原理：获取iOS底层安全区范围 返回到游戏层 对画布进行重新设置
游戏层代码：SetCanvasBounds.cs 拖入画布 和画布子物体RectTransform
文章末尾有链接

using System;
using System.Collections;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using UnityEngine;

public class SetCanvasBounds : MonoBehaviour {

#if UNITY_IOS
    [DllImport("__Internal")]
    private extern static void GetSafeAreaImpl(out float x, out float y, out float w, out float h);
#endif

    private Rect GetSafeArea()
    {
        float x, y, w, h;
#if UNITY_IOS
        GetSafeAreaImpl(out x, out y, out w, out h);
#else
        x = 0;
        y = 0;
        w = Screen.width;
        h = Screen.height;
#endif
        return new Rect(x, y, w, h);
    }

    public RectTransform canvas;
    public RectTransform panel;
    Rect lastSafeArea = new Rect(0, 0, 0, 0);

    // Use this for initialization
    void Start () {
        
    }

    void ApplySafeArea(Rect area)
    {
        var anchorMin = area.position;
        var anchorMax = area.position + area.size;
        anchorMin.x /= Screen.width;
        anchorMin.y /= Screen.height;
        anchorMax.x /= Screen.width;
        anchorMax.y /= Screen.height;
        panel.anchorMin = anchorMin;
        panel.anchorMax = anchorMax;

        lastSafeArea = area;
    }

    // Update is called once per frame
    void Update () 
    {
        Rect safeArea = GetSafeArea(); // or Screen.safeArea if you use a version of Unity that supports it

        if (safeArea != lastSafeArea)
            ApplySafeArea(safeArea);
    }
}

底层代码（也就是放在unityplugin文件夹下代码）
SafeAreaImpl.mm

#include <CoreGraphics/CoreGraphics.h>
#include "UnityAppController.h"
#include "UI/UnityView.h"

CGRect CustomComputeSafeArea(UIView* view)
{
    CGSize screenSize = view.bounds.size;
    CGRect screenRect = CGRectMake(0, 0, screenSize.width, screenSize.height);
    
    UIEdgeInsets insets = UIEdgeInsetsMake(0, 0, 0, 0);
    if ([view respondsToSelector: @selector(safeAreaInsets)])
        insets = [view safeAreaInsets];
    
    screenRect.origin.x += insets.left;
    screenRect.origin.y += insets.bottom; // Unity uses bottom left as the origin
    screenRect.size.width -= insets.left + insets.right;
    screenRect.size.height -= insets.top + insets.bottom;
    
    float scale = view.contentScaleFactor;
    screenRect.origin.x *= scale;
    screenRect.origin.y *= scale;
    screenRect.size.width *= scale;
    screenRect.size.height *= scale;
    return screenRect;
}

extern "C" void GetSafeAreaImpl(float* x, float* y, float* w, float* h)
{
    UIView* view = GetAppController().unityView;
    CGRect area = CustomComputeSafeArea(view);
    *x = area.origin.x;
    *y = area.origin.y;
    *w = area.size.width;
    *h = area.size.height;
}

编辑器：

image.png

效果：

IMG_20180115_150658.jpg