Android Fragment onAttach(Context context) 不执行

一.描述

相信很多同学在使用fragment的时候会重写onAttach(Activity activity)回调方法
应该都很清楚在Fragment直接只用getActivity（）获得上下文有时会得到空引用，所以我们大多是重写上面回调方法得到一个当前Fragment全局的上下文
，但是有些同学发现以下方法已经过时了

@Overridepublic void onAttach(Activity activity) {    super.onAttach(activity);    mAct = activity;}
所以大家都使用google新提供的方法
@Overridepublic void onAttach(Context context) {    super.onAttach(activity);    mAct = （Activity）context;}

废话少说直接贴代码

1.创建项目

首先Actvity里的布局文件

<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:tools="http://schemas.android.com/tools"    android:id="@+id/activity_main"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:orientation="vertical"    android:paddingBottom="@dimen/activity_vertical_margin"    android:paddingLeft="@dimen/activity_horizontal_margin"    android:paddingRight="@dimen/activity_horizontal_margin"    android:paddingTop="@dimen/activity_vertical_margin"    tools:context="com.designpatterns.MainActivity">    <FrameLayout        android:id="@+id/fl_content"        android:layout_width="match_parent"        android:layout_height="0dp"        android:layout_weight="1"></FrameLayout></LinearLayout>

创建Fragment的布局文件fragment_demo.xml

<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:orientation="vertical">    <TextView        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="fragment"        android:textSize="30sp" /></LinearLayout>

创建DemoFragment

package com.designpatterns.fragment;import android.annotation.TargetApi;import android.app.Activity;import android.app.Fragment;import android.content.Context;import android.os.Build;import android.os.Bundle;import android.support.annotation.Nullable;import android.util.Log;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import com.designpatterns.R;/** * Created by panda on 2017/6/30. */public class DemoFragment extends Fragment {    private Activity mAct;    @TargetApi(23)    @Override    public void onAttach(Context context) {        super.onAttach(context);        mAct = (Activity) context;        if (context == null) {            Log.i("panda", "context is null");        } else {            Log.i("panda", "context is not null");        }        Log.i("panda", "context==" + Build.VERSION.SDK_INT);    }    /*     * Deprecated on API 23     * Use onAttachToContext instead     */    @Override    public void onAttach(Activity activity) {        super.onAttach(activity);        if (activity == null) {            Log.i("panda", "activity is null");        } else {            Log.i("panda", "activity is not null");        }        Log.i("panda", "activity1==" + Build.VERSION.SDK_INT);        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.M) {            mAct = activity;            Log.i("panda", "activity2==" + Build.VERSION.SDK_INT);        }    }    @Nullable    @Override    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {        View rootView = inflater.inflate(R.layout.fragment_demo                , container, false);        return rootView;    }}

创建MainActivity

public class MainActivity extends AppCompatActivity {     private boolean isShow = false;    private DemoFragment demoFragment;    private FragmentManager manager;    private FragmentTransaction transaction;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        /**         * 拿到事务管理器并开启事务         */        manager = getFragmentManager();        transaction = manager.beginTransaction();        demoFragment = new DemoFragment();        transaction.replace(R.id.fl_content, demoFragment);        transaction.commit();    }}        

2.测试运行结果

在5.1.1手机上运行得到的结果

07-01 14:56:02.088 13338-13338/com.designpatterns I/panda: activity is not null
07-01 14:56:02.088 13338-13338/com.designpatterns I/panda: activity1==22
07-01 14:56:02.088 13338-13338/com.designpatterns I/panda: activity2==22

大家发现没有，只会执行onAttach(Activity activity)回调，而不会执行onAttach(Context context)回调，所以会导致上下文为空

在23系统版本上运行测试结果

07-01 14:57:50.499 23752-23752/com.designpatterns I/panda: activity is not null
07-01 14:57:50.499 23752-23752/com.designpatterns I/panda: activity1==23
07-01 14:57:50.499 23752-23752/com.designpatterns I/panda: context is not null
07-01 14:57:50.499 23752-23752/com.designpatterns I/panda: context==23

发现没有两个回调都执行了，而且两个回调回来的上下文都不会为空，所以有些时候不一定说过时的方法就不能用，如果是要用新方法，那么我们可以多测试下，看看选择哪种方案比较合适就用那种方式