ScrollTextView - scrolling TextView for Android

找了很就终于在国外一网站上找到一个Scroller的例子，为了日后方便查阅，在博客上记录以下。
Here are the source codes.

The key points are:
1，setHorizontallyScrolling() and use android.widget.Scroller to make the text scroll;
2，Use the attached TextPaint of the TextView to measure the length of the text in pixel;
3，setSingleLine() and setEllipsize(null) to make the text not wrapped and ellipsized;
4，Override computeScroll() to restart the scrolling when finished
Here are the source codes.

package com.dirtybear.android;

import android.content.Context;
import android.graphics.Rect;
import android.text.TextPaint;
import android.util.AttributeSet;
import android.view.animation.LinearInterpolator;
import android.widget.Scroller;
import android.widget.TextView;

public class ScrollTextView extends TextView {

// scrolling feature
private Scroller mSlr;

// milliseconds for a round of scrolling
private int mRndDuration = 250;

// the X offset when paused
private int mXPaused = 0;

// whether it's being paused
private boolean mPaused = true;

/*
* constructor
*/
public ScrollTextView(Context context) {
this(context, null);
}

/*
* constructor
*/
public ScrollTextView(Context context, AttributeSet attrs) {
this(context, attrs, android.R.attr.textViewStyle);
}

/*
* constructor
*/
public ScrollTextView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
// customize the TextView
setSingleLine();
setEllipsize(null);
setVisibility(INVISIBLE);
}

/**
* begin to scroll the text from the original position
*/
public void startScroll() {
// begin from the very right side
mXPaused = -1 * getWidth();
// assume it's paused
mPaused = true;
resumeScroll();
}

/**
* resume the scroll from the pausing point
*/
public void resumeScroll() {

if (!mPaused)
return;

// Do not know why it would not scroll sometimes
// if setHorizontallyScrolling is called in constructor.
setHorizontallyScrolling(true);

// use LinearInterpolator for steady scrolling
mSlr = new Scroller(this.getContext(), new LinearInterpolator());
setScroller(mSlr);

int scrollingLen = calculateScrollingLen();
int distance = scrollingLen - (getWidth() + mXPaused);
int duration = (new Double(mRndDuration * distance * 1.00000
/ scrollingLen)).intValue();

setVisibility(VISIBLE);
mSlr.startScroll(mXPaused, 0, distance, 0, duration);
mPaused = false;
}

/**
* calculate the scrolling length of the text in pixel
*
* @return the scrolling length in pixels
*/
private int calculateScrollingLen() {
TextPaint tp = getPaint();
Rect rect = new Rect();
String strTxt = getText().toString();
tp.getTextBounds(strTxt, 0, strTxt.length(), rect);
int scrollingLen = rect.width() + getWidth();
rect = null;
return scrollingLen;
}

/**
* pause scrolling the text
*/
public void pauseScroll() {
if (null == mSlr)
return;

if (mPaused)
return;

mPaused = true;

// abortAnimation sets the current X to be the final X,
// and sets isFinished to be true
// so current position shall be saved
mXPaused = mSlr.getCurrX();

mSlr.abortAnimation();
}

@Override
/*
* override the computeScroll to restart scrolling when finished so as that
* the text is scrolled forever
*/
public void computeScroll() {
super.computeScroll();

if (null == mSlr) return;

if (mSlr.isFinished() && (!mPaused)) {
this.startScroll();
}
}

public int getRndDuration() {
return mRndDuration;
}

public void setRndDuration(int duration) {
this.mRndDuration = duration;
}

public boolean isPaused() {
return mPaused;
}
}

Issues to be fixed:
1，Do not know why it would not scroll sometimes if setHorizontallyScrolling is called in constructor;
2，The scrolling is not smooth enough. Maybe it's because of the single thread model of Android UI;

Tested on:

Windows XP

Android SDK 1.0 rc2

Android Emulator 1.0

Android Debug Bridge version 1.0.20