怎样设置EditText内部文字被锁定不可删除和修改，而文字只能在后面输入

package com.pandastudios.pandastudios;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.widget.EditText;
/**
 * Created by loonggg on 15/8/29.
 */
public class MyEditText extends EditText{
    private String str;
    public MyEditText(Context context){
        super(context);
    }
    public MyEditText(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    public MyEditText(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }
    public MyEditText(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }
    public void setStr(String str){
        this.str = str;
    }
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Paint paint =  new Paint();
        paint.setTextSize(30);
        paint.setColor(Color.BLACK);
        paint.setAntiAlias(true);
        //编写提示文字。
        canvas.drawText(str,2,getHeight()/2+5,paint);
        super.onDraw(canvas);
    }
}

<com.pandastudios.pandastudios.MyEditText
       android:id="@+id/myEditText"
       android:layout_width="wrap_content"
       android:paddingLeft="20dp"
       android:layout_height="wrap_content" />