How does setting baselineAligned to false improve performance in LinearLayout?

问题

I was just building some UI in xml, and Lint gave me a warning and said to set android:baselineAligned to false to improve performance in ListView.

The docs for the Lint changes that added this warning say

Layout performance: Finds LinearLayouts with weights where you should set android:baselineAligned="false" for better performance, and also finds cases where you have nested weights which can cause performance problems.

Can somebody explain why this improves performance, specifically when weight is involved?

回答1:

By setting android:baselineAligned="false" , you're preventing the extra work your app's layout has to do in order to Align its children's baselines; which can obviously increase the performance. (Fewer unnecessary operations on UI => Better performance)

回答2:

how android:baselineAligned="false" help . It may not be the answer but help to get concept.

I've just managed to get 3 items (icon, text, button) centered vertically in horizontal LinearLayout.

This may seem simple, but in reality specifying android:gravity="center_vertical" as LinearLayout attribute is not enough - icon is centered, but text and button are not. This is because (presumably) text have a baseline, and centering algorithm uses it instead of 'real' vertical center. But what is worse - button (which comes next to text) is centered using text's baseline!

Specifying android:baselineAligned="false" in LinearLayout turns this off, and everything centers correctly.

回答3:

// Baseline alignment requires to measure widgets to obtain the
                // baseline offset (in particular for TextViews). The following
                // defeats the optimization mentioned above. Allow the child to
                // use as much space as it wants because we can shrink things
                // later (and re-measure).
                if (baselineAligned) {
                    final int freeSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
                    child.measure(freeSpec, freeSpec);
                }

https://github.com/android/platform_frameworks_base/blob/master/core/java/android/widget/LinearLayout.java#L1093

来源：https://stackoverflow.com/questions/9319916/how-does-setting-baselinealigned-to-false-improve-performance-in-linearlayout

标签

android

listview

android-lint