【发布时间】：2017-07-29 00:05:46
【问题描述】：
在this answer我写了
forceLayout()如果您只想重新布局自己的视图，请致电
forceLayout()内容，但不需要触发对整个视图树的重新测量 （所有父视图）。如果你有一个自定义ViewGroup那不是 改变自己的大小，但需要重新测量和重新布局 孩子们，这将是一个合适的情况forceLayout().基本上，调用
requestLayout()会导致调用parent.requestLayout()，但调用forceLayout()没有。
我记得我是通过阅读documentation 和source code 来写答案的。但是，我没有使用过forceLayout。一位用户 commented 说它没有像我描述的那样工作。
测试 forceLayout
我终于开始研究造成这种情况的原因了。我与祖父母ViewGroup、父母ViewGroup 和孩子View 建立了一个简单的项目。我为它们中的每一个都使用了自定义视图，以便我可以查看 onMeasure、onLayout 和 onDraw 中的日志语句。
当第一次从 xml 创建布局时，我得到以下日志：
ViewGroupGrandparent onMeasure called
ViewGroupParent onMeasure called
MyChildView onMeasure called
ViewGroupGrandparent onMeasure called
ViewGroupParent onMeasure called
MyChildView onMeasure called
ViewGroupGrandparent onLayout called
ViewGroupParent onLayout called
MyChildView onLayout called
MyChildView onDraw called
强制布局
这看起来是合理的输出。但是，当我随后在任何视图上单独调用forceLayout 时，我什么也得不到。如果我一次调用它们，那么子视图的 onDraw 会被调用。
孩子
childView.forceLayout();
// (no log output)
父母
viewGroupParent.forceLayout();
// (no log output)
祖父母
viewGroupGrandparent.forceLayout();
// (no log output)
大家一起
childView.forceLayout();
viewGroupParent.forceLayout();
viewGroupGrandparent.forceLayout();
// MyChildView onDraw called
请求布局
另一方面，调用requestLayout 的效果要大得多。
孩子
childView.requestLayout();
// ViewGroupGrandparent onMeasure called
// ViewGroupParent onMeasure called
// MyChildView onMeasure called
// ViewGroupGrandparent onLayout called
// ViewGroupParent onLayout called
// MyChildView onLayout called
// MyChildView onDraw called
父母
viewGroupParent.requestLayout();
// ViewGroupGrandparent onMeasure called
// ViewGroupParent onMeasure called
// ViewGroupGrandparent onLayout called
// ViewGroupParent onLayout called
祖父母
viewGroupGrandparent.requestLayout();
// ViewGroupGrandparent onMeasure called
// ViewGroupGrandparent onLayout called
问题
forceLayout什么时候有效果？为什么它似乎不像我上面的示例中那样工作？
补充代码
这是我用来进行上述测试的代码。
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.forcelayout.MainActivity">
<com.example.forcelayout.ViewGroupGrandparent
android:id="@+id/view_group_grandparent"
android:layout_width="wrap_content"
android:layout_height="wrap_content">
<com.example.forcelayout.ViewGroupParent
android:id="@+id/view_group_parent"
android:layout_width="wrap_content"
android:layout_height="wrap_content">
<com.example.forcelayout.MyChildView
android:id="@+id/child_view"
android:layout_width="100dp"
android:layout_height="100dp"/>
</com.example.forcelayout.ViewGroupParent>
</com.example.forcelayout.ViewGroupGrandparent>
<Button
android:text="Click me"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:onClick="buttonClick"/>
</LinearLayout>
MainActivity.java
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
public void buttonClick(View view) {
Log.i("TAG", "buttonClick: ");
ViewGroupGrandparent viewGroupGrandparent = (ViewGroupGrandparent) findViewById(R.id.view_group_grandparent);
ViewGroupParent viewGroupParent = (ViewGroupParent) findViewById(R.id.view_group_parent);
MyChildView childView = (MyChildView) findViewById(R.id.child_view);
childView.forceLayout();
//viewGroupParent.forceLayout();
//viewGroupGrandparent.forceLayout();
//childView.requestLayout();
//viewGroupParent.requestLayout();
//viewGroupGrandparent.requestLayout();
}
}
ViewGroupGrandparent.java
public class ViewGroupGrandparent extends LinearLayout {
public ViewGroupGrandparent(Context context) {
super(context);
}
public ViewGroupGrandparent(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public ViewGroupGrandparent(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
Log.i("TAG", "ViewGroupGrandparent onMeasure called");
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
Log.i("TAG", "ViewGroupGrandparent onLayout called");
super.onLayout(changed, l, t, r, b);
}
@Override
protected void onDraw(Canvas canvas) {
Log.i("TAG", "ViewGroupGrandparent onDraw called");
super.onDraw(canvas);
}
}
ViewGroupParent.java
public class ViewGroupParent extends LinearLayout {
public ViewGroupParent(Context context) {
super(context);
}
public ViewGroupParent(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public ViewGroupParent(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
Log.i("TAG", "ViewGroupParent onMeasure called");
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
Log.i("TAG", "ViewGroupParent onLayout called");
super.onLayout(changed, l, t, r, b);
}
@Override
protected void onDraw(Canvas canvas) {
Log.i("TAG", "ViewGroupParent onDraw called");
super.onDraw(canvas);
}
}
MyChildView.java
public class MyChildView extends View {
public MyChildView(Context context) {
super(context);
}
public MyChildView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public MyChildView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
Log.i("TAG", "MyChildView onMeasure called");
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
Log.i("TAG", "MyChildView onLayout called");
super.onLayout(changed, left, top, right, bottom);
}
@Override
protected void onDraw(Canvas canvas) {
Log.i("TAG", "MyChildView onDraw called");
super.onDraw(canvas);
}
}
【问题讨论】：
-
This looks like reasonable output我对那个输出有点困惑，因为docs 告诉：The measuring pass is implemented in measure(int, int) and is a top-down traversal of the View tree。您能否澄清一下，为什么在这种情况下它是自下而上的遍历？ -
@azizbekian，我猜这是因为我在
onMeasure方法中调用super之后有Log语句。 -
没错，现在logs are printed 的顺序正确。我认为编辑问题是有意义的，以免其他读者感到困惑。
-
@azizbekian，好的，我用重新排序的 Log 语句编辑了问题。
标签： android android-layout view