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POBest practices for dealing with expensive view height calculation?
 

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    <p>I keep running into a sizing and layout problem for custom views and I'm wondering if anyone can suggest a "best practices" approach. The problem is as follows. Imagine a custom view where the height required for the content depends on the width of the view (similar to a multi-line TextView). (Obviously, this only applies if the height isn't fixed by the layout parameters.) The catch is that for a given width, it's rather expensive to compute the content height in these custom views. In particular, it's too expensive to be computed on the UI thread, so at some point a worker thread needs to be fired up to compute the layout and when it is finished, the UI needs to be updated.</p> <p>The question is, how should this be designed? I've thought of several strategies. They all assume that whenever the height is calculated, the corresponding width is recorded.</p> <p>The first strategy is shown in this code:</p> <pre><code>protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { int width = measureWidth(widthMeasureSpec); setMeasuredDimension(width, measureHeight(heightMeasureSpec, width)); } private int measureWidth(int widthMeasureSpec) { // irrelevant to this problem } private int measureHeight(int heightMeasureSpec, int width) { int result; int specMode = MeasureSpec.getMode(measureSpec); int specSize = MeasureSpec.getSize(measureSpec); if (specMode == MeasureSpec.EXACTLY) { result = specSize; } else { if (width != mLastWidth) { interruptAnyExistingLayoutThread(); mLastWidth = width; mLayoutHeight = DEFAULT_HEIGHT; startNewLayoutThread(); } result = mLayoutHeight; if (specMode == MeasureSpec.AT_MOST &amp;&amp; result &gt; specSize) { result = specSize; } } return result; } </code></pre> <p>When the layout thread finishes, it posts a Runnable to the UI thread to set <code>mLayoutHeight</code> to the calculated height and then call <code>requestLayout()</code> (and <code>invalidate()</code>).</p> <p>A second strategy is to have <code>onMeasure</code> always use the then-current value for <code>mLayoutHeight</code> (without firing up a layout thread). Testing for changes in width and firing up a layout thread would be done by overriding <code>onSizeChanged</code>.</p> <p>A third strategy is to be lazy and wait to fire up the layout thread (if necessary) in <code>onDraw</code>.</p> <p>I would like to minimize the number of times a layout thread is launched and/or killed, while also calculating the required height as soon as possible. It would probably be good to minimize the number of calls to <code>requestLayout()</code> as well.</p> <p>From the docs, it's clear that <code>onMeasure</code> might be called several times during the course of a single layout. It's less clear (but seems likely) that <code>onSizeChanged</code> might also be called several times. So I'm thinking that putting the logic in <code>onDraw</code> might be the better strategy. But that seems contrary to the spirit of custom view sizing, so I have an admittedly irrational bias against it.</p> <p>Other people must have faced this same problem. Are there approaches I've missed? Is there a best approach?</p>
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    1. COWouldnt it be possible to use a OnGlobalLayoutListener() on the view whose height is required ?
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    2. CO@lokoko - I'm not sure I understand the suggestion. Can you elaborate? The issue is: when should I initiate the expensive layout height calculations?
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