android RelativeLayout注意

最新推荐文章于 2021-07-07 11:21:12 发布

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最新推荐文章于 2021-07-07 11:21:12 发布

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分类专栏： Android

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本文深入探讨了RelativeLayout布局中Gravity属性使用时的问题与解决方法，包括Gravity与alignBottom的使用技巧，以及RelativeLayout在滚动容器中的测量问题。通过具体实例演示，帮助开发者避免常见布局陷阱。

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最近在写代码时，偶然发现发现RelativeLayout 的setGravity并不是完全按照预想的设置进行展示，所以有一点纠结，刚开始以为是写的有问题，后来进行了一些测试发现，这个Gravity属性的确存在一些问题.

测试1：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@android:color/holo_blue_bright"
    android:gravity="bottom"
    tools:context=".MainActivity">

    <ImageView
        android:id="@+id/id_img_down"
        android:src="@mipmap/ad2_720"
        android:layout_width="300px"
        android:layout_height="400px"
        />

    <ImageView
        android:id="@+id/id_img_up"
        android:src="@mipmap/ahtv"
        android:layout_width="100px"
        android:layout_height="100px"/>
</RelativeLayout>

预想1：两张ImageView都在 RelativeLayout 的底部

结果1：
这里写图片描述

之后又尝试用了alineg_botom 都有一些小问题(进行了很多测试，由于时间紧就不详细描述了)最后终于得到想要的效果：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@android:color/holo_blue_bright"
    android:gravity="bottom"
    tools:context=".MainActivity">

    <ImageView
        android:id="@+id/id_img_down"
        android:src="@mipmap/ic_launcher"
        android:layout_width="300px"
        android:layout_height="400px"
        />

    <ImageView
        android:id="@+id/id_img_up"
        android:src="@mipmap/ahtv"
        android:layout_width="100px"
        android:layout_height="100px"
        android:layout_alignBottom="@+id/id_img_down"/>
</RelativeLayout>

后来在API文档上看到有这样一段介绍：

A Layout where the positions of the children can be described in relation to each other or to the parent. 

Note that you cannot have a circular dependency between the size of the RelativeLayout and the position of its children. For example, you cannot have a RelativeLayout whose height is set to WRAP_CONTENT and a child set to ALIGN_PARENT_BOTTOM.  

Note: In platform version 17 and lower, RelativeLayout was affected by a measurement bug that could cause child views to be measured with incorrect MeasureSpec values. (See MeasureSpec.makeMeasureSpec for more details.) This was triggered when a RelativeLayout container was placed in a scrolling container, such as a ScrollView or HorizontalScrollView. If a custom view not equipped to properly measure with the MeasureSpec mode UNSPECIFIED was placed in a RelativeLayout, this would silently work anyway as RelativeLayout would pass a very large AT_MOST MeasureSpec instead. 

This behavior has been preserved for apps that set android:targetSdkVersion="17" or older in their manifest's uses-sdk tag for compatibility. Apps targeting SDK version 18 or newer will receive the correct behavior

所以RelativeLayout在设置重力位置的时候还是有很多坑的：
1 使用Gravity 如果RelativeLayout里面有两个View，那么Gravity就不会是预想的。
2 使用align要注意相应id的view大小，对自己view大小的影响
3 还有文档上说明的circular dependency及 RelativeLayout 被放到scrolling container时的测量问题