I have an XML file with about 150 views. Yes, I know it is a lot and I did get a message from Android Studio saying I can't exceed 80 views. But I can't drop views any lower than 150. I considered using list view but it works the way I wanted it to.
The question is, will this many views make the app crash/slow the device? I've tried it on my s7 and it works perfectly fine. My lowest API is 17 which is 4.2. Wouldn't 4.2 devices be able to handle this XML without any problem?
Thanks.
The problem with having an excessively large number of Views is that Android often needs to measure, layout, and draw them, and it will traverse the entire View hierarchy to do this. If the number of Views is so large that this traversal takes more time than the screen refresh rate, you will skip frames and your UI might appear to lag or be choppy.
If not all of those Views need to be on screen at once (for example, if you are using a ScrollView to hold a very large container that the user can scroll through), then you should probably switch to using RecyclerView.
If all of those views need to be on screen at once, then you might consider writing custom Views that can display your content all at once instead of having individual Views that draw individual things. This can drastically reduce the time and complexity of the measure/layout/draw traversals.
It's difficult to suggest an approach without knowing more specifics about your UI, but hopefully that explains the issue.
I am creating an android app and I have a page that displays many cards at once. These cards have constant widths (so that two fit next to each other on the page) however they all have different heights. I want to display them in a grid formation where they fit together without any gaps like the notes app on the material design "Cards" page - http://www.google.co.uk/design/spec/components/cards.html#cards-usage -
(about 1/8th down the page). My question is, does anybody know how to do this? Any help would be appreciated.
Since we had RecyclerView, it's enough to setLayoutManager using StaggeredGridLayoutManager.
You can find more instructions here.
I have not implemented Gridview but I believe you want to manipulate its layouts, like in ListView. For a start, a link is at GridView. Look at setLayoutParams method. It will give you good ideas.
I have a ScrollView that I would like to have a variable number of children in. Easy game, use LinearLayout.
Now, I am looking for a way to make the widths of the children some proportion of the ScrollView itself (it is going to be placed in another auto-laid out view).
I have tried using LinearLayout weights (and weightSum) to keep them proportional to themselves, but then they just fill the ScrollView and don't allow scrolling.
I have android:fillViewport="true" set on the ScrollView so that it fills its container, but I can't seem to find a way to make the children some fraction of the ScrollView itself.
Basically I want ALL the children to have a fixed width of say a 3rd of the available space, so that while scrolling at most 3 items are shown, and they are proportional to the available space.
My thoughts are that I will need to create a CustomScrollView that extends ScrollView, and then #Override the onMeasure() method, is this the way, or is there an easier, more elegant solution (hopefully using the layout_weights engine already in LinearLayout)?
Any help would be much appreciated!
P.S. I had a beautiful picture drawn, but apparently you need 10 rep, lame to the n^th degree. I would love to have contributed to more questions on SO, but I need like 15 rep to vote up a question that helped me? That means I need to ask 15 questions, of which I can already find all the answers on here.
Some commonly used android layouts such as
RelativeLayout and LinearLayout (when weights are nonzero)
have onMeasure() implementations that measure their children
twice, resulting in exponential running time when nested.
This is easily verifiable by emitting Log entries
from a leaf View's onMeasure()... it gets called 2depth
times.
Can someone give a clear and specific description as to why this is?
Most importantly, is this exponential behavior due to an important part
of the overall contract, or is it just an implementation detail
that might be optimized away? If it is believed to be unavoidable,
please give an example that requires it.
Such an example would greatly help me and others
who are grumbling that the "keep your layouts shallow"
mandate is burdensome and who are wondering
whether this is being driven simply by
not-yet-optimal algorithms in the core libraries,
or whether there really is a fundamental difficulty
blocking a fix.
Perhaps a minimal example would consist
of a Button inside a LinearLayout inside another LinearLayout
(with match_parent and weight=1 everywhere, to trigger
the full exponential behavior),
with some additional parameters or circumstances
making it clear that all four of the calls
to Button.onMeasure() are indeed meaningful and necessary.
My first guess would be that only two linear-time
traversals are really needed-- the first traversal to gather everyone's
preferred sizes, the second traversal to distribute slack
and/or shrinkage.
Other layout engines in the world, such as those for Tex and Swing and HTML,
seem to be able to routinely handle very deep hierarchies
having lots of alignment constraints and stretches,
without any exponential blowup, and I imagine that's how they work.
Please note, I don't want answers explaining how
the exponential blow-up occurs-- I understand that,
and there have been several posts already where that has been
asked and answered:
Why are nested weights bad for performance? Alternatives?
Android layout measuring time doubles with each step up the hierarchy
Layout Weight warning Nested weight bad performance
Efficiency of Android Layout hierarchy
http://android-developers.blogspot.com/2009/02/android-layout-tricks-1.html
My question is whether the recursive double-measuring is
fundamentally necessary/justified,
and if so, I'd like a clear explanation/example showing why.
Edit 2013/8/22: I think maybe my question is still not getting across.
I'll try to clarify and explain my motivation, more boldly this time.
Layout is not an exponentially hard problem,
as evidenced by efficient layout engines in the world, such as those for Tex and Swing and HTML.
So, what happened with LinearLayout,
and how should the android developer community proceed in response?
I am asking not in the spirit of laying blame,
but rather to understand and decide how to best move forward.
I can think of 4 possibilities:
Fix the performance bug in the core library, without changing any contracts
Change contracts as needed, and fix the performance bug in the
core library
Write an alternative to LinearLayout, that has its essential
features (namely distributing extra space among children in specified proportions) but without the performance bug, and use it for new apps
Continue to micromanage our layouts to work around the
performance bug for the rest of our android development careers.
(4) isn't a serious option for me personally.
Furthermore it seems clear to me that
changing the behavior of LinearLayout at this point is impractical,
so I don't believe (2) is a serious option either.
That leaves (1) and (3).
I'm capable and willing to do either of those personally, but which one?
Obviously (1) is far preferable if it's possible-- so, is it possible?
That seems to be the crucial blocking question that needs to be answered
in order to determine how to move forward.
I have spent some time in the core code
and the doc and it's not becoming clear,
so that is why I'm asking the question here.
In terms of measuring children twice, it's my understanding that this is what happens with LinearLayouts particularly when weights are involved. The best explanation I've found for this comes from RomainGuy in one of his presentations.
He has a slide about this and briefly speaks to it at 17:45. Feel free to rewind to get a bit of context though. You can find the video I'm referencing here: Devoxx'10 - Dive Into Android
Basically what he says is that on the first pass they calculate the total width or height depending on orientation of the LinearLayout, add the weights of the children, and find out how much space is left over, then on the second pass with that information they are able to properly divvy out all the remaining space to all the children. Simple enough.
I'd also like to point out though that yes, while it's very true that shallow layout hierarchies have less of a performance hit, if you are adding just 1 or 2 extra layers, you probably aren't going to see a big performance impact for the user. Once it's laid out, it's done. Even in ListView's if you properly use the given "convertView", and set up ViewHolder's, you're going to get good performance.
I'd encourage you to use DDMS and do a layout dump of some of Google's apps. They are very complex, and often times surprisingly deep, but they still get good performance. Don't be stupid with your layouts, but if it saves you time, adding an extra layout isn't the end of the world.
From here: http://developer.android.com/guide/topics/ui/how-android-draws.html
A parent View may call measure() more than once on its children. For example, the parent may measure each child once with unspecified dimensions to find out how big they want to be, then call measure() on them again with actual numbers if the sum of all the children's unconstrained sizes is too big or too small (that is, if the children don't agree among themselves as to how much space they each get, the parent will intervene and set the rules on the second pass).
It seems they view the measurement pass as a dialog between parent and children. In other words they opted for maximum flexibility instead of optimization. It still seems like they could optimize the base layouts though.
I came here tonight to ask this. It's disappointing that nobody else even seems to understand your question. After thinking about it for a couple more hours, I think I might know the answer.
Consider this example:
The red box represents a LinearLayout with vertical orientation. The green box represents another LinearLayout with horizontal orientation. I would expect the layout to proceed like this:
1) The red LinearLayout would measure the heights of green LinearLayout and the large blue widget on the bottom, then compare their weights, divide any leftover vertical space accordingly, and measure the children again. (The important thing to know here is that "measuring" a view actually sets its size.)
2) The green LinearLayout would then measure the widths of its three children, compare their weights, divide the horizontal space, and "measure" again.
The catch is that in order for the red layout to measure the height of the green layout, the green layout needs to know the height of its children.
Now, you would think that LinearLayout would be smart enough to optimize away many of its calculations. For example, there is no logical reason for the green layout to measure the width of its children when determining its own height. And if its children all have a height of fill_parent, then it shouldn't need to perform any calculations at all.
But the API doesn't allow LinearLayout to be that smart. The fundamental problem is that there is no way to measure only one dimension of a view. You can get them separately after they've been measured, but the actual measurement is done by View#onMeasure(int, int). The arguments to that method are encoded with View.MeasureSpec, and there's no way to encode "ignore this dimension." So the green LinearLayout stupidly calculates both dimensions of all its children when the red layout measures it, then repeats the entire process again when it does its own layout. The widths will not have changed the second time around, but they must still be recalculated because, again, there's no way to tell the layout or its children to measure only one dimension.
So to answer your question... no, it doesn't necessarily need to be this way, at least for many common use cases. This is a deficiency of the Android API.
If Google were to add new methods to View to measure dimensions separately, the default implementation would have to rely on onMeasure(int, int), which could be even worse for performance. But if there are any unused bits in the View.MeasureSpec encoding, then it might be possible to add an "ignore" flag that would allow future versions of LinearLayout to be better optimized.
The problem in my opinion is the measure cache. As far as I could see the cache only works if there was a layout of the view in the middle, so when you perform two consecutives measures of a child (even with the same exact measure specs) in the same "onMeasure" all the children and their sub-children are measured again. If the measure cache worked fine, the second measure should be much faster as it would take the cached values and it wouldn't cause all the children hierarchy to be measured again.
I have an app that show the tv guide for a list of channels. My UI is made from a a lot of custom views with different widths that show the tv programs, all these custom views are added into a horizontal scrollview that is added into a scrollview so my views can be scroll in 2 dimensions left-right and top-down. It all works good until i add add a lot of views and it starts to slow down very much. So i need a way to recycle views like listiew does in a scrollview maybe there is a custom made scrollview that does this, or someone has an idea how to do this, its strange that scrollview isnt backed up by an adapter like gridview and listview.
I did something similar only my Views were not connected as yours but they were all different sizes.
First you need to define if your entire area (not just viewing screen) has definitive or dynamical number of your custom views.
If you have definitive number of views and their positions you should create their position map with list of Rect's (Rect has a good function whether the xy point belongs). Then you define maximum of Views which are visible on your screen. For this to work without constant loading you should have maximum visible views + at least one line of border views of total objects. After all this you should easily have your own positioning system where you load views which are in bounds of your screen + some overhead (purpose of this is that you want your users to have smooth transition while scrolling at least for some length), if you need to load some in same time you unload (read reuse/ do not dispose objects and create them onScroll events) and place them according to your needs.
And if you want to determine which views should be visible you just go through list and ask whether Views Rect intersects with your Rect of area to be loaded.
Hope this image helps a little bit more
I know it sounds a little bit confusing and difficult to implement but you did not asked a simple question :)
Hope this helps and enjoy your work.
A ScrollView that is backed up by an adapter and recycles views is a ListView, with a couple of extra optional features on top.
Maybe you want a HorizontalScrollView that is backed by an adapter? Searching for HorizontalListView will give you a few results, ex: https://github.com/dinocore1/DevsmartLib-Android.