Android系统窗口管理框架——View的生命周期

1.View的概述

This class represents the basic building block for user interface components. A View
occupies a rectangular area on the screen and is responsible for drawing and
event handling

这是官方给出的View的定义。

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。

2.View的生命周期

View有他自己的生命周期，并且它的生命周期和activity的生命周期息息相关，所以，我们和activity的生命周期一起讨论。
写一个简单的自定义View。

package com.wind.windowscontrolframework_view;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;

/**
 * Created by zhangcong on 2018/3/1.
 */

public class CustomeView extends View {
    private static final String TAG = "View";

    public CustomeView(Context context) {
        super(context);
        Log.i(TAG, "CustomeView");
    }

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

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

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

    /**
     * 布局View及其子View位置时调用
     *
     * @param changed
     * @param left
     * @param top
     * @param right
     * @param bottom
     */
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        Log.i(TAG, "onLayout");
    }

    /**
     * view的大小改变时调用
     *
     * @param w
     * @param h
     * @param oldw
     * @param oldh
     */
    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        Log.i(TAG, "onSizeChanged");
    }

    /**
     * 绘制view的时候调用
     */
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        Log.i(TAG,"measuredHeight"+getMeasuredHeight()+"measuredWidth:"+getMeasuredWidth());
        Log.i(TAG,"Height"+getHeight()+"Width:"+getWidth());
        Log.i(TAG, "onDraw");
        Paint paint = new Paint();
        paint.setColor(Color.RED);
        canvas.drawCircle(100, 100, 50, paint);
        Log.i(TAG,"measuredHeight"+getMeasuredHeight()+"measuredWidth:"+getMeasuredWidth());
        Log.i(TAG,"Height"+getHeight()+"Width:"+getWidth());
    }

    /**
     * view的焦点发生变化是调用
     *
     * @param gainFocus
     * @param direction
     * @param previouslyFocusedRect
     */
    @Override
    protected void onFocusChanged(boolean gainFocus, int direction, @Nullable Rect previouslyFocusedRect) {
        super.onFocusChanged(gainFocus, direction, previouslyFocusedRect);
        Log.i(TAG, "onFocusChanged");
    }

    /**
     * view所在的Windows发生焦点变化是调用
     *
     * @param hasWindowFocus
     */
    @Override
    public void onWindowFocusChanged(boolean hasWindowFocus) {
        super.onWindowFocusChanged(hasWindowFocus);
        Log.i(TAG, "onWindowFocusChanged");
    }

    /**
     * view被关联到窗口是调用
     */
    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();
        Log.i(TAG, "onAttachedToWindow");
    }

    /**
     * view从窗口解除是调用
     */
    @Override
    protected void onDetachedFromWindow() {
        super.onDetachedFromWindow();
        Log.i(TAG, "onDetachedFromWindow");
    }

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

    /**
     * view所在的窗口可见性发生变化时调用
     *
     * @param visibility
     */
    @Override
    protected void onWindowVisibilityChanged(int visibility) {
        super.onWindowVisibilityChanged(visibility);
        Log.i(TAG, "onWindowVisibilityChanged"+visibility);
    }
}

我们就可以看到activity和view的生命周期了；

Activity create

02-18 07:23:23.946 7866-7866/com.wind.windowscontrolframework_view I/View: CustomeView
02-18 07:23:23.946 7866-7866/com.wind.windowscontrolframework_view I/View: onFinishInflate
02-18 07:23:23.947 7866-7866/com.wind.windowscontrolframework_view I/View: onCreate
02-18 07:23:23.953 7866-7866/com.wind.windowscontrolframework_view I/View: onStart
02-18 07:23:23.961 7866-7866/com.wind.windowscontrolframework_view I/View: onResume
02-18 07:23:24.004 7866-7866/com.wind.windowscontrolframework_view I/View: onAttachedToWindow
02-18 07:23:24.004 7866-7866/com.wind.windowscontrolframework_view I/View: onWindowVisibilityChanged0
02-18 07:23:24.004 7866-7866/com.wind.windowscontrolframework_view I/View: onVisibilityChanged0
02-18 07:23:24.013 7866-7866/com.wind.windowscontrolframework_view I/View: onMeasure
02-18 07:23:24.034 7866-7866/com.wind.windowscontrolframework_view I/View: onMeasure
02-18 07:23:24.036 7866-7866/com.wind.windowscontrolframework_view I/View: onSizeChanged
02-18 07:23:24.037 7866-7866/com.wind.windowscontrolframework_view I/View: onLayout
02-18 07:23:24.084 7866-7866/com.wind.windowscontrolframework_view I/View: onWindowFocusChanged
02-18 07:23:24.094 7866-7866/com.wind.windowscontrolframework_view I/View: measuredHeight600measuredWidth:600
02-18 07:23:24.094 7866-7866/com.wind.windowscontrolframework_view I/View: Height600Width:600
02-18 07:23:24.094 7866-7866/com.wind.windowscontrolframework_view I/View: onDraw

Activity pause

02-18 11:08:15.585 7866-7866/com.wind.windowscontrolframework_view I/View: onPause
02-18 11:08:15.615 7866-7866/com.wind.windowscontrolframework_view I/View: onWindowFocusChanged
02-18 11:08:15.684 7866-7866/com.wind.windowscontrolframework_view I/View: onWindowVisibilityChanged8
02-18 11:08:15.947 7866-7866/com.wind.windowscontrolframework_view I/View: onStop

Activity destroy

02-18 11:08:15.949 7866-7866/com.wind.windowscontrolframework_view I/View: onDestroy
02-18 11:08:15.949 7866-7866/com.wind.windowscontrolframework_view I/View: onDetachedFromWindow

我们来总结一下View的声明周期随着Activity生命周期变化的情况。

我们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
}

参考 Android应用视图的载体View