计数器

/*计数器*/
mEtExplain.addTextChangedListener(new TextWatcher() {
    public int myselectionStart;
    public int myselectionEnd;

    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {
    }

    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {
    }

    @Override
    public void afterTextChanged(Editable s) {
        //文本框文字长度
        int mynumber = s.length();
        myselectionStart = mEtExplain.getSelectionStart();
        myselectionEnd = mEtExplain.getSelectionEnd();
        //删除多余输入的字（不会显示出来）
        if (mynumber > 100) {
            s.delete(myselectionStart - 1, myselectionEnd);
            mEtExplain.setText(s.toString().trim());
        }
        if (mynumber < 100 + 1 && mynumber > -1) {
            mTvExplainNum.setText(100 - mynumber + "");
        }
        //设置光标在最后
        mEtExplain.setSelection(s.length());
    }
});
/////////////////////////////布局

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginTop="4dp"
    android:orientation="vertical">

    <EditText
        android:id="@+id/et_explain"
        android:layout_width="match_parent"
        android:layout_height="100dp"
        android:layout_marginEnd="16dp"
        android:layout_marginStart="16dp"
        android:background="@null"
        android:gravity="start"
        android:hint="说明"
        android:maxLength="100"
        android:textColor="@color/c121"
        android:textSize="@dimen/fs3" />

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="46dp"
        android:layout_marginEnd="16dp"
        android:layout_marginStart="16dp">

        <TextView
            android:id="@+id/tv_explain_num"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentBottom="true"
            android:layout_alignParentEnd="true"
            android:layout_marginBottom="12dp"
            android:layout_marginEnd="12dp"
            android:text="100"
            android:textColor="@color/c125"
            android:textSize="@dimen/fs3" />
    </RelativeLayout>
</LinearLayout>