根据Flutter之Binding简单梳理得知，Flutter在runApp方法运行的时候会初始化一系列Binding，而SemanticsBinding就是其中之一。本篇博文就简单熟悉下SemanticsBinding这个Binding的作用。
mixin SemanticsBinding on BindingBase {
static SemanticsBinding get instance => _instance;
static SemanticsBinding _instance;
ui.AccessibilityFeatures get accessibilityFeatures => _accessibilityFeatures;
ui.AccessibilityFeatures _accessibilityFeatures
