New York (CNN) In a rare public appearance eight months after leaving office, former President Barack Obama took shots at his successor's plans on health care and foreign policy but didn't criticize him by name.

In a speech and question-and-answer session that closely resembled the types of appearances he favored as president, Obama called repeated efforts to repeal his signature health care law "frustrating." He defended institutions like the United Nations as essential to collective diplomacy. And he warned against nationalism and xenophobia, saying they threaten to stymie progress that has made the world more livable.

"It may be frustrating that we have to mobilize every couple months to keep our leaders from inflicting real human suffering on their constituents," Obama said of the health care battle . "But typically, that's how progress is won."

Obama's speech at an event sponsored by the Bill and Melinda Gates Foundation came during Trump's first trip to the UN General Assembly, the yearly gathering of diplomats in New York. As Obama spoke, Trump was shuttling between meetings with the leaders of the Palestinian Authority and various countries in Africa

If Trump's campaign message and governing philosophy has been focused on repairing a damaged country, Obama offered a starkly different view on Wednesday, focusing instead on the advantages Americans currently enjoy compared to moments in the past.

"We just have to reject the notion that we are suddenly gripped by forces that we can't control, and embrace the longer, more optimistic view of history and the part that we play in it," he said. "I know that statement doesn't seem to jibe with the steady stream of bad news and cynicism we're fed on television and Twitter," he said.

Obama has largely avoided criticizing his Trump since the handoff of power in January, though he has released statements on Facebook decrying decisions Trump has made over the course of his presidency.

He was sharply critical of Trump's plan to end a program shielding certain young immigrants from deportation, and he has been critical of attempts to repeal and replace the Affordable Care Act.

In other areas, however, Obama has expressed more veiled displeasure. He didn't respond directly to Trump's equivocal comments about white supremacist violence in Virginia, instead tweeting a quote from Nelson Mandela (the tweet became the most liked of all time).

The two men have not spoken since January, and their relationship is marked more by animosity than a shared respect at having both held the same office.

On Wednesday, Obama followed the classic post-presidential etiquette of not directly criticizing the sitting president -- even as he offered an example of praise for George W. Bush.

In a friendly question-and-answer session led by Bill and Melinda Gates, Obama argued that Americans should be willing to acknowledge when "someone from a different political persuasion has done something really good."

He did not cite any policy from the Trump presidency, however, but rather one from the Bush administration: the PEPFAR program to combat AIDS in Africa. Obama called it an "achievement that we needed to build on."

"I feel as if, these days within our political circles, that it's a hard thing to bring ourselves to do," Obama said.

He offered only implicit rebukes of Trump, saying there are no practical benefits to a bill Republicans are considering to replace the Affordable Care Act

Speaking in New York, Obama said the bill would increase costs and strip coverage from vulnerable Americans.

"When I see people trying to undo that hard-won progress, for the 50th or 60th time, with bills that will raise costs, reduce coverage and roll back protections for older Americans and people with pre-existing conditions...it is aggravating," Obama said.

And he decried a decision by the Trump administration to withdraw from the Paris climate accord, saying the move was misguided.

"Even if the federal government is not as engaged on these efforts as I would like," the world is still making progress on climate change, Obama said.

Obama has reemerged in public infrequently since leaving the White House in January, focused instead on writing a memoir and traveling. He has spoken at a smattering of events for his own foundation, and appeared in Europe in May with German Chancellor Angela Merkel

This fall, Obama is due to appear more, holding events for Democrats running in an off-election year, including gubernatorial candidates in New Jersey and Virginia.

The event where he appeared Tuesday was organized to unveil a new "Goalkeepers" report that assesses a set of indicators pegged to the Global Goals, targets set by the United Nations in 2015 focused on reducing poverty and improving health. The report tracks statistics on child mortality, HIV rates and other areas to identify areas of need and single out successful methods for success.