Tuesday, January 16, 2018


文章攢寫時間︰2018/01/16 11:54



When a touch event occurs, first everyone is notified of the event, starting at the Activity and going all the way to the view on top.
Then everyone is given a chance to handle the event, starting with the view on top and going all the way back to the Activity. So the Activity is the first to hear of it and the last to be given a chance to handle it.

If the Activity or some ViewGroup wants to handle the touch event right away (and not give anyone else down the line a chance at it) then it can just return true in its onInterceptTouchEvent().

If a View (or a ViewGroup) has an OnTouchListener, then the touch event is handled by OnTouchListener.onTouch(). Otherwise it is handled by onTouchEvent(). If onTouchEvent() returns true for any touch event, then the handling stops there. No one else down the line gets a chance at it.

More detailed explanation

The above diagram makes things a little more simple than they actually are. For example, between the Activity and ViewGroup A (the root layout) there is also the Window and the DecorView. I left them out above because we generally don't have to interact with them. However, I will include them below. The description below follows a touch event through the source code. You can click a link to see the actual source code.
下面的圖例也許能把整件事變得稍微簡單些,舉例來說,在Activity與ViewGroup A(根佈局)之間還有Window與DecorView。我們把它們排除在上面,因為我們一般來說不會與這兩個階層互動。不過,我會在下面的範例中引入它們。下面的描述是觸控事件的程式碼,你可以點擊連結來看源始碼。

1.The Activity's dispatchTouchEvent() is notified of a touch event. The touch event is passed in as a MotionEvent, which contains the x,y coordinates, time, type of event, and other information.

2.The touch event is sent to the Window's superDispatchTouchEvent(). Window is an abstract class. The actual implementation is PhoneWindow.

3.The next in line to get the notification is DecorView's superDispatchTouchEvent(). DecorView is what handles the status bar, navigation bar, content area, etc. It is actually just a FrameLayout subclass, which is itself a subclass of ViewGroup.
接下來我們取到的通知是DecorView的superDispatchTouchEvent()。DecorView是處理上方狀態列(status bar) 、下方導覽列(navigation bar)以及內容...等等的東西。它實際上只是一個FrameLayout的子類別,它本身就是ViewGroup的子類。

4.The next one to get the notification (correct me if I'm wrong) is the content view of your activity. That is what you set as the root layout of your activity in xml when you create the layout in the Android Studio's Layout Editor. So whether you choose a RelativeLayout, a LinearLayout, or a ConstraintLayout, they are all subclasses of ViewGroup. And ViewGroup gets notified of the touch event in dispatchTouchEvent(). This is the ViewGroup A in my diagrams above.
下一個得到通知(如果我錯了請糾正我)的是你的Activity裡的content view。這個就是你在Android Studio佈局編輯器裡編輯的、你設為根佈局的layout xml。因此,無論你選擇竹旳是RelativeLayout、LinearLayout、或是ConstraintLayout,它們都隸屬於ViewGroup的子類別。ViewGroup在dispatchTouchEvent()中獲得到觸控事件。這是我上圖裡的ViewGroup A。

5.The ViewGroup will notify any children it has of the touch event, including any ViewGroup children. This is ViewGroup B in my diagrams above.
ViewGroup將會開始通知觸控事件至底下的任何子元件項目,這也包含了ViewGroup的所有子項。在圖例中我以ViewGoup B來做舉例。

6.Anywhere along the way, a ViewGroup can short-circuit the notification process by returning true for onInterceptTouchEvent().

7.Assuming no ViewGroup cut the notifications short, the natural end of the line for the notifications is when the View's dispatchTouchEvent() get's called.

8.Now it is time, to start handling the events. If there is an OnTouchListener, then it gets the first chance at handling the touch event with onTouch(). Otherwise, the View's onTouchEvent() gets to handle it.

9.Now all the ViewGroups recursively up the line get a chance to handle the touch event in the same way that View did. Although, I didn't indicate this in the diagram above, a ViewGroup is a View subclass, so everything I described about OnTouchListener.onTouch() and onTouchEvent() also applies to ViewGroups.

10.Finally, if no one else wanted it, the Activity also gets the last chance to handle the event with onTouchEvent().

1.When would I ever need to override dispatchTouchEvent()?

Probably you wouldn't need to, unless you need to do some extra routing that doesn't occur by default. To monitor touch event notifications, you can override onInterceptTouchEvent() instead.

2.When would I ever need to override onInterceptTouchEvent()?

If you just want to spy of the touch notifications that are coming in, you can do it here and return false.

However, the main purpose of overriding this method is to let the ViewGroup handle a certain type of touch event while letting the child handle another type. For example, a ScrollView does this to handle scrolling while letting its child handle something like a Button click. Conversely, if the child view doesn't want to let its parent steal its touch event, it can call requestDisallowTouchIntercept().

3.What are the touch event types?

The main ones are 主要的有
    ACTION_DOWN - This is the start of a touch event. You should always return true for the ACTION_DOWN event in onTouchEvent if you want to handle a touch event. Otherwise, you won't get any more events delivered to you.
    ACTION_DOWN - 這是觸控事件的開始,在onTouchEvent()裡ACTION_DOWN的時候你也許永遠要返回true。否則,你不會再收到任何更多的事件。
    ACTION_MOVE - This event is continuously fired as you move your finger across the screen.
    ACTION_MOVE - 當你的手指在螢幕上不斷的滑動時,該事件會不斷的被觸發。
    ACTION_UP - This is the last event of a touch event.
    ACTION_UP - 這是觸控事件的最後一個事件。

A runner up is ACTION_CANCEL. This gets called if a ViewGroup up the tree decides to intercept the touch event.

You can view the other kinds of MotionEvents here. Since Android is multi-touch, events are also fired when other fingers ("pointers") touch the screen.