自定义viewpager可变指示器

效果图如下：


1.gif

实现方式:整体是水平的LinearLayout，大小可变的圆点是自定义控件。用属性动画控制圆点宽度的变化，实现大小动态改变的效果。
具体实现步骤：
1.可变大小的指示器实现

/**
 * 可缩小成圆放大成圆角矩形的view
 */
public class BannerItemView extends View {
    private static final String TAG = BannerItemView.class.getSimpleName();
    private Paint mPaint;
    private float rectWidth;
    private RectF rectF;
    public static final int CENTER = 0;
    public static final int LEFT = 1;
    public static final int RIGHT = 2;
    private int location = CENTER;

    public BannerItemView(Context context) {
        super(context);
        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        rectF = new RectF();
        mPaint.setColor(Color.RED);
    }

    public BannerItemView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        rectF = new RectF();
        mPaint.setColor(Color.RED);
    }

    public BannerItemView(Context context, int location) {
        this(context);
        this.location = location;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        switch (location) {
            case CENTER:
                //中间对齐
                rectF.left = getWidth() / 2 - getHeight() / 2 - rectWidth;
                rectF.right = getHeight() / 2 + getWidth() / 2 + rectWidth;
                rectF.top = 0;
                rectF.bottom = getHeight();
                break;
            case LEFT:
                rectF.left = getWidth() - getHeight() - rectWidth;
                rectF.right = getWidth();
                rectF.top = 0;
                rectF.bottom = getHeight();
                break;
            case RIGHT:
                rectF.left = 0;
                rectF.right = getHeight() + rectWidth;
                rectF.top = 0;
                rectF.bottom = getHeight();
                break;
        }
        canvas.drawRoundRect(rectF, getHeight() / 2, getHeight() / 2, mPaint);
    }

    public float getRectWidth() {
        return rectWidth;
    }

    public void setRectWidth(float rectWidth) {
        this.rectWidth = rectWidth;
        invalidate();
    }

    public int getLocation() {
        return location;
    }

    public void setLocation(int location) {
        this.location = location;
    }
}

2.可以自定义的属性

<!--viewpager指示器-->
    <declare-styleable name="BannerIndicator">
        <!--指示器之间的间隙-->
        <attr name="gap" format="dimension" />
        <!--指示器的宽度-->
        <attr name="slider_width" format="dimension" />
        <!--指示器的高度-->
        <attr name="slider_height" format="dimension" />
        <!--指示器大小变化方向-->
        <attr name="sleider_align">
            <enum name="Left" value="1" />
            <enum name="Center" value="0" />
            <enum name="Right" value="2" />
        </attr>
    </declare-styleable>

3.具体实现代码

public class BannerIndicator extends LinearLayout {

    private int dashGap;
    private int position;
    private static final String TAG = BannerIndicator.class.getSimpleName();
    private ViewPager viewPager;
    private int slider_width;
    private int slider_height;
    private int sliderAlign;

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

    public BannerIndicator(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public BannerIndicator(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        setOrientation(HORIZONTAL);
        //获取自定义属性
        TypedArray array = context.obtainStyledAttributes(attrs, R.styleable.BannerIndicator);
        dashGap = (int) array.getDimension(R.styleable.BannerIndicator_gap, Tool.dpToPx(3));
        slider_width = (int) array.getDimension(R.styleable.BannerIndicator_slider_width,
                Tool.dpToPx(12));
        slider_height = (int) array.getDimension(R.styleable.BannerIndicator_slider_height, Tool.dpToPx(4));
        sliderAlign = array.getInt(R.styleable.BannerIndicator_sleider_align, 1);
        array.recycle();
        }
    //和viewpager联动,根据viewpager页面动态生成相应的小圆点
    public void setUpWidthViewPager(ViewPager viewPager) {
        removeAllViews();
        if (viewPager == null || viewPager.getAdapter()==null||viewPager.getAdapter().getCount() < 2) {
            return;
        }
        position = 0;
        this.viewPager = viewPager;
        for (int i = 0; i < viewPager.getAdapter().getCount(); i++) {
            BannerItemView imageView = new BannerItemView(getContext(), sliderAlign);
            LinearLayout.LayoutParams layoutParams = new LinearLayout.LayoutParams(slider_width,
                    slider_height);
            //设置小圆点之间的距离
            if (i > 0) {
                layoutParams.setMargins(dashGap, 0, 0, 0);
                imageView.setAlpha(0.4f);
            } else {
                layoutParams.setMargins(0, 0, 0, 0);
                imageView.setAlpha(1);
            }
            imageView.setLayoutParams(layoutParams);
            addView(imageView);
            //默认第一个指示器增大
            setLarge(0);
        }
        //根据viewpager页面切换事件，设置指示器大小变化
        viewPager.addOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
            @Override
            public void onPageSelected(int position) {
                if (BannerIndicator.this.position != position) {
                    resetSize(BannerIndicator.this.position, position);
                    BannerIndicator.this.position = position;
                }
            }
        });
    }

    //重置指示器样式
    private void resetSize(int last, int current) {
        setLarge(current);
        setSmall(last);
    }

    //指示器增大同时设置透明度变化
    private void setLarge(int position) {
        if (getChildAt(position) instanceof BannerItemView) {
            AnimatorSet set = new AnimatorSet();
            ValueAnimator animator = getEnlarge((BannerItemView) getChildAt(position));
            ValueAnimator alpha = ObjectAnimator.ofFloat(getChildAt(position), "alpha", 0.4f, 1f);
            set.play(animator).with(alpha);
            set.setDuration(618);
            set.start();
        }
    }

  
    //放大动画
    private ValueAnimator getEnlarge(BannerItemView roundRectView) {
        return ObjectAnimator.ofFloat(roundRectView,
                "rectWidth",
                0, getOffset(roundRectView));
    }

    //根据大小变化方向获取指示器大小偏移量
    private int getOffset(BannerItemView bannerItemView) {
        int offsest = 0;
        switch (bannerItemView.getLocation()) {
            case BannerItemView.CENTER:
                offsest = (slider_width - slider_height) / 2;
                break;
            case BannerItemView.LEFT:
                offsest = slider_width - slider_height;
                break;
            case BannerItemView.RIGHT:
                offsest = slider_width - slider_height;
                break;
        }
        return offsest;
    }

    //缩小动画
    private ValueAnimator getShrink(BannerItemView roundRectView) {
        return ObjectAnimator.ofFloat(roundRectView,
                "rectWidth",
                getOffset(roundRectView), 0);
    }

    //缩小动画同事伴随透明度变化
    public void setSmall(int small) {
        if (getChildAt(small) instanceof BannerItemView) {
            AnimatorSet set = new AnimatorSet();
            ValueAnimator alpha = ObjectAnimator.ofFloat(getChildAt(position), "alpha", 1, 0.4f);
            ValueAnimator animator = getShrink((BannerItemView) getChildAt(small));
            set.play(animator).with(alpha);
            set.setDuration(618);
            set.start();
        }
    }
}

4.具体使用
4.1布局文件

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.v4.view.ViewPager
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="200dp"
        android:background="#dedede"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <com.example.demo.BannerIndicator
        android:id="@+id/indicator"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:layout_marginTop="10dp"
        app:slider_height="10dp"
        app:slider_width="24dp"
        app:sleider_align="Center"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/viewpager" />
</android.support.constraint.ConstraintLayout>

4.2.代码实现

  public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        init();
    }
    private void init(){
        ViewPager viewPager=findViewById(R.id.viewpager);
        BannerIndicator indicator=findViewById(R.id.indicator);
        final List<TestFragment>datas=new ArrayList<>();
        for (int i = 0; i <8 ; i++) {
            datas.add(new TestFragment());
        }
        viewPager.setAdapter(new FragmentPagerAdapter(getSupportFragmentManager()) {
            @Override
            public Fragment getItem(int position) {
                return datas.get(position);
            }

            @Override
            public int getCount() {
                return datas.size();
            }
        });
        indicator.setUpWidthViewPager(viewPager);
    }
}

4.3.上面用到的fragment和布局

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:text="TextView"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />


</android.support.constraint.ConstraintLayout>

public class TestFragment extends Fragment {
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_layout,container,false);
    }
}
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