在使用actionbar的时候可能会出现这样的报错
This Activity already has an action bar supplied by the window decor. Do not request Window.FEATURE_ACTION_BAR and set android:windowActionBar to false in your theme to use a Toolbar instead.
刚开始看到这个报错会觉得比较奇怪，但是仔细分析一下就能发现问题的原因了
首先在源码中（使用Android xref）找到报异常的位置，最后在activity.java找到如下代码
public void setActionBar(@Nullable Toolbar toolbar) {
final ActionBar ab = getActionBar();
if (ab instanceof WindowDecorActionBar) {
throw new IllegalStateException("This Activity already has an action bar supplied " +
"by the window decor. Do not request Window.FEATURE_ACTION_BAR and set " +
"android:windowActionBar to false in your theme to use a Toolbar instead.");
}
// If we reach here then we're setting a new action bar
// First clear out the MenuInflater to make sure that it is valid for the new Action Bar
mMenuInflater = null;
// If we have an action bar currently, destroy it
if (ab != null) {
ab.onDestroy();
}
if (toolbar != null) {
final ToolbarActionBar tbab = new ToolbarActionBar(toolbar, getTitle(), this);
mActionBar = tbab;
mWindow.setCallback(tbab.getWrappedWindowCallback());
} else {
mActionBar = null;
// Re-set the original window callback since we may have already set a Toolbar wrapper
mWindow.setCallback(this);
}
invalidateOptionsMenu();
}
能够走到这段代码，说明应用是使用toolbar来代替actionbar的
首先会获取当前的actionbar对象ab，如果已经设置toolbar，则类型为Toolbar，否则为默认的WindowDecorActionBar，然后进行类型判断
如果ab的类型为WindowDecorActionBar，表明已经设置过默认actionbar了，将WindowDecorActionBar替换成Toolbar是不被允许的，所以抛出异常
出现这种情况一般可能由以下几种原因
- 当前theme的parent含有属性<item name="android:windowActionBar">true</item>
- 当前activity的父类可能执行了对actionbar的操作，例如getActionBar，setDisplayHomeAsUpEnabled
知道问题的原因后，解决也就很容易了
