二、Fragment相关生命周期、动态添加

使用Fragment 我们可以把页面结构划分成几块，每块使用一个Fragment来管理。这样我们可以更加方便的在运行过程中动态地更新Activity中的用户界面，日后迭代更新、维护也是更加方便

注意事项： Fragment并不能单独使用，他需要嵌套在Activity 中使用，尽管他拥有自己的生命周期，但是还是会受到宿主Activity的生命周期的影响，比如Activity 被destory销毁了，他也会跟着销毁！一个Activity可以嵌套多个Fragment。

图片和部分文字转载自https://www.songyubao.com/book/primary/activity/fragment.html

1.Fragment的生命周期

四个场景用于加深对Fragment生命周期的理解

①Activity加载Fragment的时候,依次调用下面的方法: onAttach -> onCreate -> onCreateView -> onActivityCreated -> onStart ->onResume

②当我们启动一个新的页面, 此时Fragment所在的Activity不可见，会执行 onPause

③当新页面返回后，当前Activity和Fragment又可见了，会再次执行onStart和 onResume

④退出了Activity的话,那么Fragment将会被完全结束, Fragment会进入销毁状态 onPause -> onStop -> onDestoryView -> onDestory -> onDetach

onActivityCreated 已过时

常用 onCreateView 当前fragment所对应的视图对象

onResume 恢复动画/视频播放

2. Fragment的动态添加与数据传递 2.1 动态添加Fragment

class SecondActivity : AppCompatActivity(){

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_second)


        val fragment  = SecondFragment()
        //事务操作对象
        val ft:FragmentTransaction = supportFragmentManager.beginTransaction()
        ft.add(R.id.container , fragment)
        ft.commitAllowingStateLoss()
    }
}

2.2 Fragment常见的操作

val fragment = StudyFragment()
val ft = supportFragmentManager.beginTransaction()

if(!fragment.isAdded()){
  ft.add(R.id.container,fragment) //把fragment添加到事务中，当且仅当该fragment未被添加过
}
ft.show(fragment) //显示出fragment的视图
ft.hide(fragment) //隐藏fragment,使得它的视图不可见
ft.remove(fragment)//移除fragment
//替换fragment,之前添加过的fragment都会被暂时移除，把当前这个fragment添加到事务中
ft.replace(R.id.container,fragment)

//提交事务，执行对fragment的add、replace、show、hide操作
ft.commitAllowingStateLoss()

2.3 给Fragment传递数据

Activity传入数值

class SecondActivity : AppCompatActivity(){

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_second)

        val fragment  = SecondFragment()

        //给Fragment传递数据
        val bundle = Bundle()
        bundle.putInt("int_extra" , 100)
        bundle.putString("string_extra" , "string_extra")
        fragment.arguments = bundle

        //事务操作对象
        val ft:FragmentTransaction = supportFragmentManager.beginTransaction()
        ft.add(R.id.container , fragment)
        ft.commitAllowingStateLoss()
    }
}

Fragment接受数据

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)

    val intValue = arguments?.getInt("int_extra")
    val strValue = arguments?.getString("string_extra")
	
    //view是onCreateView return的view
    val textview = view as TextView
    textview.text = "${intValue} ---${strValue}"
}

思考：运行时Fragment如何获取Activity中的数据、又如何将数据传递到Activity中呢？

https://developer.android.com/guide/fragments/communicate?hl=zh-cn

设计并实现底部导航栏页面结构 Activity页面布局

此布局和如果创建项目选择带导航栏的样式相同

tips

1.占用父布局(除了导航栏)剩余的空间

 android:layout_height="0dp"
 android:layout_weight="1"

2.水平平分

android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="1"

监听选中事件

在onCreate方法中

//改变选中按钮的颜色
binding.toggleGroup.addOnButtonCheckedListener { group:MaterialButtonToggleGroup, checkedId:Int , isChecked:Boolean ->
    val childCount = group.childCount
    var selectIndex = 0
    for (index in 0 until childCount){
        val button = group.getChildAt(index) as MaterialButton
        if(button.id == checkedId){
            //选中的按钮变红
            selectIndex = index
            button.setTextColor(Color.CYAN)
            button.iconTint = ColorStateList.valueOf(Color.CYAN)
        } else {
            button.setTextColor(Color.BLACK)
            button.iconTint = ColorStateList.valueOf(Color.BLACK)
        }
    }
    switchFragment(selectIndex)
}
binding.toggleGroup.check(binding.tab1.id)

导航栏切换动态切换显示的fragment

private var tab1Fragment:SecondFragment?=null
private var tab2Fragment:SecondFragment?=null
private var tab3Fragment:SecondFragment?=null
private var shownFragment:Fragment ?= null //当前正在显示的Fragment

//切换Fragment
private fun switchFragment(selectIndex: Int) {
    //根据选中tab的id返回fragment
     val fragment = when(selectIndex){
        0->{
            if(tab1Fragment == null){
                tab1Fragment = SecondFragment()
                val bundle = Bundle()
                bundle.putString("tab" , "tab1")
                tab1Fragment!!.arguments = bundle
            }
            tab1Fragment
        }1->{
            if(tab2Fragment == null){
                tab2Fragment = SecondFragment()
                val bundle = Bundle()
                bundle.putString("tab" , "tab2")
                tab2Fragment!!.arguments = bundle
            }
            tab2Fragment
        }2->{
            if(tab3Fragment == null){
                tab3Fragment = SecondFragment()
                val bundle = Bundle()
                bundle.putString("tab" , "tab3")
                tab3Fragment!!.arguments = bundle
            }
            tab3Fragment
        }else -> {
            throw IllegalStateException("下标不符合预期")
        }
     }?:return


    val ft = supportFragmentManager.beginTransaction()
    if(!fragment.isAdded){
        ft.add(R.id.container , fragment)
    }
    ft.show(fragment)
    //避免出现多个fragment重叠
    if(shownFragment !=null){
        ft.hide(shownFragment!!)
    }
    shownFragment = fragment
    ft.commitAllowingStateLoss()
}

本例中多个fragment的生命周期

第一次点击各个tab 每个fragment onAttach----> onResume

2021-11-11 16:34:22.661 5666-5666/com.example.componentlearn E/SecondFragment: onAttach
2021-11-11 16:34:22.662 5666-5666/com.example.componentlearn E/SecondFragment: onCreate
2021-11-11 16:34:22.665 5666-5666/com.example.componentlearn E/SecondFragment: onCreateView
2021-11-11 16:34:22.675 5666-5666/com.example.componentlearn E/SecondFragment: onStart
2021-11-11 16:34:22.679 5666-5666/com.example.componentlearn E/SecondFragment: onResume
2021-11-11 16:34:24.570 5666-5666/com.example.componentlearn E/SecondFragment: onHiddenChanged : tab1-- true
2021-11-11 16:34:24.573 5666-5666/com.example.componentlearn E/SecondFragment: onAttach
2021-11-11 16:34:24.574 5666-5666/com.example.componentlearn E/SecondFragment: onCreate
2021-11-11 16:34:24.575 5666-5666/com.example.componentlearn E/SecondFragment: onCreateView
2021-11-11 16:34:24.576 5666-5666/com.example.componentlearn E/SecondFragment: onStart
2021-11-11 16:34:24.577 5666-5666/com.example.componentlearn E/SecondFragment: onResume
2021-11-11 16:34:25.737 5666-5666/com.example.componentlearn E/SecondFragment: onHiddenChanged : tab2-- true
2021-11-11 16:34:25.739 5666-5666/com.example.componentlearn E/SecondFragment: onAttach
2021-11-11 16:34:25.740 5666-5666/com.example.componentlearn E/SecondFragment: onCreate
2021-11-11 16:34:25.741 5666-5666/com.example.componentlearn E/SecondFragment: onCreateView
2021-11-11 16:34:25.741 5666-5666/com.example.componentlearn E/SecondFragment: onStart
2021-11-11 16:34:25.745 5666-5666/com.example.componentlearn E/SecondFragment: onResume

tab3切换tab2

2021-11-11 16:35:53.583 5666-5666/com.example.componentlearn E/SecondFragment: onHiddenChanged : tab3-- true
2021-11-11 16:35:53.584 5666-5666/com.example.componentlearn E/SecondFragment: onHiddenChanged : tab2-- false

tab2切换tab1

2021-11-11 16:35:55.533 5666-5666/com.example.componentlearn E/SecondFragment: onHiddenChanged : tab2-- true
2021-11-11 16:35:55.533 5666-5666/com.example.componentlearn E/SecondFragment: onHiddenChanged : tab1-- false

当且仅当activity存在多个fragment ，并且我们调用了show - hide

fragment的生命周期会调用

override fun onHiddenChanged(hidden: Boolean)

不可见为true , 可见为false

Bug记录

使用view-binding的时候 addOnButtonCheckedListener无效果

原因setContentView位置和内容写错

正确的是

private lateinit var binding: ActivitySecondBinding

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    binding = ActivitySecondBinding.inflate(layoutInflater)
    setContentView(binding.root)

二、Fragment相关生命周期、动态添加

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