Android实战技巧：Dialog

Dialog是任何系统都必须有的一个控件，作为辅助窗口，用于显示一些消息，或请求用户采取一引起操作等。

在Android中也不例外，基本使用可能参看文档。

使用时的注意事项

1. BACK键能取消掉对话框(dismiss)，但是却不会触发其onOkey和onCancel回调接口，所以如果你的对话框会改某些状态，一定要注意还有第三种方式取消对话框。

2. 尽量少用模态对话框(Model dialog)，如果Dialog.setCancellable(false)，就变成了一个模态对话框，除了程序内部把其Dismiss，否则按什么键都无法将其取消。这是极差的用户体验，对话框本身就是一种干扰，再无法取消会把用户搞疯的。所以除非特别有必要，也即当执行某个操作时不希望被打破，才可以使用模态对话框。

3. 尽量少用对话框，它对用户是一种干扰，除非需要用户做操作，或者做出选择。通常的一般性的通知用Toast或者Notification就足够了。

4. 不要使用对话框风格的Activity，也即把Activity变成一个对话框。因为这样是自已定义的布局，与系统Dialog的风格可能会不一致。最严重的是当系统风格发生变化，Dialog的子类会变化，但Activity式的对话框就不会变化。可以在ICS中找一找Activity对话框，你会发现其OK是在左边，而ICS中系统Dialog的OK都是在右边的。

5. 尽量保证Dialog对象活在Activity的生命周期之内，也即至多是在onCreate()和onDestroy()之间。

6. 要想到和测试到Activity在其Dialog.dismiss()之前死掉的情况。因为Activity必须依附于某个正在显示的Activity实例，当显示和取消的时候其Activity实例必须存在，否则就会有"IllegalArgumentException: View not attached to window manager"。

05-15 02:45:26.320: E/AndroidRuntime(1161): java.lang.IllegalArgumentException: View not attached to window manager
05-15 02:45:26.320: E/AndroidRuntime(1161):     at android.view.WindowManagerImpl.findViewLocked(WindowManagerImpl.java:355)
05-15 02:45:26.320: E/AndroidRuntime(1161):     at android.view.WindowManagerImpl.removeView(WindowManagerImpl.java:200)
05-15 02:45:26.320: E/AndroidRuntime(1161):     at android.view.Window$LocalWindowManager.removeView(Window.java:432)
05-15 02:45:26.320: E/AndroidRuntime(1161):     at android.app.Dialog.dismissDialog(Dialog.java:278)
05-15 02:45:26.320: E/AndroidRuntime(1161):     at android.app.Dialog.access$000(Dialog.java:71)
05-15 02:45:26.320: E/AndroidRuntime(1161):     at android.app.Dialog$1.run(Dialog.java:111)
05-15 02:45:26.320: E/AndroidRuntime(1161):     at android.app.Dialog.dismiss(Dialog.java:268)
05-15 02:45:26.320: E/AndroidRuntime(1161):     at com.hilton.effectiveandroid.app.DialogDemo$1.handleMessage(DialogDemo.java:26)
05-15 02:45:26.320: E/AndroidRuntime(1161):     at android.os.Handler.dispatchMessage(Handler.java:99)

7. Dialog.show()必须在主线程里调用，但Dialog.dismiss()却可以在任何线程中调用。

三种使用方式比较

1. 直接创建一个局部的Dialog对象

优点是变量是局部的容易理解和维护。缺点是Dialog对象难以控制，容易引发RuntimeException。

2. 把Dialog对象变成Activity的域

优点是Dialog对象可以重复利用，且Activity可以控制以保证Dialog不会在Activity生命周期外显示。是推荐的使用方式。

3. 用Activity的方法onCreateDialog(), showDialog()和dismissDialog()

优点是Frameworks会帮忙照看Dialog，在大多数情况下这是推荐的做法。但是对于Activity提前死掉的情况，此方法必有RuntimeException，且无法回避。

实例

public class DialogDemo extends Activity {
    private static final int DISMISS_DIALOG = 1;
    
    private ProgressDialog mBetterDialog;

    private Handler mMainHandler = new Handler() {
	@Override
	public void handleMessage(Message msg) {
	    switch (msg.what) {
	    case DISMISS_DIALOG:
		Dialog dialog = (Dialog) msg.obj;
		dialog.dismiss();
		break;
	    default:
		break;
	    }
	}
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
	super.onCreate(savedInstanceState);
	setContentView(R.layout.dialog_demo);
	
	final Button sucking = (Button) findViewById(R.id.sucking);
	sucking.setOnClickListener(new View.OnClickListener() {
	    public void onClick(View v) {
		final Activity activity = DialogDemo.this;
		final ProgressDialog dialog = new ProgressDialog(activity);
		dialog.setTitle("Worst dialogging");
		dialog.setMessage("This is the worst dialogging scheme, NEVER use it. This dialog is easy to " +
			"run out of its attached activity, yielding WindowManager#BadTokenException if the activity is gone when dismissing");
		dialog.setIndeterminate(true);
		dialog.setCancelable(true);
		// You MUST do the show in main thread anyway
		dialog.show();
		new Thread(new Runnable() {
		    public void run() {
			SystemClock.sleep(10000);
			/*
			 * IllegalArgumentException: View not attached to window manager
			 * If the activity showing the dialog was killed before dismiss() out of rotation or locale changed,
			 * the dialog will gone with activity, but when dismiss() yields "IllegalArgumentException: View not attached to 
			 * window manager".
			 * Checking isShowing() won't help.
			 * Checking activity.isFinishing() won't help, either.
			 * Dismiss it in main thread also won't give any help.
			 */
			// THIS WON't WORK
//			if (dialog.isShowing()) {
//			    dialog.dismiss();
//			}
//			if (!activity.isFinishing()) {
//			    dialog.dismiss();
//			}
			Message msg = Message.obtain();
			msg.what = DISMISS_DIALOG;
			msg.obj = dialog;
			mMainHandler.sendMessage(msg);
		    }
		}).start();
	    }
	});
	
	final Button better = (Button) findViewById(R.id.better);
	better.setOnClickListener(new View.OnClickListener() {
	    public void onClick(View v) {
		mBetterDialog = new ProgressDialog(DialogDemo.this);
		mBetterDialog.setTitle("Better dialogging");
		mBetterDialog.setMessage("This dialogging can be used. The dialog object is a field of its activity, so activity can" +
				" control it to make sure dialog only lives within activity lifecircle");
		mBetterDialog.setIndeterminate(true);
		mBetterDialog.setCancelable(true);
		// You MUST do the show in main thread anyway
		mBetterDialog.show();
		new Thread(new Runnable() {
		    public void run() {
			SystemClock.sleep(10000);
			/*
			 * This is much better, mBetterDialog is a field of its activity, so activity can take care of it in order
			 * to make sure dialog only live within activity's life circle, to avoid any unexpected exceptions.
			 */
			// THIS really works
    			if (mBetterDialog != null && mBetterDialog.isShowing()) {
    			    mBetterDialog.dismiss();
    			}
		    }
		}).start();
	    }
	});
	
	final Button optional = (Button) findViewById(R.id.optional);
	optional.setOnClickListener(new View.OnClickListener() {
	    @SuppressWarnings("deprecation")
	    public void onClick(View v) {
		showDialog(0);
		new Thread(new Runnable() {
		    public void run() {
			SystemClock.sleep(10000);
			/*
			 * This way works best for most of time, except if activity died before dismissing, exception must be
			 * thrown: "IllegalArgumentException: View not attached to window manager".
			 * Although activity takes care of its belonging dialog, there is no way to operate it manually any more.
			 * First you do not have reference to dialog object and second, any manual operation only interferences
			 * and breaks state maintained by frameworks.
			 */
			dismissDialog(0);
		    }
		}).start();
	    }
	});
    }

    @Override
    protected Dialog onCreateDialog(int id) {
	ProgressDialog d = new ProgressDialog(this);
	d.setTitle("Optional dialogging");
	d.setMessage("This dialogging scheme works best for most times, the dialogs are all taken care of by activitys and frameworks" +
			". Except for activity being killed during dialog showing");
	d.setIndeterminate(true);
	d.setCancelable(true);
	return d;
    }

    @Override
    protected void onDestroy() {
	super.onDestroy();
	// Activity is dying, all its belonging dialogs should be dismissed, of course.
	if (mBetterDialog != null && mBetterDialog.isShowing()) {
	    mBetterDialog.dismiss();
	    mBetterDialog = null;
	}
	// For dialogs showed via showDialog(int), no way to stop it in onDestroy()
//	dismissDialog(0); // cause "IllegalArgumentException: no dialog with id 0 was ever shown via Activity#showDialog"
			    // This is because Activity has to manage its dialog during onPause() and onResume() to restore
	                  // dialogs' state. So if you manually dismiss it in onDestroy(), it will cause JE.
	
//	removeDialog(0);// cause "IllegalArgumentException: no dialog with id 0 was ever shown via Activity#showDialog", when
			// dismissing in thread. 
	              // This is because Activity has to manage its dialog during onPause() and onResume() to restore
                     // dialogs' state. So if you manually dismiss it in onDestroy(), it will cause JE.
    }
}