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 was working with Android UI in Eclipse and found it to be bit hectic. Designing layout using layout managers is bit time consuming. So i was wondering whether it is possible to specify the position of the UI elements based on (x,y) system i.e top and left property which is widely used in Visual Studio IDE for VB,C# etc ?
Positioning element based on top and left property would provide much flexibility.
How would that be flexible? Yes, doing layout correctly takes time, but if you do it right, it will scale properly to any screen size. If you're using X/Y coordinates, you will be hardcoding to a specific screen size, which is an especially bad idea on Android (as there are a multitude of screen sizes available).
If you need x, y positioning, you can use a FrameLayout with foregroundGravity set to top|left, and use layout_marginLeft for the x value, and layout_marginTop for the y value.
You can use AbsoluteLayout and suppress deprecation warnings in your code, but think of how will it look on different screen sizes?
I would advise to use RelativeLayout in your case.
As far as I know, there is no built-in layout that is based on (x, y) coordinates. You might be able to find 3rd party libraries that can do this for you. However, I'm skeptical that they will provide satisfactory results. Remember that Android is deployed on a wide variety of devices which include a range of different screen sizes and resolutions. This means that you can make the UI look pretty on one device using specific coordinates but it won't look very good on other devices.
Personally, I edit my UI layouts directly in the XML files. I find that this provides me better control than using the Eclipse UI editor. You still have to learn how the layout managers themselves work.
Android tries to ensure that your layout components are arranged nicely so that they:
don't overlap with each other
don't go off the screen space
look similar on different screen sizes
etc
It gives you nice XML Attributes to help you arrange your layout. I would recommend you use RelativeLayout for this application, because it allows you to put your layout components in positions RELATIVE to each other.
Some XML attributes you can specify are given here: Android Reference, RelativeLayout.LayoutParams
I'm prone to abusing LinearLayout, every screens there's usually three or four-level deep. Layout's design usually given size in percentages. Graphic cut to pieces and not utilizing 9-Patch. All of these resulting in me using layout_weight to represent percentages almost everywhere. Today I updated ADT and Lint has this nested weights warning everywhere. Now I'm really concern about the performance if I carry this habit into a bigger application. Is there a better way to do it without changing anything from designer's side?
If I start to get too many layers of LinearLayouts I tend to switch to a RelativeLayout at the root and most of the children only 1 layer removed from root.
9-Patch resources are very helpful also. I suggest you start to make use of those more.
I am trying to implement the minimumfontsize property in android. This is common to the ios sdk. Since it is not presently available in android I was wondering if anyone can help me with a similar implementation in android?
To be more specific, I am trying to implement this property for the TextViewUI in android.Hence I basically need to implement it for this widget.
These are the ios specs of this feature:
"When drawing text that might not fit within the bounding rectangle of the label, you can use this property to prevent the receiver from reducing the font size to the point where it is no longer legible.
The default value for this property is 0.0. If you enable font adjustment for the label, you should always increase this value. This property is effective only when the numberOfLines property is set to 1."
Without knowing what you intend to do with the code, there isn't much help to be had. The closest thing to what you describe in android is setting device independent pixels (android:textSize="30dp"), which ensure that however big (or small) the text looks on your handset, it will look pretty much just like that on all other handsets, at least those of the same size/density. See R.attr and Supporting Multiple Screens
This makes no sense to me, coming from CSS. In CSS, if you specify a margin and then margin-left, the left margin will assume the more granular value.
In Android, it is the opposite. Same goes for android:radius, and I'm sure other values.
My question is: why?.. It makes no sense. Is there a single reason for doing it this way?
Edit: prompted by trying to find a solution to yet another Google ADT/Android bug http://code.google.com/p/android/issues/detail?id=7588
I have had the same frustration, but if you think about it, which value should be used? I know in your post you say that CSS uses the "more granular value" but it all boils down to pixels in the end and the result is simply two pixel values that need to be chosen between. The CSS standard chose to do it one way, Android chose the other, I don't think either approach is wrong, they are just different.