消息機制 →網絡圖片查看器

1. 子線程利用handler發送一條消息，消息被放在主線程的消息隊列裏面

2. 主線程裏面有一個looper消息的輪詢器

3. 如果輪詢器發現了新的消息，調用handlemessage的方法處理消息

MainActivity

package org.gentry.readpicture;

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

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.text.TextUtils;
import android.view.View;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.Toast;

public class MainActivity extends Activity {

	protected static final int CHANGE_UI = 1;
	protected static final int ERROR = 2;
	private EditText et_path;
	private ImageView iv;

	// 1. 主線程創建消息處理器（handler）
	private Handler handler = new Handler() {
		public void handleMessage(android.os.Message msg) {
			if (msg.what == CHANGE_UI) {
				Bitmap bitmap = (Bitmap) msg.obj;
				iv.setImageBitmap(bitmap);
			} else if (msg.what == ERROR) {
				Toast.makeText(MainActivity.this, "顯示圖片失敗", Toast.LENGTH_LONG)
						.show();
			}
		};
	};

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);

		et_path = (EditText) findViewById(R.id.et_path);
		iv = (ImageView) findViewById(R.id.iv);

	}

	public void click(View view) {
		final String path = et_path.getText().toString().trim();
		if (TextUtils.isEmpty(path)) {
			Toast.makeText(this, "圖片路徑不能爲空", Toast.LENGTH_SHORT).show();
		} else {
			// 放在子線程裏面執行（子線程不能更改主線程的UI）
			new Thread() {
				public void run() {
					// 連接服務器get請求獲取圖片
					try {
						URL url = new URL(path);
						// 根據url發送http請求
						HttpURLConnection conn = (HttpURLConnection) url
								.openConnection();
						// 設置請求的方式
						conn.setRequestMethod("GET");
						conn.setConnectTimeout(5000);
						// conn.setReadTimeout(5000);
						conn.setRequestProperty(
								"User-Agent",
								"Mozilla/4.0(compatible;MAIE 6.0; Windows NT 5.1; SV1; .NET4.OC; .NET4.OE; .NET CLR 2.0.50727; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729; Shuame)");
						// 得到服務器返回的響應碼
						int code = conn.getResponseCode();
						if (code == 200) {
							InputStream is = conn.getInputStream();
							Bitmap bitmap = BitmapFactory.decodeStream(is);
							// iv.setImageBitmap(bitmap);
							// TODO 告訴主線程一個消息，幫我更改界面。內容：bitmap
							Message msg = new Message();
							msg.what = CHANGE_UI;
							msg.obj = bitmap;
							handler.sendMessage(msg);
						} else {
							Message msg = new Message();
							msg.what = ERROR;
							handler.sendMessage(msg);
						}
					} catch (Exception e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
						Message msg = new Message();
						msg.what = ERROR;
						handler.sendMessage(msg);
					}
				};
			}.start();

		}
	}
}