ABC to Let App Users Live Stream Local Programming This week ABC will quietly revolutionize its app for iPhones and iPads with a button called "live." Users around New York and Philadelphia will be able to live stream all the programming from ABC's local stations there, the first time that any major broadcaster has turned on such a technology. The functionality will be featured at ABC's upfront presentation for advertisers on Tuesday. It is, among other things, an attempt to keep up with the rapidly changing expectations of television viewers. It also reflects the increasing role that subscriber fees play in the broadcasting business: the live stream will be available only to paying subscribers of cable and satellite providers, even though the stations" signals are available free over the public airwaves. ABC, a unit of the Walt Disney Company, said the live stream would be available in the other six cities where it owns stations sometime this summer. It is also in talks with the companies that own ABC's more than 200 affiliates to make the "live" button work in their markets. ABC finished the first of its affiliate deals, with Hearst Television, on Sunday afternoon; it said the live streams would work in Hearst's 13 markets, including Boston and Pittsburgh, in the coming months. The mobile app may prod the other broadcasters to follow ABC, much as they did seven years ago after the network started to stream full episodes of shows the morning after their TV premieres. ABC had originally planned to introduce a live-streaming feature for its apps in 2014, but decided to speed up that process this year. "We keep a very close eye on consumer demand," said Anne Sweeney, the co-chairwoman of the Disney/ABC Television Group, which includes the broadcast network. We watch how people are behaving with their devices, and we really felt that we needed to move faster. Internally the project was code-named Project Acela, a reference to the high-speed train between Boston and Washington. A team led by Albert Cheng, Ms. Sweeney's executive vice president for digital media, was given a deadline of May 14, the date of the ABC upfront. While Apple devices came first, other phones and tablets will be supported in the coming months, Mr. Cheng said. The live-stream functionality comes at a time when ABC and its broadcast rivals are trying to keep the attention of audiences that are increasingly turning to cable channels and Internet streaming services like Netflix. It gives ABC another talking point about how it is adapting to audience preferences; in this case, viewers will be able to carry "Good Morning America" with them as they move around the house in the morning, or tune into a weekend basketball game while out with friends. The live stream will work anywhere in a local market, the same way an old-fashioned TV antenna would. During a demonstration of the app in her New York office on Friday, Ms. Sweeney said she was struck by how personalized television becomes when it is live streamed to a person's phone. The app is also an implicit rebuttal to Aereo, the start-up backed by Barry Diller that is being sued by major station owners for streaming their signals to paying subscribers in New York. Ms. Sweeney reiterated her view that Aereo is illegal but said the plans for the app's live-stream feature predated the service. The app, to be named Watch ABC, in line with Disney's existing Watch Disney and Watch ESPN apps, will allow users to watch ABC shows on demand, like the network's previous app had. In the future, ABC will withhold its most recent TV episodes from the free versions of Hulu and ABC.com, further limiting access to paying subscribers of cable and satellite providers only. The mobile live stream will not carry the same ads as the television broadcast; instead, it will include the same sorts of digital ads as on ABC.com. This is in part because the Nielsen Company is not able to measure mobile viewing of live television yet. "What you see here is the same live programming," Mr. Cheng said as he used the app, "but what we are doing during the commercial break is actually inserting new ads into the stream."