需求：

需要動態更改TextView內容字型的大小，比如設定TextView只有一行，寬度只有200dp，內容超過這個之後就縮小字型顯示，只能能將字型都顯示完全；也就是動態更改TextView的字型大小，當TextView的內容比較多時縮小顯示，當TextView的內容比較少時正常顯示。

使用方式詳見官網介紹。

例子展示:

可以看出來：當文字沒有填充TextView完全時顯示的就是預設的字型，當文字能夠完全填充TextView並且一行顯示不下時，他會預設的縮小文字的字型，當文字再多時，他會預設在末尾省略。

AutofitTextView.java程式碼：

package
 
 me.grantland.widget;

import android.content.Context;
import android.util.AttributeSet;
import android.util.TypedValue;
import android.widget.TextView;

/**
 * A {@link TextView} that re-sizes its text to be no larger than the width of the view.
 *
 * @attr ref R.styleable.AutofitTextView_sizeToFit
 * @attr
 
 ref R.styleable.AutofitTextView_minTextSize
 * @attr ref R.styleable.AutofitTextView_precision
 */
public class AutofitTextView extends TextView implements AutofitHelper.OnTextSizeChangeListener {

    private AutofitHelper mHelper;

    public AutofitTextView(Context context) {
        super(context);
        init(context, null
 
, 0);
    }

    public AutofitTextView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init(context, attrs, 0);
    }

    public AutofitTextView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init(context, attrs, defStyle);
    }

    private void init(Context context, AttributeSet attrs, int defStyle) {
        mHelper = AutofitHelper.create(this, attrs, defStyle)
                .addOnTextSizeChangeListener(this);
    }

    // Getters and Setters

    /**
     * {@inheritDoc}
     */
    @Override
    public void setTextSize(int unit, float size) {
        super.setTextSize(unit, size);
        if (mHelper != null) {
            mHelper.setTextSize(unit, size);
        }
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void setLines(int lines) {
        super.setLines(lines);
        if (mHelper != null) {
            mHelper.setMaxLines(lines);
        }
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void setMaxLines(int maxLines) {
        super.setMaxLines(maxLines);
        if (mHelper != null) {
            mHelper.setMaxLines(maxLines);
        }
    }

    /**
     * Returns the {@link AutofitHelper} for this View.
     */
    public AutofitHelper getAutofitHelper() {
        return mHelper;
    }

    /**
     * Returns whether or not the text will be automatically re-sized to fit its constraints.
     */
    public boolean isSizeToFit() {
        return mHelper.isEnabled();
    }

    /**
     * Sets the property of this field (sizeToFit), to automatically resize the text to fit its
     * constraints.
     */
    public void setSizeToFit() {
        setSizeToFit(true);
    }

    /**
     * If true, the text will automatically be re-sized to fit its constraints; if false, it will
     * act like a normal TextView.
     *
     * @param sizeToFit
     */
    public void setSizeToFit(boolean sizeToFit) {
        mHelper.setEnabled(sizeToFit);
    }

    /**
     * Returns the maximum size (in pixels) of the text in this View.
     */
    public float getMaxTextSize() {
        return mHelper.getMaxTextSize();
    }

    /**
     * Set the maximum text size to the given value, interpreted as "scaled pixel" units. This size
     * is adjusted based on the current density and user font size preference.
     *
     * @param size The scaled pixel size.
     *
     * @attr ref android.R.styleable#TextView_textSize
     */
    public void setMaxTextSize(float size) {
        mHelper.setMaxTextSize(size);
    }

    /**
     * Set the maximum text size to a given unit and value. See TypedValue for the possible
     * dimension units.
     *
     * @param unit The desired dimension unit.
     * @param size The desired size in the given units.
     *
     * @attr ref android.R.styleable#TextView_textSize
     */
    public void setMaxTextSize(int unit, float size) {
        mHelper.setMaxTextSize(unit, size);
    }

    /**
     * Returns the minimum size (in pixels) of the text in this View.
     */
    public float getMinTextSize() {
        return mHelper.getMinTextSize();
    }

    /**
     * Set the minimum text size to the given value, interpreted as "scaled pixel" units. This size
     * is adjusted based on the current density and user font size preference.
     *
     * @param minSize The scaled pixel size.
     *
     * @attr ref me.grantland.R.styleable#AutofitTextView_minTextSize
     */
    public void setMinTextSize(int minSize) {
        mHelper.setMinTextSize(TypedValue.COMPLEX_UNIT_SP, minSize);
    }

    /**
     * Set the minimum text size to a given unit and value. See TypedValue for the possible
     * dimension units.
     *
     * @param unit The desired dimension unit.
     * @param minSize The desired size in the given units.
     *
     * @attr ref me.grantland.R.styleable#AutofitTextView_minTextSize
     */
    public void setMinTextSize(int unit, float minSize) {
        mHelper.setMinTextSize(unit, minSize);
    }

    /**
     * Returns the amount of precision used to calculate the correct text size to fit within its
     * bounds.
     */
    public float getPrecision() {
        return mHelper.getPrecision();
    }

    /**
     * Set the amount of precision used to calculate the correct text size to fit within its
     * bounds. Lower precision is more precise and takes more time.
     *
     * @param precision The amount of precision.
     */
    public void setPrecision(float precision) {
        mHelper.setPrecision(precision);
    }

    @Override
    public void onTextSizeChange(float textSize, float oldTextSize) {
        // do nothing
    }
}

原理：

AutofitTextView：自定義TextView並繼承系統的TextView，然後在繪製元件的時候根據getMaxLines方法獲取內容的行數若內容的行數大於1，則縮小文字的字型，然後在嘗試獲取getMaxLines方法，若內容的行數還是大於1，則縮小文字的字型，直到內容能夠一行顯示或者是字型縮小大一定的大小，這時候若縮小到一定的大小還是不能一行顯示，則尾部省略。

延伸：
TextView原始碼中：

public void setTextSize(float size) {
        setTextSize(TypedValue.COMPLEX_UNIT_SP, size);
    }
public void setTextSize(int unit, float size) {
        Context c = getContext();
        Resources r;

        if (c == null)
            r = Resources.getSystem();
        else
            r = c.getResources();

        setRawTextSize(TypedValue.applyDimension(
                unit, size, r.getDisplayMetrics()));
    }

setTextSize(int unit, int size)
第一個引數可設定如下靜態變數：
TypedValue.COMPLEX_UNIT_PX : Pixels
TypedValue.COMPLEX_UNIT_SP : Scaled Pixels
TypedValue.COMPLEX_UNIT_DIP : Device Independent Pixels

我們在使用TextView控制元件時，呼叫setTextSize方法，預設就是以sp為單位，與xml配置檔案預設儲存一致。