For example, top level LinearLayout view has 300 child view. But device screen dimension only show 11 child view once. How android compute how many child views can show once? How a view know that it will be draw?
Edited
In my work, one case like this:
An parent LinearLayout view may be has hundreds child view. In order to better performance , my solution like this:like lazy load.
List list = new ArrayList();//contain entity object that use construct View object
Default load 5 child view.
Parent LinearLayout view last child view is custom Loading View, I have override it`s onDraw() method. If loading View is draw, that means i need get next 5 child view(get next 5 object from list,and create correspond view).
I want to know how android framework handle this case?
Have u used scroll bar inside the top level LinearLayout view and add child view on that layout that's simple...
An parent LinearLayout view may be has hundreds child view. In order
to better performance , my solution like this:like lazy load.
LinearLayout with(possible) hundreds of child views kind of contradicts better performance. I think what you're looking for is a ListView with an endless adapter(a simple google search will show how to implement it, not something that difficult). This adapter will start loading views(with a loading view showing while the new content loads) as soon as you get to the last loaded element.
If you still want to use a LinearLayout:
If you just want to make the LinearLayout fill the content of the screen when it's first laid out you could post a Runnable on one of your views in the onCreate method (if this is where you'll first populate the LinearLayout). In that Runnable find the height of the LinearLayout and compare it with the combined height of its currently present children. If the combined child height is smaller then the LinearLayout height then add more views to compensate.
If the LinearLayout is in a ScrollView and you want to add additional children when the loading view becomes visible then monitor the scrolling of the ScrollView and when the loading view becomes visible add new children. Seeing when the loading view becomes visible would be done by checking how much the user has scrolled compared with the combined height of the currently present children of the LinearLayout.
Initial response:
Your question is a bit ambiguous regarding what you want to know. If you have a specific problem you should start with that.
For example, top level LinearLayout view has 300 child view. But
device screen dimension only show 11 child view once.
Please don't get yourself in a scenario like this. That number of views is to big an will result in poor performance or even the app crashing if you run out of memory(as all those views will be kept in memory).
How android compute how many child views can show once?
Each View and ViewGroup has the onMeasure method to measure itself and its children if available. The LinearLayout will have its onMeasure method called and in this method it will measure its children(with the measure method) giving them some suggestions on how big should they be(the LinearLayout receives some suggestions on how big it should be from its parent). If you want to see how this is done have a look at the source code of the LinearLayout.
How a view know that it will be draw?
I don't understand what you want to know. To draw the view on the screen its onDraw method will be called.
Related
I have problems doing a small game which adds a lot of views in a ConstraintLayout (I already tested with FrameLayout, RelativeLayout and LinearLayout. First two with same result and Linear with very rare behaviour) and changes the size and position of the views.
Each game loop (33ms) some of the views are changing it's size and position, so I do this on the LayoutParams variable applied on the View on each loop with the new size.
params.width = realWidth;
params.height = realHeight;
and I do this for the position:
view.setX(realX);
view.setY(realY);
The problem is that the change is not reflected if I didn't call view.requestLayout() and that is a huge problem because requestLayout() is repaiting the parent layout and all its childrens, slowing down the game.
How can the size changes of a view be reflected without calling requestLayout? I read that you can achieve that with view.layout(). So I changed my code with this:
view.layout((int)realX, (int)realY, (int)realX+realWidth, (int)realY+realHeight);
The problem is that it works without calling requestLayout() but the view has a very rare behaviour using layout(), and when I add more views, the views become some milliseconds invisible and appear on the left top corner, after that they appear in the correct position, but suddenly they become invisible again and again and again etc in a very rare behaviour loop.
ConstraintLayout is slow, particularly if it contains many Views and much slower if it contains nested ConstraintLayouts.
Because of you are manually setting the position for your Views, you can use a different ViewGroup than ConstraintLayouts and set the absolute position with setX(), setY(), setTop(), etc.
I have a frame layout with 2 children. A custom view and a progress bar.
Only 1 at a time is visible. Works as I need.
I want to also add some error view which will display an error and the other components would be hidden.
Is using a FrameLayout a good idea or should I switch to RelativeLayout?
If the layout works for you I would stick to FrameLayout. Since there are only 3 children (and assuming that none of them has children) the difference in performance should be really minimal (if any), but one difference I noticed digging into the source code of both FrameLayout and RelativeLayout is in the onMeasure method, FrameLayout will iterate twice all of its children, RelativeLayout iterates from 3 to 7 times (!)you can check FrameLayout onMeasure and RelativeLayout onMeasure yourself
I'm developing an app that has an UI pretty similar to Play Store. It is organized as a multiple panels one above another. First it has a panel containing a photo. Under that it has another panel containing some text and a custom background color. Under that it has another photo and finally it has a Linear Layout with vertical orientation containing a pretty long list of little views filled dynamically at runtime. I have all this inside a Scroll View, naturally.
The problem? That dynamic fill of the linear layout takes a long processor time and makes my app unresponsive during those inner views inflation. So I thought to replace the linear layout by a Recycler View. And the performance is awesome!
So? Well... Not everything is so awesome. I can't scroll the Recycler View because it's inside the Scroll View. And if I remove the Scroll View then I can't scroll the entire view (some things doesn't fit on the screen).
What's the best approach for fixing this?
It's not recommended to use a RecyclerView or ListView inside of a ScrollView precisely due to the double scrolling issues. RecyclerView is very robust and is prepared to receive headers, footers, etc. I see no reason why the entire layout could not be inside of a RecyclerView instead of a ScrollView
The ViewHolder implementation can include logic to inflate different layouts depending on what section should be next.
Pseudocode:
i.e.
if(currentAdapterItem == sectionA){
useLayoutA();
} else{
useLayoutB();
}
Just use a NestedScrollView instead of a normal ScrollView. It handles the nested scrolling quite well.
I'm making a GUI with two different parts. The first part (at the top) is composed of some banners, several fixed buttons. So I think using LinearLayout is the most straightforward way to implement. The second part is composed of several similar items grouped together which can be implemented by using ExpandableListView, I think.
However the problem is that the content exceeds the screen size. So I intend to put two of them into a ScrollView. I checked several sources, it seems that putting "ExpandableListView" inside a ScroolView is NOT possible, or not efficent, so I'm afraid...
Would you help to confirm if this is possible? efficient ?
If no, would you give me some recommendations for this layout design?
I'm indeed looking forward to your supports.
Sincerely.
If you have a fixed header at the top of a list, use ListView's header view feature.
Putting ListViews in ScrollViews fundamentally makes no sense and here is why:
ListView has one purpose: to efficiently display unbounded data sets. Since these can be extremely large (tens of thousands of items and more) you do not want to create a View for each item up front. Instead, ListView asks its Adapter for Views only for the items that currently fit in the ListView's measured space on screen. When an item's View is scrolled out of sight, ListView disconnects that View and hands it back to the adapter to fill out with new data and reuse to show other items. (This is the convertView parameter to an Adapter's getView method.)
ScrollView also has one purpose: to take a single child view and give it "infinite" vertical space to fit within. The user can then scroll up and down to see the full content.
Now given this, how many item Views would a ListView create for a 100,000 item Adapter if it had infinite height available to fill? :)
By putting a ListView inside a ScrollView you defeat ListView's key purpose. The parent ScrollView will give the ListView effectively infinite height to work with, but ListView wants to have a bounded height so that it can provide a limited window into a large data set.
Well Expandable List View itself has scrollable property by placing it in scroll view is really undesirable.As the both scroll would contradict and smooth scrolling can't be obtained in that case..
If we have any data to be shown prior or later to list...
Best way is to use header and footer view to list...
I recommend you use header and footer in your case.
I have extended LinearLayout (vertical) to create a custom compound component. This in turn contains two children:
one custom view that is drawn directly onto the view canvas.
one HorizontalScrollView->LinearView(Horizontal)->Multiple custom views.
I would now like to redraw the custom view to match the visible contents of the scroll view. The reason for this is that the long array of custom components in the scroll view are mainly static and suitable to be drawn ahead of time, while the top view is supposed to be highly dynamic and relate to whatever things are visible in the scroll view.
I hope I made the problem/idea somewhat clear. I am not att all confident this is the best approach, and I'd enjoy hearing any suggestions on alternative solutions or perhaps some idea on how to trigger a redraw-event everytime the scroll position changes in the HorizontalScrollView.
Thank!
You can have your activity listen to the scroll view adapter. in the adapter when ever the scroll position changes you execute the delegate in the Activity.
That way the activity can update the rest of the views upon scroll view change.