全面解析Activity的生命周期

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在Android应用中, Activity是最重要的组件, 其生命周期(Lifecycle)被大家所熟知.
可是, 大家须要注意一些细节, 才干在使用中得心应手.

Android的生命周期改变主要有两种情况, 一是用户參与Activity生命周期的改变, 二是系统回收或配置改动导致的改变.

本文演示样例的GitHub下载地址

Android的生命周期:

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1. 用户參与

用户最常见的两种操作, 按Home键或Back键.
(1) 点击Home键, 暂停当前Activity, 调用onPause -> onStop; 再次返回, 恢复当前页面, 调用onRestart -> onStart -> onResume.
(2) 点击Back键, 关闭当前Activity, 调用onPause -> onStop -> onDestroy; 再次启动则为重建, 从onCreate開始调用.

onStart()是Activity可见, 无法交互; onResume()是Activity可见, 能够交互. onPause是Activity无法交互, 必须运行前一个Activity的onPause完毕, 后一个Activity才干启动, 在onPause中, 不能运行复杂的操作, 否则会影响下一个Activity启动速度. onStop是Activity关闭显示.

注意: 启动透明页面仅仅调用onPause, 不会调用onStop, 即仅仅是不能够交互, 但仍在前台显示. 启动非透明页, 会调用onPause -> onStop.

运行操作, 启动透明页, 关闭透明页, 启动非透明页, 关闭非透明页.

E/DEBUG-WCL: MainActivity: onPause
E/DEBUG-WCL: MainActivity: onSaveInstanceState
E/DEBUG-WCL: TranslateActivity: onPause
E/DEBUG-WCL: MainActivity: onResume
E/DEBUG-WCL: TranslateActivity: onStop
E/DEBUG-WCL: TranslateActivity: onDestroy
E/DEBUG-WCL: MainActivity: onPause
E/DEBUG-WCL: SecondActivity: onCreate
E/DEBUG-WCL: MainActivity: onSaveInstanceState
E/DEBUG-WCL: MainActivity: onStop
E/DEBUG-WCL: SecondActivity: onPause
E/DEBUG-WCL: MainActivity: onRestart
E/DEBUG-WCL: MainActivity: onStart
E/DEBUG-WCL: MainActivity: onResume
E/DEBUG-WCL: SecondActivity: onStop
E/DEBUG-WCL: SecondActivity: onDestroy

注意透明页启动, 主页仅仅会调用onPause, 而不调用onStop.
onSaveInstanceState方法, 在关闭时, 都会运行, 但非系统调用启动, 不会运行恢复数据的操作, 即onRestoreInstanceState方法.

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2. 系统重建

在系统配置发生改变时, Activity就会重建, 因为是系统控制的关闭和启动, 会调用onSaveInstanceState保存数据, onRestoreInstanceState恢复数据.

E/DEBUG-WCL: MainActivity: onPause
E/DEBUG-WCL: MainActivity: onSaveInstanceState
E/DEBUG-WCL: MainActivity: onStop
E/DEBUG-WCL: MainActivity: onDestroy
E/DEBUG-WCL: MainActivity: onCreate
E/DEBUG-WCL: MainActivity: onStart
E/DEBUG-WCL: MainActivity: onRestoreInstanceState
E/DEBUG-WCL: MainActivity: onResume

onRestoreInstanceState在能够交互之前(onResume)运行, onSaveInstanceState在停止交互(onPause)之后运行.

系统会保存关闭之前的视图结构和暂存数据. 视图详细保存的数据项, 能够通过阅读文档, 也能够直接阅读代码. 搜索视图类的onSaveInstanceState方法, 类SavedState就是保存的数据.

TextView的onSaveInstanceState

    @Override
    public Parcelable onSaveInstanceState() {
        Parcelable superState = super.onSaveInstanceState();

        // Save state if we are forced to
        boolean save = mFreezesText;
        int start = 0;
        int end = 0;

        if (mText != null) {
            start = getSelectionStart();
            end = getSelectionEnd();
            if (start >= 0 || end >= 0) {
                // Or save state if there is a selection
                save = true;
            }
        }

        if (save) {
            SavedState ss = new SavedState(superState);
            // XXX Should also save the current scroll position!
            ss.selStart = start;
            ss.selEnd = end;

            if (mText instanceof Spanned) {
                Spannable sp = new SpannableStringBuilder(mText);

                if (mEditor != null) {
                    removeMisspelledSpans(sp);
                    sp.removeSpan(mEditor.mSuggestionRangeSpan);
                }

                ss.text = sp;
            } else {
                ss.text = mText.toString();
            }

            if (isFocused() && start >= 0 && end >= 0) {
                ss.frozenWithFocus = true;
            }

            ss.error = getError();

            if (mEditor != null) {
                ss.editorState = mEditor.saveInstanceState();
            }
            return ss;
        }

        return superState;
    }

TextView主要保存了内容Text和焦点Focus, 另一些位置和错误信息.

假设是恢复自己定义数据, 恢复时机, 能够选择onCreate或onRestoreInstanceState, 差别是onCreate须要推断savedInstanceState是否为空, 而onRestoreInstanceState的參数savedInstanceState是一定非空, 否则不会调用. 推荐使用onRestoreInstanceState.

onCreate判空

        if (savedInstanceState != null) {
            String txt = savedInstanceState.getString(EXTRA_TEXT);
            Log.e(TAG, "[onCreate]savedInstanceState: " + txt);
        }

在AndroidManifest的Activity属性中加入

android:configChanges="orientation|screenSize"

防止在旋转屏幕时, 重绘页面, 但屏幕仍能够旋转.
不会调用生命周期, 而是调用onConfigurationChanged处理旋转屏幕事件.

使用android:screenOrientation=”portrait”会使屏幕保持竖直, 无法旋转.

动画

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參考

OK, that’s all! Enjoy it!