app的引导页和banner功能，通常采用ViewPager实现，并且往往都会有指示器，显示当前所选页。本文用自定义view的方式实现一个通用的圆形指示器，继承自View。
一、效果预览
image.png
二、效果分析
上图中显示两个小圆点，表示 ViewPager有两页（不考虑无限轮播）
红色表示选中页，灰色表示未选中页
实际上红色小圆点下方也有灰色小圆点，只是被覆盖了
所以图上有两个灰色小圆点，它们统称为背景小圆点
红色小圆点称为移动小圆点
三、自定义View的属性
<declare-styleable name="CirclePagerIndicator">
<!--是否水平居中-->
<attr name="indicator_centerHorizontal" format="boolean" />
<!--滑动小圆点是否跟随-->
<attr name="indicator_follow" format="boolean" />
<!--背景小圆点颜色-->
<attr name="indicator_color" format="color" />
<!--背景小圆点描边颜色-->
<attr name="indicator_stroke_color" format="color" />
<!--背景小圆点描边宽度-->
<attr name="indicator_stroke_width" format="dimension" />
<!--移动小圆点颜色-->
<attr name="indicator_move_color" format="color" />
<!--背景小圆点半径-->
<attr name="indicator_radius" format="dimension" />
<!--移动小圆点半径-->
<attr name="indicator_move_radius" format="dimension" />
<!--小圆点间距-->
<attr name="indicator_space" format="dimension" />
</declare-styleable>
四、自定义View套路代码
public class CirclePagerIndicator extends View implements PagerIndicator {
public CirclePagerIndicator(Context context) {
this(context, null);
}
public CirclePagerIndicator(Context context, AttributeSet attrs) {
super(context, attrs);
//获取上面自定义的属性，并初始化画笔。
TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.CirclePagerIndicator);
mCenterHorizontal = a.getBoolean(R.styleable.CirclePagerIndicator_indicator_centerHorizontal, true);
//背景小圆点画笔
mBgCirclePaint.setStyle(Paint.Style.FILL);
mBgCirclePaint.setColor(a.getColor(R.styleable.CirclePagerIndicator_indicator_color, 0x0000ff));
//背景小圆点描边画笔
mBgStrokePaint.setStyle(Paint.Style.STROKE);
mBgStrokePaint.setColor(a.getColor(R.styleable.CirclePagerIndicator_indicator_stroke_color, 0x000000));
mBgStrokePaint.setStrokeWidth(a.getDimension(R.styleable.CirclePagerIndicator_indicator_stroke_width, 0));
//移动小圆点画笔
mMoveCirclePaint.setStyle(Paint.Style.FILL);
mMoveCirclePaint.setColor(a.getColor(R.styleable.CirclePagerIndicator_indicator_move_color, 0x0000ff));
//背景小圆点半径
mBgCircleRadius = a.getDimension(R.styleable.CirclePagerIndicator_indicator_radius, 10);
//移动小圆点半径
mMoveCircleRadius = a.getDimension(R.styleable.CirclePagerIndicator_indicator_move_radius, 10);
//小圆点间距
mIndicatorSpace = a.getDimension(R.styleable.CirclePagerIndicator_indicator_space, 20);
//移动小圆点是否随viewpager移动跟随
mIsFollow = a.getBoolean(R.styleable.CirclePagerIndicator_indicator_follow, true);
if (mMoveCircleRadius < mBgCircleRadius) mMoveCircleRadius = mBgCircleRadius;
a.recycle();
}
}
五、测量
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(measureWidth(widthMeasureSpec), measureHeight(heightMeasureSpec));
}
private int measureWidth(int measureSpec) {
int width;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
if ((specMode == MeasureSpec.EXACTLY) || (mViewPager == null)) {
width = specSize;
} else {
final int count = mViewPager.getAdapter().getCount();
width = (int) (getPaddingLeft() + getPaddingRight()
+ (count * 2 * mBgCircleRadius) + (mMoveCircleRadius - mBgCircleRadius) * 2 + (count - 1) * mIndicatorSpace);
if (specMode == MeasureSpec.AT_MOST) {
width = Math.min(width, specSize);
}
}
return width;
}
private int measureHeight(int measureSpec) {
int height;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
if (specMode == MeasureSpec.EXACTLY) {
height = specSize;
} else {
height = (int) (2 * mBgCircleRadius + getPaddingTop() + getPaddingBottom() + 1);
if (specMode == MeasureSpec.AT_MOST) {
height = Math.min(height, specSize);
}
}
return height;
}
六、获取自定义View尺寸
只有在onMeasure之后才能获得正确的控件宽高。所以在onSizeChanged中获取，同时当控件尺寸变化后该方法会再次执行，确保了尺寸获取的正确性。
@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
super.onSizeChanged(w, h, oldw, oldh);
if (w != oldw || h != oldh) {
//控件总宽度
mWidth = getWidth();
//控件左边距
mPaddingLeft = getPaddingLeft();
//控件右边距
mPaddingRight = getPaddingRight();
//控件顶边距
mPaddingTop = getPaddingTop();
}
}
七、绑定ViewPager
小圆点需要随着ViewPager的切换而移动，而我们都知道ViewPager中有个监听页面切换的api：addOnPageChangeListener(OnPageChangeListener listener)；。所以直接将ViewPager绑定给指示器，定义绑定方法：
@Override
public void bindViewPager(ViewPager viewPager, int initialPosition, int realSize) {
bindViewPager(viewPager, initialPosition);
this.mRealSize = realSize;
}
public void bindViewPager(ViewPager viewPager, int initialPosition) {
bindViewPager(viewPager);
setCurrentItem(initialPosition);
}
public void bindViewPager(ViewPager viewPager) {
if (mViewPager == viewPager) {
return;
}
if (viewPager.getAdapter() == null) {
throw new IllegalStateException("ViewPager does not set adapter");
}
mViewPager = viewPager;
mViewPager.addOnPageChangeListener(this);
mViewPager.getAdapter().registerDataSetObserver(mObserver);
invalidate();
}
八、处理Viewpager的页面切换监听
//*********************************OnPageChangeListener*************************************************
@Override
public void onPageScrollStateChanged(int state) {
}
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
if (mRealSize == 0) {
mCurrentPosition = position;
} else {
mCurrentPosition = position % mRealSize;
}
mPositionOffset = positionOffset;
//如果指示器跟随ViewPager缓慢滑动，那么滚动时一直绘制界面
if (mIsFollow) {
invalidate(); //实时绘制小圆点.
}
}
@Override
public void onPageSelected(int position) {
if (mRealSize == 0) {
mFollowPage = mCurrentPosition = position;
} else {
mFollowPage = mCurrentPosition = position % mRealSize; //轮播图无限, 记录的当前页.
}
invalidate();
}
//******************************************************************************************
九、重头戏，小圆点绘制
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
if (mViewPager == null) {
return;
}
//轮播图实际页面数（小圆点个数）
int count = mRealSize == 0 ? mViewPager.getAdapter().getCount() : mRealSize;
if (count == 0) {
return;
}
if (mCurrentPosition >= count) {
setCurrentItem(count - 1);
return;
}
//直径+间隔距离
final float circleAndSpace = 2 * mBgCircleRadius + mIndicatorSpace;
final float circleCenterY = mPaddingTop + mBgCircleRadius;
//第一个小圆点的圆心x坐标
float circleCenterXFirst = mPaddingLeft + mBgCircleRadius;
if (mCenterHorizontal) {
//(总长度 - 绘制圆点所占空间) / 2，居中
circleCenterXFirst += ((mWidth - mPaddingLeft - mPaddingRight) - (count * circleAndSpace - mIndicatorSpace)) / 2.0f;
}
float cX;
float cY;
float strokeRadius = mBgCircleRadius;
//如果绘制描边
if (mBgStrokePaint.getStrokeWidth() > 0) {
strokeRadius -= mBgStrokePaint.getStrokeWidth() * 1f / 2;
}
//绘制所有圆点
for (int i = 0; i < count; i++) {
cX = circleCenterXFirst + (i * circleAndSpace);//计算下个圆绘制起点偏移量
cY = circleCenterY;
//绘制背景小圆点
if (mBgCirclePaint.getAlpha() > 0) {
canvas.drawCircle(cX, cY, mBgCircleRadius, mBgCirclePaint);
}
//绘制背景小圆点描边
if (strokeRadius != mBgCircleRadius) {
canvas.drawCircle(cX, cY, strokeRadius, mBgStrokePaint);
}
}
//绘制移动的小圆点。
if (mIsFollow && mCurrentPosition == mRealSize - 1) {
if (mPositionOffset < 0.5) { //当前为最后一页, 偏移小于一半时
canvas.drawCircle(circleCenterXFirst + mCurrentPosition * circleAndSpace, circleCenterY, mMoveCircleRadius, mMoveCirclePaint);
return;
} else { //当前为最后一页, 偏移大于一半时
canvas.drawCircle(circleCenterXFirst, circleCenterY, mMoveCircleRadius, mMoveCirclePaint);
return;
}
}
float cx = (mIsFollow ? mCurrentPosition + mPositionOffset : mFollowPage) * circleAndSpace;
cX = circleCenterXFirst + cx;
cY = circleCenterY;
canvas.drawCircle(cX, cY, mMoveCircleRadius, mMoveCirclePaint);
}
十、对外提供一些方法
1、设置选中小圆点
2、刷新页面
@Override
public void setCurrentItem(int item) {
if (mViewPager == null) {
throw new IllegalStateException("indicator has not bind ViewPager");
}
if (mRealSize == 0) {
mCurrentPosition = item;
} else {
mCurrentPosition = item % mRealSize;
}
invalidate(); //调用onDraw.
}
@Override
public void notifyDataSetChanged() {
invalidate();
requestLayout();//当view的宽高不变，不会调用invalidate();
}
十一、完善，观察者设计模式
ViewPager的Adapter数据发生改变，调用Adapter的notifyDataSetChanged方法，ViewPager的数据改变，当然，与之关联的指示器控件也要刷新。怎么办？
通过查看源码，发现ViewPager的adapter中持有一个Observable对象，adapter在这里就相当于被观察者。
而ViewPager相当于观察者，它里面定义了Observer内部类，并将这个内部类对象注册给Adapter中的Observable。
很明显，非常典型的观察者模式，都是套路。（我认为这样做的好处就是解耦）
被观察者的变化，从而引起观察者的变化。不难看出，我们这里的自定义view相当于观察者，所以仿照ViewPager源码，定义一个Observer内部类如下：
//============================ 观察者设计模式 ======================================
private class IndicatorObserver extends DataSetObserver {
@Override
public void onChanged() {
notifyDataSetChanged();
}
}
那么该Observer内部类什么时候注册给adapter呢？往回看第七步的绑定Viewpager中的这句代码：
mViewPager.getAdapter().registerDataSetObserver(mObserver);
mObserver是自定义view的成员变量：
private final DataSetObserver mObserver = new IndicatorObserver
这样，自定义ViewPager指示器就完成了。
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