子视图的onMeasure和onLayout不会在其父视图的requestLayout之后被调用

Question

我正在尝试理解Android如何测量和布局视图。我创建了一个带有自定义视图和视图组的示例应用程序。

CustomViewGroup

public class CustomViewGroup extends ViewGroup {
    private static final String LOG_TAG = "CustomViewGroup";
    public CustomViewGroup(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        Log.d(LOG_TAG, String.format(Locale.ENGLISH,
                getTag() + " " +
                        "onMeasure(widthMeasureSpec=%d, heightMeasureSpec%d)",
                widthMeasureSpec, heightMeasureSpec
        ));
        final int count = getChildCount();
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != View.GONE) {
                // Make or work out measurements for children here (MeasureSpec.make...)
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
            }
        }
    }
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        Log.d(LOG_TAG, String.format(Locale.ENGLISH,
                getTag() + " " +
                        "onLayout(changed=%b, left=%d, top=%d, right=%d, bottom=%d)",
                changed, left, top, top, right, bottom
        ));
    }
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Log.d(LOG_TAG, getTag() + " " + "onDraw");
    }
}

CustomView

public class CustomView extends View {
    private static final String LOG_TAG = "CustomView";
    public CustomView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        Log.d(LOG_TAG, String.format(Locale.ENGLISH,
                getTag() + " " +
                "onLayout(changed=%b, left=%d, top=%d, right=%d, bottom=%d)",
                changed, left, top, top, right, bottom
        ));
    }
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Log.d(LOG_TAG, getTag() + " " +"onDraw");
    }
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        Log.d(LOG_TAG, String.format(Locale.ENGLISH,
                getTag() + " " +
                "onMeasure(widthMeasureSpec=%d, heightMeasureSpec=%d)",
                widthMeasureSpec, heightMeasureSpec
        ));
    }
}

MainActivity

public class MainActivity extends AppCompatActivity {
    private View mCustomGroup1;
    private View mCustomGroup2;
    private View mContainer;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Log.d("MainAct", "onCreate");
        mCustomGroup1 = findViewById(R.id.requestLayout);
        mCustomGroup2 = findViewById(R.id.requestLayout2);
        mContainer = findViewById(R.id.activity_main);
        findViewById(R.id.requestLayoutBtn).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mCustomGroup1.requestLayout();
            }
        });
        findViewById(R.id.requestLayoutBtn2).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mCustomGroup2.requestLayout();
            }
        });
        findViewById(R.id.requestLayoutContBtn).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mContainer.requestLayout();
            }
        });
    }
}

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:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="ru.dmitriev.squareorderedlayout.MainActivity">
    <Button
        android:id="@+id/requestLayoutBtn"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="requestLayout" />
    <Button
        android:id="@+id/requestLayoutBtn2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="requestLayout2" />
    <Button
        android:id="@+id/requestLayoutContBtn"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="requestLayoutCont" />
    <ru.dmitriev.squareorderedlayout.CustomViewGroup
        android:id="@+id/requestLayout"
        android:layout_width="100dp"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:background="#7f7"
        android:tag="CustomVieGroup1">
        <ru.dmitriev.squareorderedlayout.CustomView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:tag="CustomView11" />
        <ru.dmitriev.squareorderedlayout.CustomView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:tag="CustomView12" />
    </ru.dmitriev.squareorderedlayout.CustomViewGroup>
    <ru.dmitriev.squareorderedlayout.CustomViewGroup
        android:id="@+id/requestLayout2"
        android:layout_width="100dp"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:background="#f77"
        android:tag="CustomVieGroup2">
        <ru.dmitriev.squareorderedlayout.CustomView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:tag="CustomView21" />
        <ru.dmitriev.squareorderedlayout.CustomView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:tag="CustomView22" />
    </ru.dmitriev.squareorderedlayout.CustomViewGroup>
</LinearLayout>

活动开始时，在标记为CustomVieGroup1和CustomVieGroup2的视图组上调用onMeasure, onLayout和onDraw。每个视图组在其子视图组上调用这些方法。没关系。我明白了。

当我在标记为CustomVieGroup1的视图组上调用requestLayout时，onMeasure, onLayout和onDraw在CustomVieGroup1上调用。该组织对他们的孩子使用这些方法。我明白了。

当我在标记为CustomVieGroup2的视图组上调用requestLayout时，onMeasure, onLayout和onDraw在CustomVieGroup2上调用。该组织对他们的孩子使用这些方法。我也理解。

但是当我在id为activity_main的视图上调用requestLayout时，onMeasure和onLayout不会在CustomVieGroup1或CustomVieGroup2上调用。为什么?我预计onMeasure, onLayout和onDraw将在CustomVieGroup1和CustomVieGroup2(即id为activity_main的视图的子视图)上被调用

根据requestLayout的文档:

这将调度视图树的布局传递。

Answer 1

requestLayout()将导致LinearLayout (@id/activity_main)的onLayout()方法执行。然而，不能保证LinearLayout会重新布局它的子节点;这真的取决于它的执行。例如，您的LinearLayout宽度/高度设置为match_parent。当LinearLayout第二次布局时，它的宽度和高度不会改变，因为它的大小是基于它的父大小，而不是它的子大小。因为它的布局不是基于它的子元素，所以LinearLayout不需要重新布局它的子元素。这就是为什么几乎所有ViewGroup大小与其父布局匹配的布局都无效的原因。

您应该能够确认将LinearLayout (@id/activity_main)更改为'wrap_content'。通过这样做，布局的大小变得依赖于它的子元素，所以它需要重新布局它的子元素。这将导致调用自定义视图布局方法。