activity详解（二 源码篇）

Posted 2020-10-16

想要剖析一个activity灵魂深处的秘密，必然要从“召唤”它的地方开始，一步一步跟踪到它结束，那么现在让我们在源码内一步一步了解他的整个生命周期吧

一般来说，我们打开一个activity的方式是使用startActivity方法来打开一个Activity类的子类，所以查看第一个方法

    /**
     * Same as {@link #startActivity(Intent, Bundle)} with no options
     * specified.
     * 和startActivity(Intent, Bundle)方法相同，只不过没有定义Bundle
     *
     * @param intent The intent to start.
     *
     * @throws android.content.ActivityNotFoundException
     *
     * @see {@link #startActivity(Intent, Bundle)}
     * @see #startActivityForResult
     */
    @Override
    public void startActivity(Intent intent) {
        this.startActivity(intent, null);
    }

这里我们可以知道，其实在传入一个intent参数之后startActivity方法后，其实它的内部又会调用一个不同参的startActivity的重载方法，那么继续查看这个方法

    /**
     * Launch a new activity.  You will not receive any information about when
     * the activity exits.  This implementation overrides the base version,
     * providing information about
     * the activity performing the launch.  Because of this additional
     * information, the {@link Intent#FLAG_ACTIVITY_NEW_TASK} launch flag is not
     * required; if not specified, the new activity will be added to the
     * task of the caller.
     * 启动一个新的activity.而在该activity退出时，你将不会收到任何信息.
     * 它实现并重写了基础版本，提供activity准备启动时的信息，
     * 因为附加的启动flag没有被强制要求需要填写，如果没有设置的话，新的activity将会被加入到调用的task栈内.
     * （结合下文可知，启动flag，类似FLAG_ACTIVITY_NEW_TASK的，定义的是activity的启动方式，和activity的四种启动模式有关）
     *
     * <p>This method throws {@link android.content.ActivityNotFoundException}
     * if there was no Activity found to run the given Intent.
     *
     * @param intent The intent to start.
     * @param options Additional options for how the Activity should be started.
     * See {@link android.content.Context#startActivity(Intent, Bundle)
     * Context.startActivity(Intent, Bundle)} for more details.
     *
     * @throws android.content.ActivityNotFoundException
     *
     * @see {@link #startActivity(Intent)}
     * @see #startActivityForResult
     */
    @Override
    public void startActivity(Intent intent, @Nullable Bundle options) {
        if (options != null) {
            startActivityForResult(intent, -1, options);
        } else {
            // Note we want to go through this call for compatibility with
            // applications that may have overridden the method.
            startActivityForResult(intent, -1);
        }
    } 

可以看到因为使用startActivity(intent)，我们一般调用的都会是startActivityForResult(intent, -1)，那么startActivityForResult方法是什么呢

 

    /**
     * Same as calling {@link #startActivityForResult(Intent, int, Bundle)}
     * with no options.
     *
     * @param intent The intent to start.
     * @param requestCode If >= 0, this code will be returned in
     *                    onActivityResult() when the activity exits.
     * requestCode如果大于1，那么在activity结束后，会在onActivityResult()方法中返回这个code值
     * (默认传入的是-1，所以不会在activity结束后返回code值)
     *
     * @throws android.content.ActivityNotFoundException
     *
     * @see #startActivity
     */
    public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) {
        startActivityForResult(intent, requestCode, null);
    }

 

得，又是一个重载方法

    /**
     * Launch an activity for which you would like a result when it finished.
     * When this activity exits, your
     * onActivityResult() method will be called with the given requestCode.
     * Using a negative requestCode is the same as calling
     * {@link #startActivity} (the activity is not launched as a sub-activity).
     *
     * <p>Note that this method should only be used with Intent protocols
     * that are defined to return a result.  In other protocols (such as
     * {@link Intent#ACTION_MAIN} or {@link Intent#ACTION_VIEW}), you may
     * not get the result when you expect.  For example, if the activity you
     * are launching uses the singleTask launch mode, it will not run in your
     * task and thus you will immediately receive a cancel result.
     *
     * <p>As a special case, if you call startActivityForResult() with a requestCode
     * >= 0 during the initial onCreate(Bundle savedInstanceState)/onResume() of your
     * activity, then your window will not be displayed until a result is
     * returned back from the started activity.  This is to avoid visible
     * flickering when redirecting to another activity.
     *
     * <p>This method throws {@link android.content.ActivityNotFoundException}
     * if there was no Activity found to run the given Intent.
     *
     * @param intent The intent to start.
     * @param requestCode If >= 0, this code will be returned in
     *                    onActivityResult() when the activity exits.
     * @param options Additional options for how the Activity should be started.
     * See {@link android.content.Context#startActivity(Intent, Bundle)
     * Context.startActivity(Intent, Bundle)} for more details.
     *
     * @throws android.content.ActivityNotFoundException
     *
     * @see #startActivity
     */
    public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
        if (mParent == null) {
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode, options);
            if (ar != null) {
                mMainThread.sendActivityResult(
                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                    ar.getResultData());
            }
            if (requestCode >= 0) {
                // If this start is requesting a result, we can avoid making
                // the activity visible until the result is received.  Setting
                // this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
                // activity hidden during this time, to avoid flickering.
                // This can only be done when a result is requested because
                // that guarantees we will get information back when the
                // activity is finished, no matter what happens to it.
                mStartedActivity = true;
            }

            cancelInputsAndStartExitTransition(options);
            // TODO Consider clearing/flushing other event sources and events for child windows.
        } else {
            if (options != null) {
                mParent.startActivityFromChild(this, intent, requestCode, options);
            } else {
                // Note we want to go through this method for compatibility with
                // existing applications that may have overridden it.
                mParent.startActivityFromChild(this, intent, requestCode);
            }
        }
    }