Muito obrigado a https://stackoverflow.com/users/234270/speedplane . Ótima resposta!
Aqui está uma versão aprimorada de sua resposta que também cuida da altura e vem com um atributo maxFontSize para limitar o tamanho da fonte (foi útil no meu caso, então eu queria compartilhá-la):
package com.<your_package>;
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Paint;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.util.TypedValue;
import android.widget.TextView;
public class FontFitTextView extends TextView
{
private Paint mTestPaint;
private float maxFontSize;
private static final float MAX_FONT_SIZE_DEFAULT_VALUE = 20f;
public FontFitTextView(Context context)
{
super(context);
initialise(context, null);
}
public FontFitTextView(Context context, AttributeSet attributeSet)
{
super(context, attributeSet);
initialise(context, attributeSet);
}
public FontFitTextView(Context context, AttributeSet attributeSet, int defStyle)
{
super(context, attributeSet, defStyle);
initialise(context, attributeSet);
}
private void initialise(Context context, AttributeSet attributeSet)
{
if(attributeSet!=null)
{
TypedArray styledAttributes = context.obtainStyledAttributes(attributeSet, R.styleable.FontFitTextView);
maxFontSize = styledAttributes.getDimension(R.styleable.FontFitTextView_maxFontSize, MAX_FONT_SIZE_DEFAULT_VALUE);
styledAttributes.recycle();
}
else
{
maxFontSize = MAX_FONT_SIZE_DEFAULT_VALUE;
}
mTestPaint = new Paint();
mTestPaint.set(this.getPaint());
//max size defaults to the initially specified text size unless it is too small
}
/* Re size the font so the specified text fits in the text box
* assuming the text box is the specified width.
*/
private void refitText(String text, int textWidth, int textHeight)
{
if (textWidth <= 0)
return;
int targetWidth = textWidth - this.getPaddingLeft() - this.getPaddingRight();
int targetHeight = textHeight - this.getPaddingTop() - this.getPaddingBottom();
float hi = maxFontSize;
float lo = 2;
// final float threshold = 0.5f; // How close we have to be
final float threshold = 1f; // How close we have to be
mTestPaint.set(this.getPaint());
Rect bounds = new Rect();
while ((hi - lo) > threshold)
{
float size = (hi + lo) / 2;
mTestPaint.setTextSize(size);
mTestPaint.getTextBounds(text, 0, text.length(), bounds);
if (bounds.width() >= targetWidth || bounds.height() >= targetHeight)
hi = size; // too big
else
lo = size; // too small
// if (mTestPaint.measureText(text) >= targetWidth)
// hi = size; // too big
// else
// lo = size; // too small
}
// Use lo so that we undershoot rather than overshoot
this.setTextSize(TypedValue.COMPLEX_UNIT_PX, lo);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
{
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int parentWidth = MeasureSpec.getSize(widthMeasureSpec);
int height = getMeasuredHeight();
refitText(this.getText().toString(), parentWidth, height);
this.setMeasuredDimension(parentWidth, height);
}
@Override
protected void onTextChanged(final CharSequence text, final int start, final int before, final int after)
{
refitText(text.toString(), this.getWidth(), this.getHeight());
}
@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh)
{
if (w != oldw)
{
refitText(this.getText().toString(), w, h);
}
}
}
Arquivo /res/values/attr.xml correspondente:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<declare-styleable name="FontFitTextView">
<attr name="maxFontSize" format="dimension" />
</declare-styleable>
</resources>
Exemplo:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:res-auto="http://schemas.android.com/apk/res-auto"
android:id="@+id/home_Layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@drawable/background"
tools:ignore="ContentDescription" >
...
<com.<your_package>.FontFitTextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:singleLine="true"
android:text="Sample Text"
android:textSize="28sp"
res-auto:maxFontSize="35sp"/>
...
</RelativeLayout>
Para usar o novo maxFontSizeatributo, não se esqueça de adicionar xmlns:res-auto="http://schemas.android.com/apk/res-auto"como mostra o exemplo.