RelativeLayout measure方法报NullPointerException

问题描述

之前开发中碰到这样一个bug，部分机型会有崩溃现象，错误内容如下：

- E/AndroidRuntime( 3579): FATAL EXCEPTION: main
- E/AndroidRuntime( 3579): java.lang.NullPointerException
- E/AndroidRuntime( 3579):    at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:431)
- E/AndroidRuntime( 3579):    at android.view.View.measure(View.java:8462)

很奇怪，明明没有什么特别的地方，但是在Android4.3的机器上就是会有崩溃~~~

问题分析

在stackoverflow上发现有人碰到过相同的问题。

问题原因
RelativeLayout的onMeasure方法中有这样一段代码：

if (isWrapContentWidth) {
            // Width already has left padding in it since it was calculated by looking at
            // the right of each child view
            width += mPaddingRight;

            if (mLayoutParams != null && mLayoutParams.width >= 0) {
                width = Math.max(width, mLayoutParams.width);
            }

            width = Math.max(width, getSuggestedMinimumWidth());
            width = resolveSize(width, widthMeasureSpec);

看起来没有问题，但是对比Android 4.3源码，

if (mLayoutParams.width >= 0) {
      width = Math.max(width, mLayoutParams.width);
}

这也就是为什么会在Android4.3手机上出现崩溃的原因之一。
再深入探究一下mLayoutParams这个参数为什么会是null。首先看代码的使用方法：

contentView = (ViewGroup) LayoutInflater.
               rom(context).inflate(R.layout.select_text_oprate_window, null);

这句代码的意思是，从xml中实例化View视图，但是父视图为空，所以从xml实例化的View视图没有办法attach到View层次树中。那么这个和mLayoutParams有什么关系？？~~~
首先看看mLayoutParams这个参数表示什么意思：

/**
     * The layout parameters associated with this view and used by the parent
     * {@link android.view.ViewGroup} to determine how this view should be
     * laid out.
     * {@hide}
     */
    protected ViewGroup.LayoutParams mLayoutParams;

从注释中可以看出，这个参数表示子View布局的期望值，实际值由父View，或者说整个View树决定。

 /**
     * Get the LayoutParams associated with this view. All views should have
     * layout parameters. These supply parameters to the <i>parent</i> of this
     * view specifying how it should be arranged. There are many subclasses of
     * ViewGroup.LayoutParams, and these correspond to the different subclasses
     * of ViewGroup that are responsible for arranging their children.
     *
     * This method may return null if this View is not attached to a parent
     * ViewGroup or {@link #setLayoutParams(android.view.ViewGroup.LayoutParams)}
     * was not invoked successfully. When a View is attached to a parent
     * ViewGroup, this method must not return null.
     *
     * @return The LayoutParams associated with this view, or null if no
     *         parameters have been set yet
     */
    @ViewDebug.ExportedProperty(deepExport = true, prefix = "layout_")
    public ViewGroup.LayoutParams getLayoutParams() {
        return mLayoutParams;
    }

在来看这个函数的get方法。注释里面写了两条：1、mLayoutParams这个参数必须设置；2、当View视图attach到父视图（View层次树），mLayoutParams这个参数肯定不会为null。

问题解决

回到项目代码，

contentView = (ViewGroup) LayoutInflater.
             rom(context).inflate(R.layout.select_text_oprate_window, null);

首先，这种inflate方法没有父布局，所以也没有attach这一说~~~
再者，项目代码中没有显示调用contentView.setLayoutParams(params)方法。按照源码中的说法，这个参数可能报NullPointerException。（为什么是可能？？！！）
解决方法：

采用一般建议的inflate方法

contentView = (ViewGroup) LayoutInflater.
             rom(context).inflate(R.layout.select_text_oprate_window, parent,false);

显示setLayoutParams

contentView = (ViewGroup) LayoutInflater.
                from(context).inflate(R.layout.select_text_oprate_window, null);

 ViewGroup.LayoutParams params = new ViewGroup.LayoutParams(
                ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
        contentView.setLayoutParams(params);

RelativeLayout measure方法报NullPointerException