How is the number of views limited?

Question

I want to figure out the main effectivity issues of the Android layouts and views. I'm doing a research now but maybe somebody has answers already.

I have a RelativeLayout that is populated with views dynamically. Basically, the application loads a forum thread in XML and then renders the discussion tree so each message is displayed in its own group of views. I've decided not to use WebView because I want to implement some client-side functions that should be easier to do on custom views than on HTML page.

However this approach has a major flaw: it's heavy.

The first problem (which I've solved) was nesting of views. This is not an issue now, my layout is almost flat so the maximum depth is 10-12 (counting from the very top PhoneWindow$DecorView, the actual depth depends on data).

Now I've reached the next limit that is somehow connected to (or caused by) resource consumption of the views. After loading the data, the application hangs for a while to build the layout (creates the views and populates them with the data), and the hang time seems to grow linear with the data size; and if the data is large enough, the view will never appear (eventually the system suggests closing the application because it isn't responding).

Now the questions:

1. Does memory consumption depend significantly on the view class? In other words, is there any major difference between a Button and a TextView, or an ImageView? I can attach a click handler to any view so they don't differ much in usage.

2. Do background images affect the performance? If the same image is set in N views, will it make the layout N times heavier? (I understand that this question may look silly but anyway.)

3. Are nine-patch images significantly heavier than regular ones? What is better: to create N views where each has some background images, or to make one view that is N times wider and has a repeating background?

4. Given some layouts, what should be optimized first: overall number of views, nesting levels, or something else?

5. The most interesting. Is that possible to measure or at least estimate the resources consumed by the activity and its views? If I make some change, how could I see that I'm going the right way?

UPDATE

Thanks to User117, some questions that I asked above are now answered. I've used the Hierarchy Viewer and optimized my layout: compared to what I had before, the overall number of views is now reduced almost twice, and the nesting is also reduced.

However the application still hangs on a large forum thread.

UPDATE 2

I've connected the debugger to my device and found that the application gets out of memory.

But what is very unexpected for me is that the error occurs after I populate the layout. The sequence is as follows:

1. All my views are added. I can see a slight slow down as they are being added.
2. Almost nothing happens for a couple of seconds. During that time, a few info messages are spawned in the log, they are identical: [global] Loaded time zone names for en_US in XXXXms, the only difference is number of milliseconds.
3. The out of memory error message is spawned: [dalvikvm-heap] Out of memory on a N-byte allocation (the actual size varies). The long error reporting starts.

What does this mean? Looks like the rendering have its own demands that may be considerable.

UPDATE 3

At last I've found the core issue. Here is a screenshot from my application, see an explanation below the image.

Each message consists of a round button that shows or hides the replies and a red content frame to the right of the button. This is very simple and requires only 6 views including the layouts.

The problem is the indentation with these connection lines that show which message is related to which.

In my current implementation, the indentation is built of small ImageView's, each containing a square image that shows either empty space, or a vertical line, or a T-like connector, or a L-like corner. All these views are aligned to each other within the large RelativeLayout that holds the whole discussion tree.

This works fine for small and medium trees (up to few hundreds of messages), but when I try to load a large tree (2K+ messages), I get the result explained in UPDATE 2 above.

Obviously, I have two problems here. I spawn large number of views that all consume memory, and these views are ImageView's that require more memory for rendering because they render a bitmap and therefore create graphics caches (according to explanation given by User117 in the comments).

I tried disabling loading the images into the indentation views but got no effect. It seems like adding that huge number of views is quite enough to eat all available memory.

My other idea was to create an indentation image for each message that would contain all pipes and corners, so each message would have the only indentation view instead of 10 or 20. But this is even more consuming: I've got out of memory in the middle of populating the layout. (I cached the images in a map so two bitmaps with identical sequence of images weren't created, that didn't help.)

So I'm coming to conclusion that I'm in a dead end. Is it ever possible to draw all these lines at once?

Answer 1

1. Different View's are different kinds of Object. Some only draw() light weight stuff, some can hold large Bitmap Objects, and handler Objects and so on. So, yes different View's will consume different amount of RAM.

2. If same Bitmap object is shared among views, There's only one Object in RAM, each View will have a reference variable pointing to that object. But, not so when View draws: Drawing same Bitmap n times at n places on screen will consume n times CPU and generate n different bitmap_cache for each View.

3. Each side of a 9-patch image is actually bigger by 2 pixels from the original image. They are not much different as a file. When they are drawn, both can be scaled and will take almost equal space. The only difference is that 9-Patch are scaled differently.

   1. Setting the background of the larger, parent view is better when the child views are transparent, and background will show through.

   2. You can save a small image and set a tiled background so that it can fill a large area.

4. Nesting is to be optimized first, because all of the views might not be visible at a given time, let's say only a few views are visible in scrolling layout. Then you can cut down on total number of views used. Take cues from ListView: Given that user will be only seeing a sub set of total data at a time, it re-cycles the views. and saves a lot of resources.

5. SDK provides Hierarchy Viewer tool for this purpose. It shows a full tree structure of a running Layout, also places red flags for the sluggish parts of the layout.

A good layout is that which:

1. Easy to be measured (avoid complex weighted widths, heights and alignments). For example instead of doing layout_gravity="left" for each each child, parent can have gravity="left".

2. Has less depth, each overlapping view adds another layer to be composited while screen is drawn. Each nested View Structure, asks for a chained layout call.

3. Is smart and re-cycles Views rather than create all of them.

4. Reuses its parts: tags like <merge> and <include>.

Update: Answers to this question, show many approaches for a tree view in android.