类似拨号盘的EditText，随文字数量的增加，动态改变文字的大小，比较简单。

public class AutoSizeEditText extends EditText {
    private static float DEFAULT_MIN_TEXT_SIZE = 2; // 最小的字体大小
    private float minTextSize, maxTextSize;
    public AutoSizeEditText(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    private void init() {
        maxTextSize = this.getTextSize();// 获取单个字体的像素
        minTextSize = DEFAULT_MIN_TEXT_SIZE;
    }

    public void resizeText(TextView textView, float originalTextSize, float minTextSize) {
        final Paint paint = textView.getPaint();
        final int width = textView.getWidth();
        if (width == 0) {
            return;
        }
        textView.setTextSize(TypedValue.COMPLEX_UNIT_PX, originalTextSize);
        float ratio = width / paint.measureText(textView.getText().toString());
        if (ratio <= 1.0f) {
            textView.setTextSize(
                    TypedValue.COMPLEX_UNIT_PX, Math.max(minTextSize, originalTextSize * ratio));
        }
    }

    @Override
    protected void onTextChanged(CharSequence text, int start, int before,
                                 int after) {
        super.onTextChanged(text, start, before, after);
        resizeText(this, maxTextSize, minTextSize);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        resizeText(this, maxTextSize, minTextSize);
    }

}