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; 
  } 
 
}