01. View基础概念@[toc] 预习 View和activity有什么区别？为什么都是提供视图并且与用户进行交互？

@[toc] 在这里插入图片描述

预习

View和activity有什么区别？为什么都是提供视图并且与用户进行交互？

01. View的介绍

1.1 什么是View

View 代表了一个矩形区域，可以用来显示内容或响应用户的交互。

1.2 常见的View的子类

View 是抽象类，所有可见的 UI 元素，都是 View 的子类。常见的 View 类型包括：

TextView：用于显示文本内容。
Button：一个可点击的按钮。
ImageView：用于显示图片。
EditText：一个可以输入文本的编辑框。
CheckBox：一个带勾选框的选择项。

1.3 View和ViewGroup

ViewGroup 是 View 的子类，他很特殊，不像Button那些子类一样用来显示内容，他用来充当一个容器，他内部可以包含多个子 View 或子 ViewGroup，以此来实现复杂的 UI 布局。ViewGroup和View共同实现了view树，如下图：

在这里插入图片描述

同时ViewGroup 也是 Android 中布局控件的父类，所有布局类（如 LinearLayout、RelativeLayout、FrameLayout）都继承自 ViewGroup。以下是一些常见的 ViewGroup 子类及其功能：

LinearLayout：线性布局，以水平或垂直的顺序排列子视图。
RelativeLayout：相对布局，根据子视图之间的相对位置进行排列。
FrameLayout：帧布局，子视图会堆叠显示，适合显示单一子视图。
RecyclerView：用于显示大量数据的高性能列表，可以实现复杂的列表布局。

02. View 的生命周期

2.1 View的核心生命周期

View 的生命周期主要包括测量、布局、绘制等步骤。

onMeasure()：测量 View 的宽度和高度。
onLayout()：确定 View 的位置，尤其在 ViewGroup 中更为重要。
onDraw()：绘制 View 的内容。

2.2 View的详细生命周期

在View中有诸多回调方法，它们在View的不同生命周期阶段调用，常用的有以下方法。写一个简单的自定义View来观察View与Activity的生命周期变化。

public class CustomView extends View {

    private static final String TAG = "View";

    public CustomView(Context context) {
        super(context);
        Log.d(TAG, "CustomView()");
    }

    public CustomView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        Log.d(TAG, "CustomView()");
    }

    public CustomView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        Log.d(TAG, "CustomView()");
    }

    /**
     * View在xml文件里加载完成时调用
     */
    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        Log.d(TAG, "View onFinishInflate()");
    }

    /**
     * 测量View及其子View大小时调用
     */
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        Log.d(TAG, "View onMeasure()");
    }

    /**
     * 布局View及其子View大小时调用
     */
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        Log.d(TAG, "View onLayout() left = " + left + " top = " + top + " right = " + right + " bottom = " + bottom);
    }

    /**
     * View大小发生改变时调用
     */
    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        Log.d(TAG, "View onSizeChanged() w = " + w + " h = " + h + " oldw = " + oldw + " oldh = " + oldh);
    }

    /**
     * 绘制View及其子View大小时调用
     */
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Log.d(TAG, "View onDraw()");
    }

    /**
     * 物理按键事件发生时调用
     */
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        Log.d(TAG, "View onKeyDown() event = " + event.getAction());
        return super.onKeyDown(keyCode, event);
    }

    /**
     * 物理按键事件发生时调用
     */
    @Override
    public boolean onKeyUp(int keyCode, KeyEvent event) {
        Log.d(TAG, "View onKeyUp() event = " + event.getAction());
        return super.onKeyUp(keyCode, event);
    }

    /**
     * 触摸事件发生时调用
     */
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.d(TAG, "View onTouchEvent() event =  " + event.getAction());
        return super.onTouchEvent(event);
    }

    /**
     * View获取焦点或者失去焦点时调用
     */
    @Override
    protected void onFocusChanged(boolean gainFocus, int direction, @Nullable Rect previouslyFocusedRect) {
        super.onFocusChanged(gainFocus, direction, previouslyFocusedRect);
        Log.d(TAG, "View onFocusChanged() gainFocus = " + gainFocus);
    }

    /**
     * View所在窗口获取焦点或者失去焦点时调用
     */
    @Override
    public void onWindowFocusChanged(boolean hasWindowFocus) {
        super.onWindowFocusChanged(hasWindowFocus);
        Log.d(TAG, "View onWindowFocusChanged() hasWindowFocus = " + hasWindowFocus);
    }

    /**
     * View被关联到窗口时调用
     */
    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();
        Log.d(TAG, "View onAttachedToWindow()");
    }

    /**
     * View从窗口分离时调用
     */
    @Override
    protected void onDetachedFromWindow() {
        super.onDetachedFromWindow();
        Log.d(TAG, "View onDetachedFromWindow()");
    }

    /**
     * View的可见性发生变化时调用
     */
    @Override
    protected void onVisibilityChanged(@NonNull View changedView, int visibility) {
        super.onVisibilityChanged(changedView, visibility);
        Log.d(TAG, "View onVisibilityChanged() visibility = " + visibility);
    }

    /**
     * View所在窗口的可见性发生变化时调用
     */
    @Override
    protected void onWindowVisibilityChanged(int visibility) {
        super.onWindowVisibilityChanged(visibility);
        Log.d(TAG, "View onWindowVisibilityChanged() visibility = " + visibility);
    }
}

2.3 Activity与View的生命周期

Activity create

2024-08-22 13:48:02.423 31303-31303/org.jjjjjjava.freshview D/Test--View--: CustomView()
2024-08-22 13:48:02.423 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onFinishInflate()
2024-08-22 13:48:02.618 31303-31303/org.jjjjjjava.freshview D/Test--Activity--: onCreate
2024-08-22 13:48:02.627 31303-31303/org.jjjjjjava.freshview D/Test--Activity--: onStart
2024-08-22 13:48:02.630 31303-31303/org.jjjjjjava.freshview D/Test--Activity--: onResume
2024-08-22 13:48:02.670 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onAttachedToWindow()
2024-08-22 13:48:02.670 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onWindowVisibilityChanged() visibility = 0
2024-08-22 13:48:02.670 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onVisibilityChanged() visibility = 0
2024-08-22 13:48:02.687 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onMeasure()
2024-08-22 13:48:02.709 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onMeasure()
2024-08-22 13:48:02.713 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onSizeChanged() w = 1080 h = 30 oldw = 0 oldh = 0
2024-08-22 13:48:02.714 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onLayout() left = 0 top = 0 right = 1080 bottom = 30
2024-08-22 13:48:02.993 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onDraw()
2024-08-22 13:48:03.058 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onWindowFocusChanged() hasWindowFocus = true

Activity pause

2024-08-22 13:50:02.843 31303-31303/org.jjjjjjava.freshview D/Test--Activity--: onPause
2024-08-22 13:50:02.866 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onWindowFocusChanged() hasWindowFocus = false
2024-08-22 13:50:03.343 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onWindowVisibilityChanged() visibility = 8
2024-08-22 13:50:03.369 31303-31303/org.jjjjjjava.freshview D/Test--Activity--: onStop
2024-08-22 13:50:03.375 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onVisibilityChanged() visibility = 4

Activity resume

2024-08-22 13:50:57.229 31303-31303/org.jjjjjjava.freshview D/Test--Activity--: onStart
2024-08-22 13:50:57.232 31303-31303/org.jjjjjjava.freshview D/Test--Activity--: onResume
2024-08-22 13:50:57.233 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onVisibilityChanged() visibility = 0
2024-08-22 13:50:57.237 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onWindowVisibilityChanged() visibility = 0
2024-08-22 13:50:57.277 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onDraw()
2024-08-22 13:50:57.299 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onWindowFocusChanged() hasWindowFocus = true

Activity destory

2024-08-22 13:51:46.013 31303-31303/org.jjjjjjava.freshview D/Test--Activity--: onStop
2024-08-22 13:51:46.014 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onVisibilityChanged() visibility = 4
2024-08-22 13:51:46.019 31303-31303/org.jjjjjjava.freshview D/Test--Activity--: onDestroy
2024-08-22 13:51:46.020 31303-31303/org.jjjjjjava.freshview D/Test--View--: View onDetachedFromWindow()

来总结一下View的声明周期随着Activity生命周期变化的情况。

01. View基础概念