利用HorizontalScrollView自己写一个viewPager指示器

目前滑动指示器最著名的是JakeWarton的ViewpagerIndicator，用别人的东西固然方便，但是也带来很多使用上的疑惑，这篇博客，我们使用HorizontalScrollView自己写一个viewPager指示器。

这里首先说一下很多自己写的indicator只限于可视范围内不能移动的指示器，所以tab的数量有限，一般最多五个就已经很拥挤了，可是我们发现开源的ViewpagerIndicator有一个很棒的效果就是不用限定tab的个数，并且当前选中的tab将处于中间位置（两边不可滑动范围除外），这一点我便想到了利用HorizontalScrollView来实现这个效果。而tab的显示我使用TextView就行动态加载，然后把tab放到HorizontalScrollView中，在这里注意一个问题，那就是HorizontalScrollView本身的子view个数是有限定的，只能是一个，这一点跟scrollview一样，源码是这样的：

@Override
public void addView(View child) {
if (getChildCount() > 0) {
throw new IllegalStateException("HorizontalScrollView can host only one direct child");
}

super.addView(child);
}

我们可以看到如果getChildCount()的个数大于零，就会抛出异常，所以这里我使用一个LinearLayout先包裹所有的textview然后再把LinearLayout放入HorizontalScrollView中，这样就不会抛异常了。代码如下：

LinearLayout linearLayout = new LinearLayout(context);
linearLayout.setOrientation(LinearLayout.HORIZONTAL);
linearLayout.setLayoutParams(new LinearLayout.LayoutParams(count*tabWidth, LinearLayout.LayoutParams.MATCH_PARENT));
for (int i = 0; i < count; i++)
{
TextView tv = new TextView(getContext());
LinearLayout.LayoutParams lp = new LinearLayout.LayoutParams(tabWidth,
LinearLayout.LayoutParams.MATCH_PARENT);
tv.setGravity(Gravity.CENTER);
tv.setTextColor(COLOR_TEXT_NORMAL);
tv.setText(titles[i]);
tv.setTextSize(TypedValue.COMPLEX_UNIT_SP, 16);//字体大小
tv.setLayoutParams(lp);
final int finalI = i;
tv.setOnClickListener(new OnClickListener()
{
@Override
public void onClick(View v)
{
if(viewPager!=null){
viewPager.setCurrentItem(finalI);
}
}
});
linearLayout.addView(tv);
}
addView(linearLayout);

而HorizontalScrollView中那个下划线效果，通过dispatchDraw方法实现

@Override
protected void dispatchDraw(Canvas canvas)
{
super.dispatchDraw(canvas);
canvas.save();
canvas.translate(mTranslationX, getHeight() - lineheight);
canvas.drawLine(0, 0, tabWidth, 0, mPaint);//（startX, startY, stopX, stopY, paint）
canvas.restore();
}

其滑动效果则通过监听viewpager滑动的OnPageChangeListener接口中的onPageScrolled函数实现，我们知道onPageScrolled有三个参数起源吗如下：

/**
* This method will be invoked when the current page is scrolled, either as part
* of a programmatically initiated smooth scroll or a user initiated touch scroll.
*
* @param position Position index of the first page currently being displayed.
*                 Page position+1 will be visible if positionOffset is nonzero.
* @param positionOffset Value from [0, 1) indicating the offset from the page at position.
* @param positionOffsetPixels Value in pixels indicating the offset from position.
*/
void onPageScrolled(int position, float positionOffset, int positionOffsetPixels);

通过源码可知，第一个参数是当前的位置，第二个参数比较有意思，是指滑动距离相对于整个viewpager宽度的百分比，第三个是滑动的真正距离。这里我们利用第二个参数实现跟随滑动效果。这里我封装了一个函数如下：

public void scroll(int position, float offset)
{
mTranslationX = tabWidth * (position + offset);
scrollTo((int)mTranslationX-(SCREEN_WIDTH-tabWidth)/2, 0);
invalidate();
}

解释一下，我们将position和offse（百分比）传进去首先计算下划线应该滑动到的初始位置，然后利用scrollTo函数将HorizontalScrollView进行移动，最后重绘，于是就到达了滑动跟随的效果。全部源码如下：

package com.zp.scrolltest;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.support.v4.view.ViewPager;
import android.util.AttributeSet;
import android.util.TypedValue;
import android.view.Gravity;
import android.view.View;
import android.widget.HorizontalScrollView;
import android.widget.LinearLayout;
import android.widget.TextView;

/**
* Created by ez on 2017/5/4.
*/

public class MyIndicator extends HorizontalScrollView implements ViewPager.OnPageChangeListener{

private static final int COLOR_TEXT_NORMAL = 0xFF000000;
private static final int COLOR_INDICATOR_COLOR = Color.BLACK;

private Context context;
private  int tabWidth;
private String[] titles;
private int count;
private Paint mPaint;
private float mTranslationX;
private ViewPager viewPager;
private int SCREEN_WIDTH;
private float lineheight = 2.0f;

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

public MyIndicator(Context context, AttributeSet attrs) {
super(context, attrs);
init(context);
}

public MyIndicator(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init(context);
}

private void init(Context context){
this.context = context;
mPaint = new Paint();
mPaint.setColor(COLOR_INDICATOR_COLOR);
mPaint.setStrokeWidth(lineheight);//底部指示线的宽度
setHorizontalScrollBarEnabled(false);
SCREEN_WIDTH = context.getResources().getDisplayMetrics().widthPixels;
}

public void setLineheight(float height){
this.lineheight = height;
mPaint.setStrokeWidth(lineheight);//底部指示线的宽度
}

public void setViewPager(ViewPager viewPager){
this.viewPager = viewPager;
viewPager.addOnPageChangeListener(this);
}

public void setTitles(String[] titles){
this.titles = titles;
count = titles.length;
tabWidth = SCREEN_WIDTH/4;
generateTitleView();
}

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
super.onSizeChanged(w, h, oldw, oldh);
tabWidth = SCREEN_WIDTH/4;
}

@Override
protected void dispatchDraw(Canvas canvas)
{
super.dispatchDraw(canvas);
canvas.save();
canvas.translate(mTranslationX, getHeight() - lineheight);
canvas.drawLine(0, 0, tabWidth, 0, mPaint);//（startX, startY, stopX, stopY, paint）
canvas.restore();
}

public void scroll(int position, float offset)
{
mTranslationX = tabWidth * (position + offset);
scrollTo((int)mTranslationX-(SCREEN_WIDTH-tabWidth)/2, 0);
invalidate();
}

private void generateTitleView()
{
if (getChildCount() > 0)
this.removeAllViews();
count = titles.length;

LinearLayout linearLayout = new LinearLayout(context);
linearLayout.setOrientation(LinearLayout.HORIZONTAL);
linearLayout.setLayoutParams(new LinearLayout.LayoutParams(count*tabWidth, LinearLayout.LayoutParams.MATCH_PARENT));
for (int i = 0; i < count; i++)
{
TextView tv = new TextView(getContext());
LinearLayout.LayoutParams lp = new LinearLayout.LayoutParams(tabWidth,
LinearLayout.LayoutParams.MATCH_PARENT);
tv.setGravity(Gravity.CENTER);
tv.setTextColor(COLOR_TEXT_NORMAL);
tv.setText(titles[i]);
tv.setTextSize(TypedValue.COMPLEX_UNIT_SP, 16);//字体大小
tv.setLayoutParams(lp);
final int finalI = i;
tv.setOnClickListener(new OnClickListener()
{
@Override
public void onClick(View v)
{
if(viewPager!=null){
viewPager.setCurrentItem(finalI);
}
}
});
linearLayout.addView(tv);
}
addView(linearLayout);
}

@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
scroll(position, positionOffset);
}

@Override
public void onPageSelected(int position) {

}

@Override
public void onPageScrollStateChanged(int state) {

}
}

最后说一下使用，首先在XML中像普通组件一样使用：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_main"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.zp.scrolltest.MainActivity">

<com.zp.scrolltest.MyIndicator
android:layout_width="wrap_content"
android:layout_height="40dp"
android:background="@android:color/holo_blue_bright"
android:id="@+id/indicador"/>

<android.support.v4.view.ViewPager
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/pager"/>

</LinearLayout>

接下来在Java代码中：

indicador = (MyIndicator) findViewById(R.id.indicador);
titles = new String[]{"a", "b", "c", "d", "e", "f"};
indicador.setTitles(titles);

viewPager = (ViewPager) findViewById(R.id.pager);

viewPager.setAdapter(mAdapter);
indicador.setViewPager(viewPager);

以上就是全部实现。