关于FLAG_ACTIVITY_NEW_TASK的使用

参考文章： https://blog.csdn.net/u010389391/article/details/78558475

Context调用startActivity, 有部分情况会报出如下错误：

Caused by: android.util.AndroidRuntimeException: Calling startActivity() from outside of an Activity context requires the FLAG_ACTIVITY_NEW_TASK flag. Is this really what you want?
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翻看源码根据Android的委托机制，ContextWrapper最后是调用了ContextImpl的startActivity方法

@Override
    public void startActivity(Intent intent, Bundle options) {
        warnIfCallingFromSystemProcess();
        if ((intent.getFlags()&Intent.FLAG_ACTIVITY_NEW_TASK) == 0) {
            throw new AndroidRuntimeException(
                    "Calling startActivity() from outside of an Activity "
                    + " context requires the FLAG_ACTIVITY_NEW_TASK flag."
                    + " Is this really what you want?");
        }
        mMainThread.getInstrumentation().execStartActivity(
                getOuterContext(), mMainThread.getApplicationThread(), null,
                (Activity) null, intent, -1, options);
    }

而这样的错误在Activity是不会发生的，原因是Activity重写了这部分的代码

@Override
    public void startActivity(Intent intent) {
        this.startActivity(intent, null);
    }
@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);
        }
    }

public void startActivityForResult(Intent intent, int requestCode, @Nullable Bundle options) {
        if (mParent == null) {
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode, options);

    }

总结：

在Activity上下文之外启动Activity需要给Intent设置FLAG_ACTIVITY_NEW_TASK标志，不然会报异常。