Android 下拉刷新(使用异步任务和ViewTreeObserver监听)

隐藏和显示异步任务,主要是通过控制内边距Pandding来实现

package com.siyehuazhilian.pushdownrefresh;

import android.app.Activity;
import android.graphics.Bitmap;
import android.os.Bundle;
import android.os.Handler;
import android.view.MotionEvent;
import android.view.ViewTreeObserver;
import android.view.ViewTreeObserver.OnPreDrawListener;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.RelativeLayout;
import android.widget.Toast;

import com.siyehuazhilian.utils.DownImageTask;

public class MainActivity extends Activity {
	private RelativeLayout relativeLayout;
	private LinearLayout linearLayout;
	private ImageView imageView;

	private int refreshHeight;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		relativeLayout = (RelativeLayout) findViewById(R.id.relativelayout);
		linearLayout = (LinearLayout) findViewById(R.id.linearlayout_refresh);
		imageView = (ImageView) findViewById(R.id.imageview);

		ViewTreeObserver observer = linearLayout.getViewTreeObserver();
		observer.addOnPreDrawListener(new OnPreDrawListener() {

			@Override
			public boolean onPreDraw() {
				if (refreshHeight == 0) {
					refreshHeight = linearLayout.getMeasuredHeight();
					relativeLayout.setPadding(0, -refreshHeight, 0, 0);
				}
				return true;
			}
		});
	}

	private int startY;
	private int distance;

	@Override
	public boolean onTouchEvent(MotionEvent event) {
		switch (event.getAction()) {
		case MotionEvent.ACTION_DOWN:
			startY = (int) event.getRawY();
			break;
		case MotionEvent.ACTION_MOVE:
			distance = (int) ((event.getRawY() - startY) / 1.5);
			if (distance < linearLayout.getHeight() && distance > 0) {
				// distance = refreshHeight;
				relativeLayout.setPadding(0, linearLayout.getHeight()
						- distance, 0, 0);
			}
			break;
		case MotionEvent.ACTION_UP:
			// 手指离开,判断是满足刷新的要求
			// 要求distance要大于等于linearLayout.getHeight()的一半,
			// 如果大于,就让刷新界面完全展示出来并开启下载任务,如果小于,就重新隐藏刷新界面
			if (distance > linearLayout.getHeight() / 2) {
				relativeLayout.setPadding(0, 0, 0, 0);
				DownImageTask downImageTask = new DownImageTask(handler);
				downImageTask
						.execute("http://a.hiphotos.baidu.com/image/w%3D2048/sign=487f5fb067380cd7e61ea5ed957cad34/a6efce1b9d16fdfa575066eeb68f8c5494ee7bf6.jpg");
			} else {
				relativeLayout.setPadding(0, -linearLayout.getHeight(), 0, 0);
			}

			break;

		default:
			break;
		}
		return super.onTouchEvent(event);
	}

	private Handler handler = new Handler() {
		public void handleMessage(android.os.Message msg) {
			switch (msg.what) {
			case 1:
				Bitmap bitmap = (Bitmap) msg.obj;
				imageView.setImageBitmap(bitmap);
				Toast.makeText(MainActivity.this, "更新完毕", 1000).show();
				// 下载完成还是要隐藏刷新的界面的
				relativeLayout.setPadding(0, -linearLayout.getHeight(), 0, 0);
				break;
			default:
				break;
			}
		};
	};

}

异步任务

package com.siyehuazhilian.utils;

import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Handler;
import android.os.Message;

public class DownImageTask extends AsyncTask<String, Integer, Bitmap> {
	private Handler handler;

	public DownImageTask(Handler handler) {
		this.handler = handler;
	}

	@Override
	protected Bitmap doInBackground(String... parems) {
		Bitmap bitmap = null;
		try {
			URL url = new URL(parems[0]);
			HttpURLConnection connection = (HttpURLConnection) url
					.openConnection();
			InputStream inputStream = connection.getInputStream();
			bitmap = BitmapFactory.decodeStream(inputStream);
		} catch (Exception e) {
			e.printStackTrace();
		}

		return bitmap;
	}

	@Override
	protected void onPostExecute(Bitmap result) {
		super.onPostExecute(result);
		Message message = handler.obtainMessage(1, result);
		message.sendToTarget();
	}

}