public class MainActivity extends Activity {
 private String str = "";
 
 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
 
  for (int i = 0; i < 200; i++) {
   str = str + i + " ";
  }
  final ExpandableTextView etv = (ExpandableTextView) findViewById(R.id.etv);
  etv.setText(str);
 
  Button btn = (Button) findViewById(R.id.btn);
 
  btn.setOnClickListener(new OnClickListener() {
 
   @Override
   public void onClick(View v) {
    boolean b = etv.getExpandablestatus();
 
    b = !b;
    etv.setExpandable(b);
 
   }
  });
 
 }
 
}

package com.ganchuanpu.ExpandableTextView;
 
import android.content.Context;
import android.util.AttributeSet;
import android.view.ViewTreeObserver;
import android.view.ViewTreeObserver.OnPreDrawListener;
import android.widget.TextView;
import android.widget.Toast;
 
public class ExpandableTextView extends TextView {
 
 // 最大行，默认显示3行
 private final int MAX = 3;
 // 完全展开需要的行数
 private int lines;
 
 private ExpandableTextView mExpandableTextView;
 
 private boolean expandablestatus = false;
 
 // 构造方法用两个参数的
 public ExpandableTextView(Context context, AttributeSet attrs) {
  super(context, attrs);
  mExpandableTextView = this;
  init();
 
 }
 
 private void init() {
  // 在view绘制之前的时候执行，在onDraw之前
  ViewTreeObserver mViewTreeObserver = this.getViewTreeObserver();
  mViewTreeObserver.addOnPreDrawListener(new OnPreDrawListener() {
 
   @Override
   public boolean onPreDraw() {
    // 避免重复监听
    mExpandableTextView.getViewTreeObserver().removeOnPreDrawListener(this);
    // 获得内容行数
    lines = getLineCount();
 
    return true;
   }
  });
  setExpandable(false);
 
 }
 // 是否展开或者收缩，
 // true，展开；
 // false，不展开
 
 public void setExpandable(boolean isExpand) {
  if (isExpand) {
   setMaxLines(lines + 1);
  } else
   setMaxLines(MAX);
 
  expandablestatus = isExpand;
 }
 
 public boolean getExpandablestatus() {
  return expandablestatus;
 }
 
}