6.2视图的载体View

目录介绍

00.View简单介绍
01.View的生命周期
02.生命周期作用

00.View简单介绍

先看一段介绍
- This class represents the basic building block for user interface components. A Viewoccupies a rectangular area on the screen and is responsible for drawing and event handling.
View是屏幕上的一块矩形区域，负责界面的绘制与触摸事件的处理，它是一种界面层控件的抽象，所有的控件都继承自View。
- View是Android显示框架中较为复杂的一环，首先是它的生命周期会随着Activity的生命周期进行变化，掌握View的生命周期对我们自定义View有着重要的意义。另一个方面View从ViewRoot.performTraversals()开始经历measure、layout、draw三个流程最终显示在用户面前，用户在点击屏幕时，点击事件随着Activity传入Window，最终由ViewGroup/View进行分发处理。

01.View的生命周期

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);
    }
}

1.1 Activity与View的生命周期

Activity create

2019-08-22 13:48:02.423 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: CustomView()
2019-08-22 13:48:02.423 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onFinishInflate()
2019-08-22 13:48:02.618 31303-31303/org.yczbj.ycrefreshview D/小杨测试--Activity--: onCreate
2019-08-22 13:48:02.627 31303-31303/org.yczbj.ycrefreshview D/小杨测试--Activity--: onStart
2019-08-22 13:48:02.630 31303-31303/org.yczbj.ycrefreshview D/小杨测试--Activity--: onResume
2019-08-22 13:48:02.670 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onAttachedToWindow()
2019-08-22 13:48:02.670 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onWindowVisibilityChanged() visibility = 0
2019-08-22 13:48:02.670 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onVisibilityChanged() visibility = 0
2019-08-22 13:48:02.687 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onMeasure()
2019-08-22 13:48:02.709 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onMeasure()
2019-08-22 13:48:02.713 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onSizeChanged() w = 1080 h = 30 oldw = 0 oldh = 0
2019-08-22 13:48:02.714 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onLayout() left = 0 top = 0 right = 1080 bottom = 30
2019-08-22 13:48:02.993 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onDraw()
2019-08-22 13:48:03.058 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onWindowFocusChanged() hasWindowFocus = true

Activity pause

2019-08-22 13:50:02.843 31303-31303/org.yczbj.ycrefreshview D/小杨测试--Activity--: onPause
2019-08-22 13:50:02.866 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onWindowFocusChanged() hasWindowFocus = false
2019-08-22 13:50:03.343 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onWindowVisibilityChanged() visibility = 8
2019-08-22 13:50:03.369 31303-31303/org.yczbj.ycrefreshview D/小杨测试--Activity--: onStop
2019-08-22 13:50:03.375 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onVisibilityChanged() visibility = 4

Activity resume

2019-08-22 13:50:57.229 31303-31303/org.yczbj.ycrefreshview D/小杨测试--Activity--: onStart
2019-08-22 13:50:57.232 31303-31303/org.yczbj.ycrefreshview D/小杨测试--Activity--: onResume
2019-08-22 13:50:57.233 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onVisibilityChanged() visibility = 0
2019-08-22 13:50:57.237 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onWindowVisibilityChanged() visibility = 0
2019-08-22 13:50:57.277 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onDraw()
2019-08-22 13:50:57.299 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onWindowFocusChanged() hasWindowFocus = true

Activity destory

2019-08-22 13:51:46.013 31303-31303/org.yczbj.ycrefreshview D/小杨测试--Activity--: onStop
2019-08-22 13:51:46.014 31303-31303/org.yczbj.ycrefreshview D/小杨测试--View--: View onVisibilityChanged() visibility = 4
2019-08-22 13:51:46.019 31303-31303/org.yczbj.ycrefreshview D/小杨测试--Activity--: onDestroy
2019-08-22 13:51:46.020 31303-31303/org.yczbj.ycrefreshview D/小杨测试--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。

02.生命周期作用

了解这些生命周期方法有什么作用呢？🤔其实这些方法在我们自定义View的时候发挥着很大的作用，我们来举几种应用场景。

场景1：在Activity启动时获取View的宽高，但是在onCreate、onStart和onResume均无法获取正确的结果。这是因为在Activity的这些方法里，Viewed绘制可能还没有完成，我们可以在View的生命周期方法里获取。

@Override
public void onWindowFocusChanged(boolean hasFocus) {
    super.onWindowFocusChanged(hasFocus);
    if(hasFocus){
        int width = view.getMeasuredWidth();
        int height = view.getMeasuredHeight();
    }
}

场景2：在Activity生命周期发生变化时，View也要做响应的处理，典型的有VideoView保存进度和恢复进度。

@Override
protected void onVisibilityChanged(@NonNull View changedView, int visibility) {
    super.onVisibilityChanged(changedView, visibility);
    //TODO do something if activity lifecycle changed if necessary
    //Activity onResume()
    if(visibility == VISIBLE){
        
    }
    //Activity onPause()
    else {
        
    }
}

@Override
public void onWindowFocusChanged(boolean hasWindowFocus) {
    super.onWindowFocusChanged(hasWindowFocus);

    //TODO do something if activity lifecycle changed if necessary
    //Activity onResume()
    if (hasWindowFocus) {
    }
    //Activity onPause()
    else {
    }
}

场景3：释放线程、资源

@Override
protected void onDetachedFromWindow() {
    super.onDetachedFromWindow();
    //TODO release resources, thread, animation
}

其他介绍

01.关于博客汇总链接

02.关于我的博客

github：https://github.com/yangchong211
知乎：https://www.zhihu.com/people/yczbj/activities
简书：http://www.jianshu.com/u/b7b2c6ed9284
csdn：http://my.csdn.net/m0_37700275
喜马拉雅听书：http://www.ximalaya.com/zhubo/71989305/
开源中国：https://my.oschina.net/zbj1618/blog
泡在网上的日子：http://www.jcodecraeer.com/member/content_list.php?channelid=1
邮箱：yangchong211@163.com
阿里云博客：https://yq.aliyun.com/users/article?spm=5176.100- 239.headeruserinfo.3.dT4bcV
segmentfault头条：https://segmentfault.com/u/xiangjianyu/articles
掘金：https://juejin.im/user/5939433efe88c2006afa0c6e