android: View的getWidth() 和 getMeasureWidth()方法的区别

View的高宽是由View本身和Parent容器共同决定的。
 getMeasuredWidth() 和 getWidth() 分别对应于视图绘制的 measure 和 layout 阶段。getMeasuredWidth()获取的是View原始的大小，也就是这个View在XML文件中配置或者是代码中设置的大小。getWidth()获取的是这个View最终显示的大小，这个大小有可能等于原始的大小，也有可能不相等。比如说，在父布局的onLayout()方法或者该View的onDraw()方法里调用measure(0, 0)，二者的结果可能会不同（measure中的参数可以自己定义）。

getWidth()

/**
     * Return the width of the your view.
     * @return The width of your view, in pixels.
     */
    @ViewDebug.ExportedProperty(category = "layout")
    public final int getWidth() {
        return mRight - mLeft;
    }

从源码上看， getWidth() 是根据 mRight 和 mLeft 之间的差值计算出来的，需要在布局之后才能确定它们的坐标，也就是说布局后在onLayout()方法里才能调用getWidth()来获取。因此，getWidth()获取的宽度是在View设定好布局后整个View的宽度。

getMeasuredWidth()

/**
     * Like {@link #getMeasuredWidthAndState()}, but only returns the
     * raw width component (that is the result is masked by
     * {@link #MEASURED_SIZE_MASK}).
     *
     * @return The raw measured width of this view.
     */
    public final int getMeasuredWidth() {
        return mMeasuredWidth & MEASURED_SIZE_MASK;
    }

从源码上看， getMeasuredWidth() 获取的是 mMeasuredWidth 的这个值。这个值是一个8位的十六进制的数字，高两位表示的是这个measure阶段的Mode的值，具体可以查看MeasureSpec的原理。这里 mMeasuredWidth & MEASURED_SIZE_MASK 表示的是测量阶段结束之后，View真实的值。而且这个值会在调用了 setMeasuredDimensionRaw() 函数之后会被设置。所以 getMeasuredWidth() 的值是measure阶段结束之后得到的View的原始的值。

    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

    protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int opticalWidth  = insets.left + insets.right;
            int opticalHeight = insets.top  + insets.bottom;

            measuredWidth  += optical ? opticalWidth  : -opticalWidth;
            measuredHeight += optical ? opticalHeight : -opticalHeight;
        }
        setMeasuredDimensionRaw(measuredWidth, measuredHeight);
    }

    private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
        mMeasuredWidth = measuredWidth;
        mMeasuredHeight = measuredHeight;

        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
    }

总结一下， getMeasuredWidth 是measure阶段获得的View的原始宽度， getWidth 是layout阶段完成后，其在父容器中所占的最终宽度。

什么时候不同呢？

首先自定义一个父布局： CustomView

package com.yongdaimi.android.androidapitest.view;

import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.widget.LinearLayout;

import androidx.annotation.Nullable;

public class CustomView extends LinearLayout {

    public static final String TAG = "xp";

    private View mFirstView;

    private int mFirstViewWidth;
    private int mFirstViewHeight;

    private int mFirstViewMeasureWidth;
    private int mFirstViewMeasureHeight;

    public CustomView(Context context) {
        this(context, null);
    }

    public CustomView(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public CustomView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    private void init() {
        setOrientation(LinearLayout.HORIZONTAL);
    }


    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        mFirstView = getChildAt(0);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);

        mFirstViewWidth = mFirstView.getWidth();
        mFirstViewHeight = mFirstView.getHeight();

        mFirstViewMeasureWidth = mFirstView.getMeasuredWidth();
        mFirstViewMeasureHeight = mFirstView.getMeasuredHeight();


        Log.i(TAG, "onSizeChanged: mFirstViewWidth: " + mFirstViewWidth + ", mFirstViewHeight: " + mFirstViewHeight
                + ", mFirstViewMeasureWidth: " + mFirstViewMeasureWidth + ", mFirstViewMeasureHeight: " + mFirstViewMeasureHeight);
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);
        mFirstView.layout(-100, 0, -50, 50);

        mFirstViewWidth = mFirstView.getWidth();
        mFirstViewHeight = mFirstView.getHeight();

        mFirstViewMeasureWidth = mFirstView.getMeasuredWidth();
        mFirstViewMeasureHeight = mFirstView.getMeasuredHeight();

        Log.i(TAG, "onLayout: mFirstViewWidth: " + mFirstViewWidth + ", mFirstViewHeight: " + mFirstViewHeight
                + ", mFirstViewMeasureWidth: " + mFirstViewMeasureWidth + ", mFirstViewMeasureHeight: " + mFirstViewMeasureHeight);
    }

}

然后在activity的布局文件使用这个布局并添加一个新的控件：

<?xml version="1.0" encoding="utf-8"?>
<com.yongdaimi.android.androidapitest.view.CustomView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/tv_first"
        android:layout_width="60dip"
        android:layout_height="60dip"
        android:background="@android:color/holo_green_light"
        />

</com.yongdaimi.android.androidapitest.view.CustomView>

运行：

2020-09-04 10:56:51.202 30999-30999/com.yongdaimi.android.androidapitest I/xp: onSizeChanged: mFirstViewWidth: 0, mFirstViewHeight: 0, mFirstViewMeasureWidth: 180, mFirstViewMeasureHeight: 180
2020-09-04 10:56:51.204 30999-30999/com.yongdaimi.android.androidapitest I/xp: onLayout: mFirstViewWidth: 50, mFirstViewHeight: 50, mFirstViewMeasureWidth: 180, mFirstViewMeasureHeight: 180

可以看到，只要在代码里重新修改了子控件的摆放位置，getWidth和getMeasureWidth的值就会不同。

如何在onCreate中拿到View的宽度和高度？

在onCreate()中获取View的高宽有三种方法：

1. View.post(runnable)

view.post(new Runnable() {
            @Override
            public void run() {
                int width = view.getWidth();
                int measuredWidth = view.getMeasuredWidth();
                Log.i(TAG, " " + width);
                Log.i(TAG, "measuredWidth: " + measuredWidth);
            }
        });

利用Handler通信机制，发送一个Runnable到MessageQueue中，当View布局处理完成时，自动发送消息，通知UI进程。借此机制，巧妙获取View的高宽属性，代码简洁，相比ViewTreeObserver监听处理，还不需要手动移除观察者监听事件。

2. ViewTreeObserver.addOnGlobalLayoutListener()

监听View的 onLayout() 绘制过程，一旦layout触发变化，立即回调 onLayoutChange 方法。
注意，使用完也要主要调用 removeOnGlobalListener() 方法移除监听事件。避免后续每一次发生全局View变化均触发该事件，影响性能。

ViewTreeObserver vto = view.getViewTreeObserver();
        vto.addOnGlobalLayoutListener(new OnGlobalLayoutListener() {
            @Override
            public void onGlobalLayout() {
                view.getViewTreeObserver().removeGlobalOnLayoutListener(this);
                Log.i(TAG, " " + view.getWidth());
                Log.i(TAG, "height: " + view.getHeight());
            }
        });

3. View.measure(int widthMeasureSpec, int heightMeasureSpec)

除了在onCreate()中获得View的高宽，还可以在Activity的onWindowFocusChanged() 方法中获得高宽。