = null;
    private int tabTypefaceStyle = Typeface.BOLD;
    private int lastScrollX = 0;
    private int tabBackgroundResId = R.drawable.background_tab;

然后是构造方法，这里面获取了自定义属性的值，以及初始化了相关画笔，代码如下：

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

        setFillViewport(true);
        setWillNotDraw(false);

        tabsContainer = new LinearLayout(context);
        tabsContainer.setOrientation(LinearLayout.HORIZONTAL);
        tabsContainer.setLayoutParams(new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));
        addView(tabsContainer);

        DisplayMetrics dm = getResources().getDisplayMetrics();

        scrollOffset = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, scrollOffset, dm);
        indicatorHeight = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, indicatorHeight, dm);
        underlineHeight = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, underlineHeight, dm);
        dividerPadding = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dividerPadding, dm);
        tabPadding = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, tabPadding, dm);
        dividerWidth = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dividerWidth, dm);
        tabTextSize = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_SP, tabTextSize, dm);

        // get system attrs (android:textSize and android:textColor)

        TypedArray a = context.obtainStyledAttributes(attrs, ATTRS);

        tabTextSize = a.getDimensionPixelSize(0, tabTextSize);
        tabTextColor = a.getColor(1, tabTextColor);

        a.recycle();

        // get custom attrs

        a = context.obtainStyledAttributes(attrs, R.styleable.PagerSlidingTabStrip);

        indicatorColor = a.getColor(R.styleable.PagerSlidingTabStrip_pstsIndicatorColor, indicatorColor);
        Log.d("alan","indicatorColor---------~~~~"+indicatorColor);
        underlineColor = a.getColor(R.styleable.PagerSlidingTabStrip_pstsUnderlineColor, underlineColor);
        dividerColor = a.getColor(R.styleable.PagerSlidingTabStrip_pstsDividerColor, dividerColor);
        indicatorHeight = a.getDimensionPixelSize(R.styleable.PagerSlidingTabStrip_pstsIndicatorHeight, indicatorHeight);
        underlineHeight = a.getDimensionPixelSize(R.styleable.PagerSlidingTabStrip_pstsUnderlineHeight, underlineHeight);
        dividerPadding = a.getDimensionPixelSize(R.styleable.PagerSlidingTabStrip_pstsDividerPadding, dividerPadding);
        tabPadding = a.getDimensionPixelSize(R.styleable.PagerSlidingTabStrip_pstsTabPaddingLeftRight, tabPadding);
        tabBackgroundResId = a.getResourceId(R.styleable.PagerSlidingTabStrip_pstsTabBackground, tabBackgroundResId);
        shouldExpand = a.getBoolean(R.styleable.PagerSlidingTabStrip_pstsShouldExpand, shouldExpand);
        scrollOffset = a.getDimensionPixelSize(R.styleable.PagerSlidingTabStrip_pstsScrollOffset, scrollOffset);
        textAllCaps = a.getBoolean(R.styleable.PagerSlidingTabStrip_pstsTextAllCaps, textAllCaps);

        a.recycle();

        rectPaint = new Paint();
        rectPaint.setAntiAlias(true);
        rectPaint.setStyle(Style.FILL);

        dividerPaint = new Paint();
        dividerPaint.setAntiAlias(true);
        dividerPaint.setStrokeWidth(dividerWidth);
        //当Tab个数不超过屏幕宽度时
        defaultTabLayoutParams = new LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.MATCH_PARENT);
        //当Tab个数超过屏幕宽度时
        expandedTabLayoutParams = new LinearLayout.LayoutParams(0, LayoutParams.MATCH_PARENT, 1.0f);

        if (locale == null) {
            locale = getResources().getConfiguration().locale;
        }
    }

解释：HorizontalScrollView里面只有一个子控件，就是tabsContainer，一个线性布局，这个用来装多个TextView用来做为Tab标签，tabsContainer有两种设定宽高的方式，一个是当Tab很少，不足以超过屏幕时是这个

//当Tab个数不超过屏幕宽度时
        defaultTabLayoutParams = new LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.MATCH_PARENT);

当Tab很多，超过屏幕宽度了时，就是按权重了

//当Tab个数超过屏幕宽度时
        expandedTabLayoutParams = new LinearLayout.LayoutParams(0, LayoutParams.MATCH_PARENT, 1.0f);

当外部调用setViewPager时，会走notifyDataSetChanged()方法，这是notifyDataSetChanged方法

public void notifyDataSetChanged() {

        tabsContainer.removeAllViews();

        tabCount = pager.getAdapter().getCount();

        for (int i = 0; i < tabCount; i++) {
            //Tab是图标类
            if (pager.getAdapter() instanceof IconTabProvider) {
                addIconTab(i, ((IconTabProvider) pager.getAdapter()).getPageIconResId(i));
            } else {//如果Tab是文字类
                addTextTab(i, pager.getAdapter().getPageTitle(i).toString());
            }

        }

        updateTabStyles();

        getViewTreeObserver().addOnGlobalLayoutListener(new OnGlobalLayoutListener() {

            @SuppressWarnings("deprecation")
            @SuppressLint("NewApi")
            @Override
            public void onGlobalLayout() {

                if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN) {
                    getViewTreeObserver().removeGlobalOnLayoutListener(this);
                } else {
                    getViewTreeObserver().removeOnGlobalLayoutListener(this);
                }

                currentPosition = pager.getCurrentItem();
                scrollToChild(currentPosition, 0);
            }
        });

    }

即在线性布局里循环添加TextView,通过addTextTab和addTab方法进行添加，具体如下：

/**
     * 添加每一个文字Tab
     */
    private void addTextTab(final int position, String title) {

        TextView tab = new TextView(getContext());
        tab.setText(title);
        tab.setGravity(Gravity.CENTER);
        tab.setSingleLine();

        addTab(position, tab);
    }

private void addTab(final int position, View tab) {
        tab.setFocusable(true);
        tab.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                pager.setCurrentItem(position);
            }
        });

        tab.setPadding(tabPadding, 0, tabPadding, 0);
        tabsContainer.addView(tab, position, shouldExpand ? expandedTabLayoutParams : defaultTabLayoutParams);
    }

add完毕后又通过updateTabStyles这个方法来给TextView设置背景和文字属性等：

private void updateTabStyles() {

        for (int i = 0; i < tabCount; i++) {

            View v = tabsContainer.getChildAt(i);

            v.setBackgroundResource(tabBackgroundResId);

            if (v instanceof TextView) {

                TextView tab = (TextView) v;
                tab.setTextSize(TypedValue.COMPLEX_UNIT_PX, tabTextSize);
                tab.setTypeface(tabTypeface, tabTypefaceStyle);
                tab.setTextColor(tabTextColor);

                // setAllCaps() is only available from API 14, so the upper case is made manually if we are on a
                // pre-ICS-build
                if (textAllCaps) {
                    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
                        tab.setAllCaps(true);
                    } else {
                        tab.setText(tab.getText().toString().toUpperCase(locale));
                    }
                }
            }
        }

    }

还要根据ViewPager的位置滑动Tab到指定位置，是这么做的：

private void scrollToChild(int position, int offset) {

        if (tabCount == 0) {
            return;
        }

        int newScrollX = tabsContainer.getChildAt(position).getLeft() + offset;

        if (position > 0 || offset > 0) {
            newScrollX -= scrollOffset;
        }

        if (newScrollX != lastScrollX) {
            lastScrollX = newScrollX;
            scrollTo(newScrollX, 0);
        }

    }

在onDraw方法里面，绘制Tab底部指针和底线：

@Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        if (isInEditMode() || tabCount == 0) {
            return;
        }

        final int height = getHeight();

        // draw indicator line

        rectPaint.setColor(indicatorColor);
//        Log.d("alan","indicatorColor---->"+indicatorColor);

        // default: line below current tab
        View currentTab = tabsContainer.getChildAt(currentPosition);
        float lineLeft = currentTab.getLeft();
        float lineRight = currentTab.getRight();

        // if there is an offset, start interpolating left and right coordinates between current and next tab
        if (currentPositionOffset > 0f && currentPosition < tabCount - 1) {

            View nextTab = tabsContainer.getChildAt(currentPosition + 1);
            final float nextTabLeft = nextTab.getLeft();
            final float nextTabRight = nextTab.getRight();

            lineLeft = (currentPositionOffset * nextTabLeft + (1f - currentPositionOffset) * lineLeft);
            lineRight = (currentPositionOffset * nextTabRight + (1f - currentPositionOffset) * lineRight);
        }
        //画Tab下面的指针
        canvas.drawRect(lineLeft, height - indicatorHeight, lineRight, height, rectPaint);

        // draw underline

        rectPaint.setColor(underlineColor);
        canvas.drawRect(0, height - underlineHeight, tabsContainer.getWidth(), height, rectPaint);

        // draw divider

        dividerPaint.setColor(dividerColor);
        for (int i = 0; i < tabCount - 1; i++) {
            View tab = tabsContainer.getChildAt(i);
            canvas.drawLine(tab.getRight(), dividerPadding, tab.getRight(), height - dividerPadding, dividerPaint);
        }
    }

到现在，就分析完了，认为主要难度在于滑动的距离计算，还有一些坐标计算，通过这个源码分析，还是能学到很多自定义控件知识的