/**
     * 獲取textview最大能顯示幾個字
     * @param text 文本內容
     * @param size 文本字體大小
     * @param maxWidth textview的最大寬度
     * @return
     */
    private float getLineMaxNumber(String text, float size,float maxWidth) {
        if (null == text || "".equals(text)){
            return 0;
        }
        Paint paint = new Paint();
        paint.setTextSize(size);
        //得到文本內容總體長度
        float textWidth = paint.measureText(text);
        // textWidth
        float width = textWidth / text.length();
        float total = maxWidth / width;
        return total;
    }

/**
     * 獲取textview一行最大能顯示幾個字(需要在TextView測量完成之后)
     *
     * @param text     文本內容
     * @param paint    textview.getPaint()
     * @param maxWidth textview.getMaxWidth()/或者是指定的數(shù)值,如200dp
     */
    private int getLineMaxNumber(String text, TextPaint paint, int maxWidth) {
        if (null == text || "".equals(text)) {
            return 0;
        }
        StaticLayout staticLayout = new StaticLayout(text, paint, maxWidth, Layout.Alignment.ALIGN_NORMAL
                , 1.0f, 0, false);
        //獲取第一行最后顯示的字符下標
        return staticLayout.getLineEnd(0);
    }

  <TextView
        android:id="@+id/textView"
        android:layout_width="300dp"
        android:layout_height="wrap_content"
        android:ellipsize="end"
        android:singleLine="true" />

String str = "If you really want to hear about it, the first thing you'll probably want to know";
這樣就會導致顯示不全，像這樣：
If you really want to hear about it, the first thin...

所以，如果你想得到已顯示的字符個數(shù)，或者未顯示的字符個數(shù)，那么其中的關鍵是如何計算每一個字符的寬度。
然后遍歷這個字符串，當前n個字符寬度總和，超過TextView寬度時，就得到了已顯示的字符個數(shù)。

String str = "If you really want to hear about it, the first thing you'll probably want to know";
mTextView = (TextView) findViewById(R.id.textView);

// 計算TextView寬度：xml中定義的寬度300dp，轉換成px
float textViewWidth = convertDpToPixel(300);
float dotWidth = getCharWidth(mTextView, '.');
Log.d(TAG, "TextView width " + textViewWidth);

int sumWidth = 0;
for (int index=0; index<str.length(); index++) {
    // 計算每一個字符的寬度
    char c = str.charAt(index);
    float charWidth = getCharWidth(mTextView, c);
    sumWidth += charWidth;
    Log.d(TAG, "#" + index + ": " + c + ", width=" + charWidth + ", sum=" + sumWidth);
    
    if (sumWidth + dotWidth*3 >= textViewWidth) {
        Log.d(TAG, "TextView shows #" + index + " char: " + str.substring(0, index));
        break;
    }
}

// Dp轉Px
private float convertDpToPixel(float dp){
    Resources resources = getResources();
    DisplayMetrics metrics = resources.getDisplayMetrics();
    float px = dp * (metrics.densityDpi / 160f);
    return px;
}

// 計算每一個字符的寬度
public float getCharWidth(TextView textView, char c) {
    textView.setText(String.valueOf(c));
    textView.measure(0, 0);
    return textView.getMeasuredWidth();
}